essay about the process of photosynthesis

ENCYCLOPEDIC ENTRY

Photosynthesis.

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar.

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Learning materials, instructional links.

• Photosynthesis (Google doc)

Most life on Earth depends on photosynthesis .The process is carried out by plants, algae, and some types of bacteria, which capture energy from sunlight to produce oxygen (O 2 ) and chemical energy stored in glucose (a sugar). Herbivores then obtain this energy by eating plants, and carnivores obtain it by eating herbivores.

The process

During photosynthesis, plants take in carbon dioxide (CO 2 ) and water (H 2 O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose. The plant then releases the oxygen back into the air, and stores energy within the glucose molecules.

Chlorophyll

Inside the plant cell are small organelles called chloroplasts , which store the energy of sunlight. Within the thylakoid membranes of the chloroplast is a light-absorbing pigment called chlorophyll , which is responsible for giving the plant its green color. During photosynthesis , chlorophyll absorbs energy from blue- and red-light waves, and reflects green-light waves, making the plant appear green.

Light-dependent Reactions vs. Light-independent Reactions

While there are many steps behind the process of photosynthesis, it can be broken down into two major stages: light-dependent reactions and light-independent reactions. The light-dependent reaction takes place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light- dependent reaction. The chlorophyll absorbs energy from the light waves, which is converted into chemical energy in the form of the molecules ATP and NADPH . The light-independent stage, also known as the Calvin cycle , takes place in the stroma , the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light- independent reaction. During this stage, energy from the ATP and NADPH molecules is used to assemble carbohydrate molecules, like glucose, from carbon dioxide.

C3 and C4 Photosynthesis

Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis, including C3 photosynthesis and C4 photosynthesis. C3 photosynthesis is used by the majority of plants. It involves producing a three-carbon compound called 3-phosphoglyceric acid during the Calvin Cycle, which goes on to become glucose. C4 photosynthesis, on the other hand, produces a four-carbon intermediate compound, which splits into carbon dioxide and a three-carbon compound during the Calvin Cycle. A benefit of C4 photosynthesis is that by producing higher levels of carbon, it allows plants to thrive in environments without much light or water. The National Geographic Society is making this content available under a Creative Commons CC-BY-NC-SA license . The License excludes the National Geographic Logo (meaning the words National Geographic + the Yellow Border Logo) and any images that are included as part of each content piece. For clarity the Logo and images may not be removed, altered, or changed in any way.

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Science News Explores

Explainer: how photosynthesis works.

Plants make sugar and oxygen with the power of water, carbon dioxide and sunlight

Green plants take in light from the sun and turn water and carbon dioxide into the oxygen we breathe and the sugars we eat.

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By Bethany Brookshire

October 28, 2020 at 6:30 am

Take a deep breath. Then thank a plant. If you eat fruit, vegetables, grains or potatoes, thank a plant too.  Plants and algae provide us with the oxygen we need to survive, as well as the carbohydrates we use for energy. They do it all through photosynthesis.

Photosynthesis is the process of creating sugar and oxygen from carbon dioxide, water and sunlight. It happens through a long series of chemical reactions. But it can be summarized like this: Carbon dioxide, water and light go in. Glucose, water and oxygen come out. (Glucose is a simple sugar.)

Photosynthesis can be split into two processes. The “photo” part refers to reactions triggered by light. “Synthesis” — the making of the sugar — is a separate process called the Calvin cycle.

Both processes happen inside a chloroplast. This is a specialized structure, or organelle, in a plant cell. The structure contains stacks of membranes called thylakoid membranes. That’s where the light reaction begins.

Let the light shine in

When light hits a plant’s leaves, it shines on chloroplasts and into their thylakoid membranes. Those membranes are filled with chlorophyll , a green pigment. This pigment absorbs light energy. Light travels as electromagnetic waves . The wavelength — distance between waves — determines energy level. Some of those wavelengths are visible to us as the colors we see . If a molecule, such as chlorophyll, has the right shape, it can absorb the energy from some wavelengths of light.

Chlorophyll can absorb light we see as blue and red. That’s why we see plants as green. Green is the wavelength plants reflect, not the color they absorb.

While light travels as a wave, it also can be a particle called a photon . Photons have no mass. They do, however, have a small amount of light energy.

When a photon of light from the sun bounces into a leaf, its energy excites a chlorophyll molecule. That photon starts a process that splits a molecule of water. The oxygen atom that splits off from the water instantly bonds with another, creating a molecule of oxygen, or O 2 . The chemical reaction also produces a molecule called ATP and another molecule called NADPH. Both of these allow a cell to store energy. The ATP and NADPH also will take part in the synthesis part of photosynthesis.

Notice that the light reaction makes no sugar. Instead, it supplies energy — stored in the ATP and NADPH — that gets plugged into the Calvin cycle. This is where sugar is made.

But the light reaction does produce something we use: oxygen. All the oxygen we breathe is the result of this step in photosynthesis, carried out by plants and algae (which are not plants ) the world over.

Give me some sugar

The next step takes the energy from the light reaction and applies it to a process called the Calvin cycle. The cycle is named for Melvin Calvin, the man who discovered it.

The Calvin cycle is sometimes also called the dark reaction because none of its steps require light. But it still happens during the day. That’s because it needs the energy produced by the light reaction that comes before it.

While the light reaction takes place in the thylakoid membranes, the ATP and NADPH it produces end up in the stroma. This is the space inside the chloroplast but outside the thylakoid membranes.

The Calvin cycle has four major steps:

• carbon fixation : Here, the plant brings in CO 2 and attaches it to another carbon molecule, using rubisco. This is an enzyme , or chemical that makes reactions move faster. This step is so important that rubisco is the most common protein in a chloroplast — and on Earth. Rubisco attaches the carbon in CO 2 to a five-carbon molecule called ribulose 1,5-bisphosphate (or RuBP). This creates a six-carbon molecule, which immediately splits into two chemicals, each with three carbons.
• reduction : The ATP and NADPH from the light reaction pop in and transform the two three-carbon molecules into two small sugar molecules. The sugar molecules are called G3P. That’s short for glyceraldehyde 3-phosphate (GLIH- sur-AAL-duh-hide 3-FOS-fayt).
• carbohydrate formation : Some of that G3P leaves the cycle to be converted into bigger sugars such as glucose (C 6 H 12 O 6 ).
• regeneration : With more ATP from the continuing light reaction, leftover G3P picks up two more carbons to become RuBP. This RuBP pairs up with rubisco again. They are now ready to start the Calvin cycle again when the next molecule of CO 2 arrives.

At the end of photosynthesis, a plant ends up with glucose (C 6 H 12 O 6 ), oxygen (O 2 ) and water (H 2 O). The glucose molecule goes on to bigger things. It can become part of a long-chain molecule, such as cellulose; that’s the chemical that makes up cell walls. Plants also can store the energy packed in a glucose molecule within larger starch molecules. They can even put the glucose into other sugars — such as fructose — to make a plant’s fruit sweet.

All of these molecules are carbohydrates — chemicals containing carbon, oxygen and hydrogen. (CarbOHydrate makes it easy to remember.) The plant uses the bonds in these chemicals to store energy. But we use the these chemicals too. Carbohydrates are an important part of the foods we eat, particularly grains, potatoes, fruits and vegetables.

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Photosynthesis As A Biological Process Essay

Introduction.

Photosynthesis is a biological process in which plants utilize the available carbon dioxide in the atmosphere to give out oxygen. There is also the presence of a green pigment called chlorophyll is involved in the transfer of unutilized energy to utilizable chemical energy. Mostly the process of photosynthesis involves the utilization of water to release oxygen that we depend on for our lives. Plants which are the only photosynthetic organism to have leaves are viewed as a solar collector packed with photosynthetic cells. For this process to occur, the following raw material should be available; water and carbon dioxide which after entering the leaf cell it produces oxygen found in the atmosphere. During the process water from the soil is taken up by the roots all the way to the leaves via the xylem. In order for the plants not to dry out they use the stoma so that they can exchange gases. Stomata are the only way in which oxygen can get their way out of the leaf. However during this process a great amount of water is lost. This can be witnessed by the cottonwood trees in dry seasons by loosing a total of 100 gallons daily(Kramer & Kozlowski, 1960).

When you consider this process we can classify plants to be carbon sinks because they play a great role of utilizing the carbon dioxide found in oceans and atmosphere. Plants are also involved in production of carbon dioxide through respiration and used by photosynthesis they too convert energy absorbed from the sunlight into chemical energy with covalent bonds and other carbon dioxide sources includes animals. Carbonates in the ocean are formed so that they can balance the presence carbon dioxide and oxygen in the atmosphere. (Smith, 1984).

Carbon dioxide plays different roles in the plants life cycle. Though in many debates it has never been revealed how higher level of carbon dioxide will benefit the Earth. This is true because food crops, flowers and trees depend mostly on carbon dioxide. According to the Voluminous scientist, evidence shows that when the amount of carbon dioxide in the atmosphere rises above the current level the rate of plant growth will increase and enlarge due to more efficient photosynthesis and reduced water loss. Extreme temperatures will not harm plants, there will be faster growth rates and pollutants and excessive nutrients will not injure plants. Increased carbon dioxide in the atmosphere is projected to increase plant productivity, increases the size of a leaf and thickness, the heights of a stem and seed production. This will also lead to an increase in the both numbers and sizes of fruits and flowers (Smith, 1984).

It is also important to note that, though plants through the process of photosynthesis produces oxygen, they will only survive for a few days without oxygen even if everything is provided. If this goes on for sometimes they cannot stay alive. Plants differ from animals due to their abilities to make their own nutrients through the process of photosynthesis. Through this carbohydrates is produced and it’s broken down by plants to get energy. During this process food is created and a reaction is needed so that the created food can be broken down into usable form, and this process requires oxygen, water and nutrients (Wittwer, 1992).

The above discussed can also be applied to people where by they cannot survive without plants. Plants and animals are the two main kingdoms of life. The Earth consists of more than 300,000 species of plant and they can create their own food by means of energy from sunlight. All oxygen is generated by plants. They also make life on the Earth possible by providing humans with food as well as building material. This plant kingdom has different species which can be grouped into; mosses and liverworts, ferns, cone plants and flowering plants (Wittwer, 1992).

According to me life is made possible by plants for example forests and grasslands which supplies oxygen. According Scientists and conservationists if deforestation goes on without control the survival system on the Earth will be injured. In addition to this plants also act as source of food to the people for example fruits, leaves, roots and tuber, seeds and barks too. Plants can also be seen contributing to the survival of the people whereby they make seeds which are transported to different places of the world spreading it. They are sources of energy, People also depend on plants by exchanging gifts in form of flowers, plants be of assistance when it comes to people surviving the harsh conditions.

Plants reduce the amount of noise in the urban setting and add the aesthetic value to the environment. They also contribute towards the ecology of an area by their roots stabilizing the soils which prevent soil erosion. They also reduce the speed of wind which is mostly used by farmers and provide them with income.

With all this, I would conclude that it will be impossible to say that people can survive without plants. This is so because people need oxygen, food, shelter, building material et cetera which is provided by plants. Therefore I will urge everyone to protect all the trees found on Earth by avoiding degrading activities for example; deforestating and polluting forested areas. By doing so, we will be promoting a healthy life to everyone (Wittwer, 1992).

Kramer, P.J. & Kozlowski, T. (1960). Physiology of trees. New York, NY: McGraw Hill.

Smith, W.H. (1984). Pollutant uptake by plants: In air pollution and plant life. New York, NY: John Wiley.

Wittwer, S.H. (1992). Rising carbon dioxide is great for plants. Journal of Biology, 12(6), 1-9.

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1. IvyPanda . "Photosynthesis As A Biological Process." April 1, 2022. https://ivypanda.com/essays/photosynthesis-as-a-biological-process/.

Bibliography

IvyPanda . "Photosynthesis As A Biological Process." April 1, 2022. https://ivypanda.com/essays/photosynthesis-as-a-biological-process/.

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The Process of Photosynthesis

Introduction.

Photosynthesis is fundamental to the energy flow process in living organisms. “Plants are the primary producers and they make use of sunlight to produce sugars for energy production.” (Govindjee, 1997, p. 45) Excess nutrients are stored and the plants are eaten up by herbivores and omnivores which rely on the energy stored in the plant cells to keep alive. The herbivores are subsequently consumed by the omnivores and carnivores; this process continues from one living organism to another creating a food chain that sustains life on earth.

Problem statement

The energy flow process is fundamental to the sustenance of life on earth. For survival, each organism requires nutrition; the nutrition is sourced from another organism as food substrates except for the green plants which manufacture their food using sunlight and minerals. Many types of research have been conducted and reveal that the process of photosynthesis is the main process that sustains life. This experimental study is designed to ascertain how the process of photosynthesis leads to energy production and how it is affected by variation in light intensity and wavelength.

Relevance of the question

This study is essential for a proper understanding of the role of plants as primary producers in the food chain process.

Literature review

The photosynthesis process occurs primarily in the leaves with little taking place in the stem for some plants. The main parts of the leaf in which are involved in the process include; “the upper and lower epidermis, the mesophyll, the vascular bundles and the stomates.” (Photosynthesis, 2000) The epidermis lacks chlorophyll and therefore photosynthesis does not occur there, the epidermis only acts to protect the internal part of the leaf. “The stomates are tiny holes in the epidermis through which gaseous exchange takes place.” (Photosynthesis, 2000, para.2)

Through the stomates, Co2 enters, and O2 leaves. “The vascular bundles constitute the plants transport system through which water and other nutrients are moved around the plant.” (Photosynthesis, 2000, para.2)

Chlorophyll is composed of the outer and inner membranes, “there is also an inter membrane space stroma and thylakoids which are stacked in grana. The chlorophyll is normally built in the membrane of the thylakoids.” (Photosynthesis, 2000, para.3)

Thus, photosynthesis is the main process in which the “radiant light energy is absorbed by the chloroplast’s pigment, chlorophyll and converted into chemical energy in the molecular form of ATP and NADH.” (Photosynthesis, 2000) The energy is then utilized in driving the Calvin cycle in the production of three-carbon sugars. The three-carbon sugars are subsequently converted to various types of carbohydrates.

According to Govindjee, it is currently possible to carry out photolysis in the laboratory. The reaction was first performed by Robert Hill in 1937 and thus became known as the Hill Reaction. The Hill Reaction requires only intact isolated chloroplasts rather than the entire intact plant cell. However, since isolated chloroplasts do not reduce carbon dioxide directly, it is essential to provide a hydrogen acceptor for the reduction process and permit electron transport to take place. The hydrogen acceptor that was chosen for this experiment is the synthetic compound 2,6-dichlorophenol-indophenol or DPIP , which is blue in the oxidized form and colorless when reduced. As indicated above, the source of hydrogen for the reduction in water. This hydrogen can reduce DPIP and turn it from a blue substance to a colorless one.

Oxygen is also formed, but it is not monitored in this experiment. The change in color of the DPIP solution, which is directly proportional to the number of hydrogen produced in the Hill Reaction, can be assayed using colorimetric spectrophotometry. The more hydrogen produced, the more DPIP that is reduced and the more colorless the solution containing DPIP becomes. Therefore, the course of this reaction can be assessed by the bleaching of the artificial hydrogen acceptor. (1997, p. 144))

Light + CO2 + 2H2O → n (CH2O) + H2O + O2

The experiment entailed the use of both boiled and fresh chloroplast suspension. The boiled chloroplast was used as a control and photosynthetic activities were monitored in the fresh chloroplast. The results were measured using a spectrophotometer and tabulated. The same procedure was repeated with variations in the light intensity and wavelength.

• Spectrophotometer
• Spectrophotometer cuvettes
• Phosphate buffer (pH6.5)
• Chloroplast suspension
• Sodium hydrosulfite
• Distilled water
• Wrapping foil
• Digital timer
• DPIP solution

The source of hydrogen for the reduction in water. This hydrogen can reduce DPIP and turn it from a blue substance to a colorless one. Oxygen is also formed, but it is not monitored in this experiment. The change in color of the DPIP solution, which is directly proportional to the number of hydrogen produced in the Hill Reaction, can be assayed using colorimetric spectrophotometry. The more hydrogen produced, the more DPIP that is reduced and the more colorless the solution containing DPIP becomes. Therefore, the course of this reaction can be assessed by the bleaching of the artificial hydrogen acceptor. The experimental procedure was selected because it has a high specificity and therefore results will have a low error margin. The chemicals used are readily available were purchased in various forms and reconstituted in the laboratory before the practice according to the manufacture’s guidelines.

• Obtain fresh and boiled chloroplast suspension prepared before the practical time. The boiling of the suspension will render the chloroplasts non-functional and will serve as one of the experimental controls. Allow the solution to return to room temperature before use. The chloroplast is boiled preferably at 100 degrees for 5 to 7 minutes.
• Obtain four clean spectrophotometer cuvettes. Label them 1-4 and prepare them as follows: Cuvette 1: 3.0 ml of phosphate buffer (pH 6.5) and 1.0 ml of boiled chloroplast suspension, Cuvette 2: 3.0 ml of phosphate buffer (pH 6.5) and 1.0 ml of chloroplast suspension, Cuvette 3: 3.0 ml of phosphate buffer (pH 6.5) and 1.0 ml of chloroplast suspension, Cuvette 4: 3.0 ml of phosphate buffer (pH 6.5) and 1.0 ml of chloroplast suspension
• Add 50 μl of the 0.1% DPIP solution to cuvette 4 only. Shake it to mix the solution well. Then add a few (very few) crystals of sodium hydrosulfite (Na2S2O4) to it. Sodium hydrosulfite reduces the DPIP and removes the blue color.
• Be sure that the spectrophotometer is set to 600nm. Use the ‘reduced’ cuvette 4 as a blank to zero the spectrophotometer.
• Add the same amount of DPIP that you added to cuvette 4 to cuvette 1. Mix the solution well and take a reading as quickly as possible using the spectrophotometer, because cuvettes with functional chloroplasts should immediately start producing hydrogen that reduces the DPIP. Record this value as ‘time zero’. Start the digital timer.
• Repeat step 5 for cuvettes 2 and 3. Record the ‘time zero’ values and their starting time points.
• Immediately after taking the first readings, wrap cuvette 3 completely in aluminum foil and place cuvettes 1-3 in a beaker of water at room temperature. Set the light source 10 inches from the cuvettes and turn it on to the highest setting. Note that the water in the beaker serves as a thermal buffer to prevent any experimental artifact due to warming by the source light.
• Every two minutes record the optical density of cuvettes 1 and 2. Leave cuvette 3 in the dark until the end of the procedure.
• Continue taking readings until the optical density reading of cuvette 2 does not change.
• Remove cuvette 3 from its foil wrapping. Quickly read the optical density of this cuvette. Take two-minute readings until the optical density reading does not change for two or three-time points. Make a graphical plot with time as the independent variable and optical density as the dependent variable.

Note: Different colors of light indicate the difference in wavelength thus the use of various colors of cellophane to measure wavelength.

Dependent, independent, and controlled variables

The independent variable is the time of exposure whereby the duration of the reaction is changed to determine the effect of time on the DPIP reduction (visualized as the clearing of the blue color). The dependent variable is the optical density which varies depending on the duration the cuvette was left to react. The controlled variables include the use of the same amount of constituent chemical for each tube, temperature of the reactions where all the tubes are reacted at room temperature.

Threat reduction to internal validity was done by:

Taking measurements immediately after the timed duration had expired; was done to prevent errors in the measurement of the optical density. Providing the same conditions for all measurements taken i.e. the spectrophotometer was blanked by the same solution for all the readings which was done at 600nm. All the necessary conditions for the materials were provided prior to and during the experiment.

Plants form an important part of the food chain. “Green plants manufacture their on food using the photosynthesis process.” (Photosynthesis, 2000, para.1) Sunlight is an important aspect of this process; the practical examines the process of photosynthesis, particularly the role played by light. Therefore:

HA: Light is a fundamental factor for the photosynthetic process where it’s used to reduce carbon dioxide and break the water molecule. In this experiment, the hydrogen from the water molecule reacts with DCIP to reduce its blue color. The spectrophotometer is used to measure the intensity of the reduced color.

Ho: Light is not a fundamental factor in the process of photosynthesis and does not reduce carbon dioxide or break the water molecule to produce hydrogen used in the experiment to reduce DCIP’s blue color that is measured using a spectrophotometer.

Use of appropriate methods, tools, and technologies to collect quantitative data

In this experiment quantitative data was collected as optical density readings. The readings were taken at 600 nm after the spectrophotometer was blanked using cuvette 4 which had 3.0 ml of phosphate buffer (pH 6.5) 1.0 ml of chloroplast suspension and 50 μl of the 0.1% DPIP solution added and the reaction left to complete( color clears). The optical density reading for each cuvette was recorded against the time duration in the lab notebook.

Data Collection

Data collection is an important aspect for the success of any experiment, for this particular experiment, the data was collected by the use of a spectrophotometer. Every reaction was timed according to the manual and on expiry of the time duration, the optical density readings were taken using the spectrophotometer and recorded in the laboratory notebook for use in the tabulation and plotting of graphs for analysis.

Experiment: the hill reaction.

Results of the Experiment

The results for the experimental were plotted as below

This experiment was designed to investigate the photosynthesis process in which light is utilized for energy production. In the hill reaction, the rate of photosynthesis remained the same for cuvettes 1 and 3, there was no evidence of photosynthetic activity in the fourth cuvette. In cuvette 5, the photosynthetic activity decreased with time. From the experiments, it is deduced that photosynthesis occurs in two stages, the light-dependent, and the light-independent stage. Generally, “in the first stage energy is captured and stored in the form of ATP and NADPH.” (Photosynthesis, 2000)

In the second stage light-independent stage the energy stored is used to process sugars using carbon. The results of this experiment agree with the hypothesis that light is a fundamental factor for the photosynthetic process where it’s used to reduce carbon dioxide and break the water molecule. In this experiment, the hydrogen from the water molecule reacts with DCIP to reduce its blue color. The spectrophotometer is used to measure the intensity of the reduced color

The experiment was carried out successfully and the results indicate that indeed plants manufacture their own food using sunlight as a source of energy. Therefore sunlight is an important factor in this process. However, the success of the practical was due to the experimental design which provided all the necessary material and conditions for the laboratory model of photosynthesis. A good experimental design gives results with minimal errors and thus can be used to conclude. “Validity refers how closeness the values are for repeated measures.”(Govindjee, 1997, p. 67) Replication of the above experiment gives results from which comparison can be drawn to give a test of validity for this experimental design.

The experiment can be replicated by another person provided he/she formulates a suitable experimental design that will include the identification, manipulation, and measurement of all the parameters in the investigation. The experimental design must be reliable and carefully selected to reduce the error margin to minimum values as this is an important aspect of this experiment.

Reference list

Govindjee, G. (1997). Experiments in plant biology. Berlin: Springer.

Photosynthesis. (2000). Web.

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AP®︎/College Biology

Course: ap®︎/college biology   >   unit 3.

• Photosynthesis

Intro to photosynthesis

• Breaking down photosynthesis stages
• Conceptual overview of light dependent reactions
• The light-dependent reactions
• The Calvin cycle
• Photosynthesis evolution
• Photosynthesis review

Introduction

What is photosynthesis.

• Energy. The glucose molecules serve as fuel for cells: their chemical energy can be harvested through processes like cellular respiration and fermentation , which generate adenosine triphosphate— ATP ‍   , a small, energy-carrying molecule—for the cell’s immediate energy needs.
• Fixed carbon. Carbon from carbon dioxide—inorganic carbon—can be incorporated into organic molecules; this process is called carbon fixation , and the carbon in organic molecules is also known as fixed carbon . The carbon that's fixed and incorporated into sugars during photosynthesis can be used to build other types of organic molecules needed by cells.

The ecological importance of photosynthesis

• Photoautotrophs use light energy to convert carbon dioxide into organic compounds. This process is called photosynthesis.
• Chemoautotrophs extract energy from inorganic compounds by oxidizing them and use this chemical energy, rather than light energy, to convert carbon dioxide into organic compounds. This process is called chemosynthesis.
• Photoheterotrophs obtain energy from sunlight but must get fixed carbon in the form of organic compounds made by other organisms. Some types of prokaryotes are photoheterotrophs.
• Chemoheterotrophs obtain energy by oxidizing organic or inorganic compounds and, like all heterotrophs, get their fixed carbon from organic compounds made by other organisms. Animals, fungi, and many prokaryotes and protists are chemoheterotrophs.

Leaves are sites of photosynthesis

The light-dependent reactions and the calvin cycle.

• The light-dependent reactions take place in the thylakoid membrane and require a continuous supply of light energy. Chlorophylls absorb this light energy, which is converted into chemical energy through the formation of two compounds, ATP ‍   —an energy storage molecule—and NADPH ‍   —a reduced (electron-bearing) electron carrier. In this process, water molecules are also converted to oxygen gas—the oxygen we breathe!
• The Calvin cycle , also called the light-independent reactions , takes place in the stroma and does not directly require light. Instead, the Calvin cycle uses ATP ‍   and NADPH ‍   from the light-dependent reactions to fix carbon dioxide and produce three-carbon sugars—glyceraldehyde-3-phosphate, or G3P, molecules—which join up to form glucose.

Photosynthesis vs. cellular respiration

Attribution.

• “ Overview of Photosynthesis ” by OpenStax College, Biology, CC BY 3.0 . Download the original article for free at http://cnx.org/contents/5bb72d25-e488-4760-8da8-51bc5b86c29d@8 .
• “ Overview of Photosynthesis ” by OpenStax College, Concepts of Biology, CC BY 3.0 . Download the original article for free at http://cnx.org/contents/[email protected] .

Works cited:

• "Great Oxygenation Event." Wikipedia. Last modified July 17, 2016. https://en.wikipedia.org/wiki/Great_Oxygenation_Event .

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An overview of photosynthesis

How the photosystems work, other electron transfer chain components, abbreviations, competing interests, recommended reading and key publications, photosynthesis.

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Matthew P. Johnson; Photosynthesis. Essays Biochem 31 October 2016; 60 (3): 255–273. doi: https://doi.org/10.1042/EBC20160016

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Photosynthesis sustains virtually all life on planet Earth providing the oxygen we breathe and the food we eat; it forms the basis of global food chains and meets the majority of humankind's current energy needs through fossilized photosynthetic fuels. The process of photosynthesis in plants is based on two reactions that are carried out by separate parts of the chloroplast. The light reactions occur in the chloroplast thylakoid membrane and involve the splitting of water into oxygen, protons and electrons. The protons and electrons are then transferred through the thylakoid membrane to create the energy storage molecules adenosine triphosphate (ATP) and nicotinomide–adenine dinucleotide phosphate (NADPH). The ATP and NADPH are then utilized by the enzymes of the Calvin–Benson cycle (the dark reactions), which converts CO 2 into carbohydrate in the chloroplast stroma. The basic principles of solar energy capture, energy, electron and proton transfer and the biochemical basis of carbon fixation are explained and their significance is discussed.

Introduction

Photosynthesis is the ultimate source of all of humankind's food and oxygen, whereas fossilized photosynthetic fuels provide ∼87% of the world's energy. It is the biochemical process that sustains the biosphere as the basis for the food chain. The oxygen produced as a by-product of photosynthesis allowed the formation of the ozone layer, the evolution of aerobic respiration and thus complex multicellular life.

Oxygenic photosynthesis involves the conversion of water and CO 2 into complex organic molecules such as carbohydrates and oxygen. Photosynthesis may be split into the ‘light’ and ‘dark’ reactions. In the light reactions, water is split using light into oxygen, protons and electrons, and in the dark reactions, the protons and electrons are used to reduce CO 2 to carbohydrate (given here by the general formula CH 2 O). The two processes can be summarized thus:

Light reactions:

Dark reactions:

The positive sign of the standard free energy change of the reaction (Δ G °) given above means that the reaction requires energy ( an endergonic reaction ). The energy required is provided by absorbed solar energy, which is converted into the chemical bond energy of the products ( Box 1 ).

Photosynthesis converts ∼200 billion tonnes of CO 2 into complex organic compounds annually and produces ∼140 billion tonnes of oxygen into the atmosphere. By facilitating conversion of solar energy into chemical energy, photosynthesis acts as the primary energy input into the global food chain. Nearly all living organisms use the complex organic compounds derived from photosynthesis as a source of energy. The breakdown of these organic compounds occurs via the process of aerobic respiration, which of course also requires the oxygen produced by photosynthesis.

Unlike photosynthesis, aerobic respiration is an exergonic process (negative Δ G °) with the energy released being used by the organism to power biosynthetic processes that allow growth and renewal, mechanical work (such as muscle contraction or flagella rotation) and facilitating changes in chemical concentrations within the cell (e.g. accumulation of nutrients and expulsion of waste). The use of exergonic reactions to power endergonic ones associated with biosynthesis and housekeeping in biological organisms such that the overall free energy change is negative is known as ‘ coupling’.

Photosynthesis and respiration are thus seemingly the reverse of one another, with the important caveat that both oxygen formation during photosynthesis and its utilization during respiration result in its liberation or incorporation respectively into water rather than CO 2 . In addition, glucose is one of several possible products of photosynthesis with amino acids and lipids also being synthesized rapidly from the primary photosynthetic products.

The consideration of photosynthesis and respiration as opposing processes helps us to appreciate their role in shaping our environment. The fixation of CO 2 by photosynthesis and its release during breakdown of organic molecules during respiration, decay and combustion of organic matter and fossil fuels can be visualized as the global carbon cycle ( Figure 1 ).

The global carbon cycle

The relationship between respiration, photosynthesis and global CO 2 and O 2 levels.

At present, this cycle may be considered to be in a state of imbalance due to the burning of fossil fuels (fossilized photosynthesis), which is increasing the proportion of CO 2 entering the Earth's atmosphere, leading to the so-called ‘greenhouse effect’ and human-made climate change.

Oxygenic photosynthesis is thought to have evolved only once during Earth's history in the cyanobacteria. All other organisms, such as plants, algae and diatoms, which perform oxygenic photosynthesis actually do so via cyanobacterial endosymbionts or ‘chloroplasts’. An endosymbiotoic event between an ancestral eukaryotic cell and a cyanobacterium that gave rise to plants is estimated to have occurred ∼1.5 billion years ago. Free-living cyanobacteria still exist today and are responsible for ∼50% of the world's photosynthesis. Cyanobacteria themselves are thought to have evolved from simpler photosynthetic bacteria that use either organic or inorganic compounds such a hydrogen sulfide as a source of electrons rather than water and thus do not produce oxygen.

The site of photosynthesis in plants

In land plants, the principal organs of photosynthesis are the leaves ( Figure 2 A). Leaves have evolved to expose the largest possible area of green tissue to light and entry of CO 2 to the leaf is controlled by small holes in the lower epidermis called stomata ( Figure 2 B). The size of the stomatal openings is variable and regulated by a pair of guard cells, which respond to the turgor pressure (water content) of the leaf, thus when the leaf is hydrated, the stomata can open to allow CO 2 in. In contrast, when water is scarce, the guard cells lose turgor pressure and close, preventing the escape of water from the leaf via transpiration.

Location of the photosynthetic machinery

( A ) The model plant Arabidopsis thaliana . ( B ) Basic structure of a leaf shown in cross-section. Chloroplasts are shown as green dots within the cells. ( C ) An electron micrograph of an Arabidopsis chloroplast within the leaf. ( D ) Close-up region of the chloroplast showing the stacked structure of the thylakoid membrane.

Within the green tissue of the leaf (mainly the mesophyll) each cell (∼100 μm in length) contains ∼100 chloroplasts (2–3 μm in length), the tiny organelles where photosynthesis takes place. The chloroplast has a complex structure ( Figure 2 C, D) with two outer membranes (the envelope), which are colourless and do not participate in photosynthesis, enclosing an aqueous space (the stroma) wherein sits a third membrane known as the thylakoid, which in turn encloses a single continuous aqueous space called the lumen.

The light reactions of photosynthesis involve light-driven electron and proton transfers, which occur in the thylakoid membrane, whereas the dark reactions involve the fixation of CO 2 into carbohydrate, via the Calvin–Benson cycle, which occurs in the stroma ( Figure 3 ). The light reactions involve electron transfer from water to NADP + to form NADPH and these reactions are coupled to proton transfers that lead to the phosphorylation of adenosine diphosphate (ADP) into ATP. The Calvin–Benson cycle uses ATP and NADPH to convert CO 2 into carbohydrates ( Figure 3 ), regenerating ADP and NADP + . The light and dark reactions are therefore mutually dependent on one another.

Division of labour within the chloroplast

The light reactions of photosynthesis take place in the thylakoid membrane, whereas the dark reactions are located in the chloroplast stroma.

Photosynthetic electron and proton transfer chain

The light-driven electron transfer reactions of photosynthesis begin with the splitting of water by Photosystem II (PSII). PSII is a chlorophyll–protein complex embedded in the thylakoid membrane that uses light to oxidize water to oxygen and reduce the electron acceptor plastoquinone to plastoquinol. Plastoquinol in turn carries the electrons derived from water to another thylakoid-embedded protein complex called cytochrome b 6 f (cyt b 6 f ). cyt b 6 f oxidizes plastoquinol to plastoquinone and reduces a small water-soluble electron carrier protein plastocyanin, which resides in the lumen. A second light-driven reaction is then carried out by another chlorophyll protein complex called Photosystem I (PSI). PSI oxidizes plastocyanin and reduces another soluble electron carrier protein ferredoxin that resides in the stroma. Ferredoxin can then be used by the ferredoxin–NADP + reductase (FNR) enzyme to reduce NADP + to NADPH. This scheme is known as the linear electron transfer pathway or Z-scheme ( Figure 4 ).

The photosynthetic electron and proton transfer chain

The linear electron transfer pathway from water to NADP + to form NADPH results in the formation of a proton gradient across the thylakoid membrane that is used by the ATP synthase enzyme to make ATP.

The Z-scheme, so-called since it resembles the letter ‘Z’ when turned on its side ( Figure 5 ), thus shows how the electrons move from the water–oxygen couple (+820 mV) via a chain of redox carriers to NADP + /NADPH (−320 mV) during photosynthetic electron transfer. Generally, electrons are transferred from redox couples with low potentials (good reductants) to those with higher potentials (good oxidants) (e.g. during respiratory electron transfer in mitochondria) since this process is exergonic (see Box 2 ). However, photosynthetic electron transfer also involves two endergonic steps, which occur at PSII and at PSI and require an energy input in the form of light. The light energy is used to excite an electron within a chlorophyll molecule residing in PSII or PSI to a higher energy level; this excited chlorophyll is then able to reduce the subsequent acceptors in the chain. The oxidized chlorophyll is then reduced by water in the case of PSII and plastocyanin in the case of PSI.

Z-scheme of photosynthetic electron transfer

The main components of the linear electron transfer pathway are shown on a scale of redox potential to illustrate how two separate inputs of light energy at PSI and PSII result in the endergonic transfer of electrons from water to NADP + .

The water-splitting reaction at PSII and plastoquinol oxidation at cyt b 6 f result in the release of protons into the lumen, resulting in a build-up of protons in this compartment relative to the stroma. The difference in the proton concentration between the two sides of the membrane is called a proton gradient. The proton gradient is a store of free energy (similar to a gradient of ions in a battery) that is utilized by a molecular mechanical motor ATP synthase, which resides in the thylakoid membrane ( Figure 4 ). The ATP synthase allows the protons to move down their concentration gradient from the lumen (high H + concentration) to the stroma (low H + concentration). This exergonic reaction is used to power the endergonic synthesis of ATP from ADP and inorganic phosphate (P i ). This process of photophosphorylation is thus essentially similar to oxidative phosphorylation, which occurs in the inner mitochondrial membrane during respiration.

An alternative electron transfer pathway exists in plants and algae, known as cyclic electron flow. Cyclic electron flow involves the recycling of electrons from ferredoxin to plastoquinone, with the result that there is no net production of NADPH; however, since protons are still transferred into the lumen by oxidation of plastoquinol by cyt b 6 f , ATP can still be formed. Thus photosynthetic organisms can control the ratio of NADPH/ATP to meet metabolic need by controlling the relative amounts of cyclic and linear electron transfer.

Light absorption by pigments

Photosynthesis begins with the absorption of light by pigments molecules located in the thylakoid membrane. The most well-known of these is chlorophyll, but there are also carotenoids and, in cyanobacteria and some algae, bilins. These pigments all have in common within their chemical structures an alternating series of carbon single and double bonds, which form a conjugated system π–electron system ( Figure 6 ).

Major photosynthetic pigments in plants

The chemical structures of the chlorophyll and carotenoid pigments present in the thylakoid membrane. Note the presence in each of a conjugated system of carbon–carbon double bonds that is responsible for light absorption.

The variety of pigments present within each type of photosynthetic organism reflects the light environment in which it lives; plants on land contain chlorophylls a and b and carotenoids such as β-carotene, lutein, zeaxanthin, violaxanthin, antheraxanthin and neoxanthin ( Figure 6 ). The chlorophylls absorb blue and red light and so appear green in colour, whereas carotenoids absorb light only in the blue and so appear yellow/red ( Figure 7 ), colours more obvious in the autumn as chlorophyll is the first pigment to be broken down in decaying leaves.

Basic absorption spectra of the major chlorophyll and carotenoid pigments found in plants

Chlorophylls absorb light energy in the red and blue part of the visible spectrum, whereas carotenoids only absorb light in the blue/green.

Light, or electromagnetic radiation, has the properties of both a wave and a stream of particles (light quanta). Each quantum of light contains a discrete amount of energy that can be calculated by multiplying Planck's constant, h (6.626×10 −34 J·s) by ν, the frequency of the radiation in cycles per second (s −1 ):

The frequency (ν) of the light and so its energy varies with its colour, thus blue photons (∼450 nm) are more energetic than red photons (∼650 nm). The frequency (ν) and wavelength (λ) of light are related by:

where c is the velocity of light (3.0×10 8 m·s −1 ), and the energy of a particular wavelength (λ) of light is given by:

Thus 1 mol of 680 nm photons of red light has an energy of 176 kJ·mol −1 .

The electrons within the delocalized π system of the pigment have the ability to jump up from the lowest occupied molecular orbital (ground state) to higher unoccupied molecular electron orbitals (excited states) via the absorption of specific wavelengths of light in the visible range (400–725 nm). Chlorophyll has two excited states known as S 1 and S 2 and, upon interaction of the molecule with a photon of light, one of its π electrons is promoted from the ground state (S 0 ) to an excited state, a process taking just 10 −15 s ( Figure 8 ). The energy gap between the S 0 and S 1 states is spanned by the energy provided by a red photon (∼600–700 nm), whereas the energy gap between the S 0 and S 2 states is larger and therefore requires a more energetic (shorter wavelength, higher frequency) blue photon (∼400–500 nm) to span the energy gap.

Jablonski diagram of chlorophyll showing the possible fates of the S 1 and S 2 excited states and timescales of the transitions involved

Photons with slightly different energies (colours) excite each of the vibrational substates of each excited state (as shown by variation in the size and colour of the arrows).

Upon excitation, the electron in the S 2 state quickly undergoes losses of energy as heat through molecular vibration and undergoes conversion into the energy of the S 1 state by a process called internal conversion. Internal conversion occurs on a timescale of 10 −12 s. The energy of a blue photon is thus rapidly degraded to that of a red photon. Excitation of the molecule with a red photon would lead to promotion of an electron to the S 1 state directly. Once the electron resides in the S 1 state, it is lower in energy and thus stable on a somewhat longer timescale (10 −9 s). The energy of the excited electron in the S 1 state can have one of several fates: it could return to the ground state (S 0 ) by emission of the energy as a photon of light (fluorescence), or it could be lost as heat due to internal conversion between S 1 and S 0 . Alternatively, if another chlorophyll is nearby, a process known as excitation energy transfer (EET) can result in the non-radiative exchange of energy between the two molecules ( Figure 9 ). For this to occur, the two chlorophylls must be close by (<7 nm), have a specific orientation with respect to one another, and excited state energies that overlap (are resonant) with one another. If these conditions are met, the energy is exchanged, resulting in a mirror S 0 →S 1 transition in the acceptor molecule and a S 1 →S 0 transition in the other.

Basic mechanism of excitation energy transfer between chlorophyll molecules

Two chlorophyll molecules with resonant S 1 states undergo a mirror transition resulting in the non-radiative transfer of excitation energy between them.

Light-harvesting complexes

In photosynthetic systems, chlorophylls and carotenoids are found attached to membrane-embedded proteins known as light-harvesting complexes (LHCs). Through careful binding and orientation of the pigment molecules, absorbed energy can be transferred among them by EET. Each pigment is bound to the protein by a series of non-covalent bonding interactions (such as, hydrogen bonds, van der Waals interactions, hydrophobic interaction and co-ordination bonds between lone pair electrons of residues such as histidine in the protein and the Mg 2+ ion in chlorophyll); the protein structure is such that each bound pigment experiences a slightly different environment in terms of the surrounding amino acid side chains, lipids, etc., meaning that the S 1 and S 2 energy levels are shifted in energy with respect to that of other neighbouring pigment molecules. The effect is to create a range of pigment energies that act to ‘funnel’ the energy on to the lowest-energy pigments in the LHC by EET.

Reaction centres

A photosystem consists of numerous LHCs that form an antenna of hundreds of pigment molecules. The antenna pigments act to collect and concentrate excitation energy and transfer it towards a ‘special pair’ of chlorophyll molecules that reside in the reaction centre (RC) ( Figure 10 ). Unlike the antenna pigments, the special pair of chlorophylls are ‘redox-active’ in the sense that they can return to the ground state (S 0 ) by the transfer of the electron residing in the S 1 excited state (Chl*) to another species. This process is known as charge separation and result in formation of an oxidized special pair (Chl + ) and a reduced acceptor (A − ). The acceptor in PSII is plastoquinone and in PSI it is ferredoxin. If the RC is to go on functioning, the electron deficiency on the special pair must be made good, in PSII the electron donor is water and in PSI it is plastocyanin.

Basic structure of a photosystem

Light energy is captured by the antenna pigments and transferred to the special pair of RC chlorophylls which undergo a redox reaction leading to reduction of an acceptor molecule. The oxidized special pair is regenerated by an electron donor.

It is worth asking why photosynthetic organisms bother to have a large antenna of pigments serving an RC rather than more numerous RCs. The answer lies in the fact that the special pair of chlorophylls alone have a rather small spatial and spectral cross-section, meaning that there is a limit to the amount of light they can efficiently absorb. The amount of light they can practically absorb is around two orders of magnitude smaller than their maximum possible turnover rate, Thus LHCs act to increase the spatial (hundreds of pigments) and spectral (several types of pigments with different light absorption characteristics) cross-section of the RC special pair ensuring that its turnover rate runs much closer to capacity.

Photosystem II

PSII is a light-driven water–plastoquinone oxidoreductase and is the only enzyme in Nature that is capable of performing the difficult chemistry of splitting water into protons, electrons and oxygen ( Figure 11 ). In principle, water is an extremely poor electron donor since the redox potential of the water–oxygen couple is +820 mV. PSII uses light energy to excite a special pair of chlorophylls, known as P680 due to their 680 nm absorption peak in the red part of the spectrum. P680* undergoes charge separation that results in the formation of an extremely oxidizing species P680 + which has a redox potential of +1200 mV, sufficient to oxidize water. Nonetheless, since water splitting involves four electron chemistry and charge separation only involves transfer of one electron, four separate charge separations (turnovers of PSII) are required to drive formation of one molecule of O 2 from two molecules of water. The initial electron donation to generate the P680 from P680 + is therefore provided by a cluster of manganese ions within the oxygen-evolving complex (OEC), which is attached to the lumen side of PSII ( Figure 12 ). Manganese is a transition metal that can exist in a range of oxidation states from +1 to +5 and thus accumulates the positive charges derived from each light-driven turnover of P680. Progressive extraction of electrons from the manganese cluster is driven by the oxidation of P680 within PSII by light and is known as the S-state cycle ( Figure 12 ). After the fourth turnover of P680, sufficient positive charge is built up in the manganese cluster to permit the splitting of water into electrons, which regenerate the original state of the manganese cluster, protons, which are released into the lumen and contribute to the proton gradient used for ATP synthesis, and the by-product O 2 . Thus charge separation at P680 provides the thermodynamic driving force, whereas the manganese cluster acts as a catalyst for the water-splitting reaction.

Basic structure of the PSII–LHCII supercomplex from spinach

The organization of PSII and its light-harvesting antenna. Protein is shown in grey, with chlorophylls in green and carotenoids in orange. Drawn from PDB code 3JCU

S-state cycle of water oxidation by the manganese cluster (shown as circles with roman numerals representing the manganese ion oxidation states) within the PSII oxygen-evolving complex

Progressive extraction of electrons from the manganese cluster is driven by the oxidation of P680 within PSII by light. Each of the electrons given up by the cluster is eventually repaid at the S 4 to S 0 transition when molecular oxygen (O 2 ) is formed. The protons extracted from water during the process are deposited into the lumen and contribute to the protonmotive force.

The electrons yielded by P680* following charge separation are not passed directly to plastoquinone, but rather via another acceptor called pheophytin, a porphyrin molecule lacking the central magnesium ion as in chlorophyll. Plastoquinone reduction to plastoquinol requires two electrons and thus two molecules of plastoquinol are formed per O 2 molecule evolved by PSII. Two protons are also taken up upon formation of plastoquinol and these are derived from the stroma. PSII is found within the thylakoid membrane of plants as a dimeric RC complex surrounded by a peripheral antenna of six minor monomeric antenna LHC complexes and two to eight trimeric LHC complexes, which together form a PSII–LHCII supercomplex ( Figure 11 ).

Photosystem I

PSI is a light-driven plastocyanin–ferredoxin oxidoreductase ( Figure 13 ). In PSI, the special pair of chlorophylls are known as P700 due to their 700 nm absorption peak in the red part of the spectrum. P700* is an extremely strong reductant that is able to reduce ferredoxin which has a redox potential of −450 mV (and is thus is, in principle, a poor electron acceptor). Reduced ferredoxin is then used to generate NADPH for the Calvin–Benson cycle at a separate complex known as FNR. The electron from P700* is donated via another chlorophyll molecule and a bound quinone to a series of iron–sulfur clusters at the stromal side of the complex, whereupon the electron is donated to ferredoxin. The P700 species is regenerated form P700 + via donation of an electron from the soluble electron carrier protein plastocyanin.

Basic structure of the PSI–LHCI supercomplex from pea

The organization of PSI and its light-harvesting antenna. Protein is shown in grey, with chlorophylls in green and carotenoids in orange. Drawn from PDB code 4XK8.

PSI is found within the thylakoid membrane as a monomeric RC surrounded on one side by four LHC complexes known as LHCI. The PSI–LHCI supercomplex is found mainly in the unstacked regions of the thylakoid membrane ( Figure 13 ).

Plastoquinone/plastoquinol

Plastoquinone is a small lipophilic electron carrier molecule that resides within the thylakoid membrane and carries two electrons and two protons from PSII to the cyt b 6 f complex. It has a very similar structure to that of the molecule ubiquinone (coenzyme Q 10 ) in the mitochondrial inner membrane.

Cytochrome b 6 f complex

The cyt b 6 f complex is a plastoquinol–plastocyanin oxidoreductase and possess a similar structure to that of the cytochrome bc 1 complex (complex III) in mitochondria ( Figure 14 A). As with Complex III, cyt b 6 f exists as a dimer in the membrane and carries out both the oxidation and reduction of quinones via the so-called Q-cycle. The Q-cycle ( Figure 14 B) involves oxidation of one plastoquinol molecule at the Qp site of the complex, both protons from this molecule are deposited in the lumen and contribute to the proton gradient for ATP synthesis. The two electrons, however, have different fates. The first is transferred via an iron–sulfur cluster and a haem cofactor to the soluble electron carrier plastocyanin (see below). The second electron derived from plastoquinol is passed via two separate haem cofactors to another molecule of plastoquinone bound to a separate site (Qn) on the complex, thus reducing it to a semiquinone. When a second plastoquinol molecule is oxidized at Qp, a second molecule of plastocyanin is reduced and two further protons are deposited in the lumen. The second electron reduces the semiquinone at the Qn site which, concomitant with uptake of two protons from the stroma, causes its reduction to plastoquinol. Thus for each pair of plastoquinol molecules oxidized by the complex, one is regenerated, yet all four protons are deposited into the lumen. The Q-cycle thus doubles the number of protons transferred from the stroma to the lumen per plastoquinol molecule oxidized.

( A ) Structure drawn from PDB code 1Q90. ( B ) The protonmotive Q-cycle showing how electrons from plastoquinol are passed to both plastocyanin and plastoquinone, doubling the protons deposited in the lumen for every plastoquinol molecule oxidized by the complex.

Plastocyanin

Plastocyanin is a small soluble electron carrier protein that resides in the thylakoid lumen. The active site of the plastocyanin protein binds a copper ion, which cycles between the Cu 2+ and Cu + oxidation states following its oxidation by PSI and reduction by cyt b 6 f respectively.

Ferredoxin is a small soluble electron carrier protein that resides in the chloroplast stroma. The active site of the ferredoxin protein binds an iron–sulfur cluster, which cycles between the Fe 2+ and Fe 3+ oxidation states following its reduction by PSI and oxidation by the FNR complex respectively.

Ferredoxin–NADP + reductase

The FNR complex is found in both soluble and thylakoid membrane-bound forms. The complex binds a flavin–adenine dinucleotide (FAD) cofactor at its active site, which accepts two electrons from two molecules of ferredoxin before using them reduce NADP + to NADPH.

ATP synthase

The ATP synthase enzyme is responsible for making ATP from ADP and P i ; this endergonic reaction is powered by the energy contained within the protonmotive force. According to the structure, 4.67 H + are required for every ATP molecule synthesized by the chloroplast ATP synthase. The enzyme is a rotary motor which contains two domains: the membrane-spanning F O portion which conducts protons from the lumen to the stroma, and the F 1 catalytic domain that couples this exergonic proton movement to ATP synthesis.

Membrane stacking and the regulation of photosynthesis

Within the thylakoid membrane, PSII–LHCII supercomplexes are packed together into domains known as the grana, which associate with one another to form grana stacks. PSI and ATP synthase are excluded from these stacked PSII–LHCII regions by steric constraints and thus PSII and PSI are segregated in the thylakoid membrane between the stacked and unstacked regions ( Figure 15 ). The cyt b 6 f complex, in contrast, is evenly distributed throughout the grana and stromal lamellae. The evolutionary advantage of membrane stacking is believed to be a higher efficiency of electron transport by preventing the fast energy trap PSI from ‘stealing’ excitation energy from the slower trap PSII, a phenomenon known as spillover. Another possible advantage of membrane stacking in thylakoids may be the segregation of the linear and cyclic electron transfer pathways, which might otherwise compete to reduce plastoquinone. In this view, PSII, cyt b 6 f and a sub-fraction of PSI closest to the grana is involved in linear flow, whereas PSI and cyt b 6 f in the stromal lamellae participates in cyclic flow. The cyclic electron transfer pathway recycles electrons from ferredoxin back to plastoquinone and thus allows protonmotive force generation (and ATP synthesis) without net NADPH production. Cyclic electron transfer thereby provides the additional ATP required for the Calvin–Benson cycle (see below).

Lateral heterogeneity in thylakoid membrane organization

( A ) Electron micrograph of the thylakoid membrane showing stacked grana and unstacked stromal lamellae regions. ( B ) Model showing the distribution of the major complexes of photosynthetic electron and proton transfer between the stacked grana and unstacked stromal lamellae regions.

‘Dark’ reactions: the Calvin–Benson cycle

CO 2 is fixed into carbohydrate via the Calvin–Benson cycle in plants, which consumes the ATP and NADPH produced during the light reactions and thus in turn regenerates ADP, P i and NADP + . In the first step of the Calvin–Benson cycle ( Figure 16 ), CO 2 is combined with a 5-carbon (5C) sugar, ribulose 1,5-bisphosphate in a reaction catalysed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). The reaction forms an unstable 6C intermediate that immediately splits into two molecules of 3-phosphoglycerate. 3-Phosphoglycerate is first phosphorylated by 3-phosphoglycerate kinase using ATP to form 1,3-bisphosphoglycerate. 1,3-Bisphosphoglycerate is then reduced by glyceraldehyde 3-phosphate dehydrogenase using NADPH to form glyceraldehyde 3-phosphate (GAP, a triose or 3C sugar) in reactions, which are the reverse of glycolysis. For every three CO 2 molecules initially combined with ribulose 1,5-bisphopshate, six molecules of GAP are produced by the subsequent steps. However only one of these six molecules can be considered as a product of the Calvin–Benson cycle since the remaining five are required to regenerate ribulose 1,5-bisphosphate in a complex series of reactions that also require ATP. The one molecule of GAP that is produced for each turn of the cycle can be quickly converted by a range of metabolic pathways into amino acids, lipids or sugars such as glucose. Glucose in turn may be stored as the polymer starch as large granules within chloroplasts.

The Calvin–Benson cycle

Overview of the biochemical pathway for the fixation of CO 2 into carbohydrate in plants.

A complex biochemical ‘dance’ ( Figure 16 ) is then involved in the regeneration of three ribulose 1,5-bisphosphate (5C) from the remaining five GAP (3C) molecules. The regeneration begins with the conversion of two molecules of GAP into dihydroxyacetone phosphate (DHAP) by triose phosphate isomerase; one of the DHAP molecules is the combined with another GAP molecule to make fructose 1,6-bisphosphate (6C) by aldolase. The fructose 1,6-bisphosphate is then dephosphorylated by fructose-1,6-bisphosphatase to yield fructose 6-phosphate (6C) and releasing P i . Two carbons are then removed from fructose 6-phosphate by transketolase, generating erythrose 4-phosphate (4C); the two carbons are transferred to another molecule of GAP generating xylulose 5-phosphate (5C). Another DHAP molecule, formed from GAP by triose phosphate isomerase is then combined with the erythrose 4-phosphate by aldolase to form sedoheptulose 1,7-bisphosphate (7C). Sedoheptulose 1,7-bisphosphate is then dephosphorylated to sedoheptulose 7-phosphate (7C) by sedoheptulose-1,7-bisphosphatase releasing P i . Sedoheptulose 7-phosphate has two carbons removed by transketolase to produce ribose 5-phosphate (5C) and the two carbons are transferred to another GAP molecule producing another xylulose 5-phosphate (5C). Ribose 5-phosphate and the two molecules of xylulose 5-phosphate (5C) are then converted by phosphopentose isomerase to three molecules of ribulose 5-phosphate (5C). The three ribulose 5-phosphate molecules are then phosphorylated using three ATP by phosphoribulokinase to regenerate three ribulose 1,5-bisphosphate (5C).

Overall the synthesis of 1 mol of GAP requires 9 mol of ATP and 6 mol of NADPH, a required ratio of 1.5 ATP/NADPH. Linear electron transfer is generally thought to supply ATP/NADPH in a ratio of 1.28 (assuming an H + /ATP ratio of 4.67) with the shortfall of ATP believed to be provided by cyclic electron transfer reactions. Since the product of the Calvin cycle is GAP (a 3C sugar) the pathway is often referred to as C 3 photosynthesis and plants that utilize it are called C 3 plants and include many of the world's major crops such as rice, wheat and potato.

Many of the enzymes involved in the Calvin–Benson cycle (e.g. transketolase, glyceraldehyde-3-phosphate dehydrogenase and aldolase) are also involved in the glycolysis pathway of carbohydrate degradation and their activity must therefore be carefully regulated to avoid futile cycling when light is present, i.e. the unwanted degradation of carbohydrate. The regulation of the Calvin–Benson cycle enzymes is achieved by the activity of the light reactions, which modify the environment of the dark reactions (i.e. the stroma). Proton gradient formation across the thylakoid membrane during the light reactions increases the pH and also increases the Mg 2+ concentration in the stroma (as Mg 2+ flows out of the lumen as H + flows in to compensate for the influx of positive charges). In addition, by reducing ferredoxin and NADP + , PSI changes the redox state of the stroma, which is sensed by the regulatory protein thioredoxin. Thioredoxin, pH and Mg 2+ concentration play a key role in regulating the activity of the Calvin–Benson cycle enzymes, ensuring the activity of the light and dark reactions is closely co-ordinated.

It is noteworthy that, despite the complexity of the dark reactions outlined above, the carbon fixation step itself (i.e. the incorporation of CO 2 into carbohydrate) is carried out by a single enzyme, Rubisco. Rubisco is a large multisubunit soluble protein complex found in the chloroplast stroma. The complex consists of eight large (56 kDa) subunits, which contain both catalytic and regulatory domains, and eight small subunits (14 kDa), which enhance the catalytic function of the L subunits ( Figure 17 A). The carboxylation reaction carried out by Rubisco is highly exergonic (Δ G °=−51.9 kJ·mol- 1 ), yet kinetically very slow (just 3 s −1 ) and begins with the protonation of ribulose 1,5-bisphosphate to form an enediolate intermediate which can be combined with CO 2 to form an unstable 6C intermediate that is quickly hydrolysed to yield two 3C 3-phosphoglycerate molecules. The active site in the Rubisco enzyme contains a key lysine residue, which reacts with another (non-substrate) molecule of CO 2 to form a carbamate anion that is then able to bind Mg 2+ . The Mg 2+ in the active site is essential for the catalytic function of Rubisco, playing a key role in binding ribulose 1,5-bisphosphate and activating it such that it readily reacts with CO 2.. Rubisco activity is co-ordinated with that of the light reactions since carbamate formation requires both high Mg 2+ concentration and alkaline conditions, which are provided by the light-driven changes in the stromal environment discussed above ( Figure 17 B).

( A ) Structure of the Rubisco enzyme (the large subunits are shown in blue and the small subunits in green); four of each type of subunit are visible in the image. Drawn from PDB code 1RXO. ( B ) Activation of the lysine residue within the active site of Rubisco occurs via elevated stromal pH and Mg 2+ concentration as a result of the activity of the light reactions.

In addition to carboxylation, Rubisco also catalyses a competitive oxygenation reaction, known as photorespiration, that results in the combination of ribulose 1,5-bisphosphate with O 2 rather than CO 2 . In the oxygenation reaction, one rather than two molecules of 3-phosphoglycerate and one molecule of a 2C sugar known as phosphoglycolate are produced by Rubisco. The phosphoglycolate must be converted in a series of reactions that regenerate one molecule of 3-phosphoglycerate and one molecule of CO 2 . These reactions consume additional ATP and thus result in an energy loss to the plant. Although the oxygenation reaction of Rubisco is much less favourable than the carboxylation reaction, the relatively high concentration of O 2 in the leaf (250 μM) compared with CO 2 (10 μM) means that a significant amount of photorespiration is always occurring. Under normal conditions, the ratio of carboxylation to oxygenation is between 3:1 and 4:1. However, this ratio can be decreased with increasing temperature due to decreased CO 2 concentration in the leaf, a decrease in the affinity of Rubisco for CO 2 compared with O 2 and an increase in the maximum rate of the oxygenation reaction compared with the carboxylation reaction. The inefficiencies of the Rubisco enzyme mean that plants must produce it in very large amounts (∼30–50% of total soluble protein in a spinach leaf) to achieve the maximal photosynthetic rate.

CO 2 -concentrating mechanisms

To counter photorespiration, plants, algae and cyanobacteria have evolved different CO 2 -concentrating mechanisms CCMs that aim to increase the concentration of CO 2 relative to O 2 in the vicinity of Rubisco. One such CCM is C 4 photosynthesis that is found in plants such as maize, sugar cane and savanna grasses. C 4 plants show a specialized leaf anatomy: Kranz anatomy ( Figure 18 ). Kranz, German for wreath, refers to a bundle sheath of cells that surrounds the central vein within the leaf, which in turn are surrounded by the mesophyll cells. The mesophyll cells in such leaves are rich in the enzyme phosphoenolpyruvate (PEP) carboxylase, which fixes CO 2 into a 4C carboxylic acid: oxaloaceatate. The oxaloacetate formed by the mesophyll cells is reduced using NADPH to malate, another 4C acid: malate. The malate is then exported from the mesophyll cells to the bundle sheath cells, where it is decarboxylated to pyruvate thus regenerating NADPH and CO 2 . The CO 2 is then utilized by Rubisco in the Calvin cycle. The pyruvate is in turn returned to the mesophyll cells where it is phosphorylated using ATP to reform PEP ( Figure 19 ). The advantage of C 4 photosynthesis is that CO 2 accumulates at a very high concentration in the bundle sheath cells that is then sufficient to allow Rubisco to operate efficiently.

Diagram of a C 4 plant leaf showing Kranz anatomy

The C 4 pathway (NADP + –malic enzyme type) for fixation of CO 2

Plants growing in hot, bright and dry conditions inevitably have to have their stomata closed for large parts of the day to avoid excessive water loss and wilting. The net result is that the internal CO 2 concentration in the leaf is very low, meaning that C 3 photosynthesis is not possible. To counter this limitation, another CCM is found in succulent plants such as cacti. The Crassulaceae fix CO 2 into malate during the day via PEP carboxylase, store it within the vacuole of the plant cell at night and then release it within their tissues by day to be fixed via normal C 3 photosynthesis. This is termed crassulacean acid metabolism (CAM).

This article is a reviewed, revised and updated version of the following ‘Biochemistry Across the School Curriculum’ (BASC) booklet: Weaire, P.J. (1994) Photosynthesis . For further information and to provide feedback on this or any other Biochemical Society education resource, please contact [email protected]. For further information on other Biochemical Society publications, please visit www.biochemistry.org/publications .

adenosine diphosphate

adenosine triphosphate

carbohydrate

cytochrome b 6 f

dihydroxyacetone phosphate

excitation energy transfer

ferredoxin–NADP + reductase

glyceraldehyde 3-phosphate

light-harvesting complex

nicotinomide–adenine dinucleotide phosphate

phosphoenolpyruvate

inorganic phosphate

reaction centre

ribulose-1,5-bisphosphate carboxylase/oxygenase

I thank Professor Colin Osborne (University of Sheffield, Sheffield, U.K.) for useful discussions on the article, Dr Dan Canniffe (Penn State University, Pennsylvania, PA, U.S.A.) for providing pure pigment spectra and Dr P.J. Weaire (Kingston University, Kingston-upon-Thames, U.K.) for his original Photosynthesis BASC article (1994) on which this essay is partly based.

The Author declares that there are no competing interests associated with this article.

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• What is Photosynthesis

When you get hungry, you grab a snack from your fridge or pantry. But what can plants do when they get hungry? You are probably aware that plants need sunlight, water, and a home (like soil) to grow, but where do they get their food? They make it themselves!

Plants are called autotrophs because they can use energy from light to synthesize, or make, their own food source. Many people believe they are “feeding” a plant when they put it in soil, water it, or place it outside in the Sun, but none of these things are considered food. Rather, plants use sunlight, water, and the gases in the air to make glucose, which is a form of sugar that plants need to survive. This process is called photosynthesis and is performed by all plants, algae, and even some microorganisms. To perform photosynthesis, plants need three things: carbon dioxide, water, and sunlight.

Just like you, plants need to take in gases in order to live. Animals take in gases through a process called respiration. During the respiration process, animals inhale all of the gases in the atmosphere, but the only gas that is retained and not immediately exhaled is oxygen. Plants, however, take in and use carbon dioxide gas for photosynthesis. Carbon dioxide enters through tiny holes in a plant’s leaves, flowers, branches, stems, and roots. Plants also require water to make their food. Depending on the environment, a plant’s access to water will vary. For example, desert plants, like a cactus, have less available water than a lilypad in a pond, but every photosynthetic organism has some sort of adaptation, or special structure, designed to collect water. For most plants, roots are responsible for absorbing water. 

The last requirement for photosynthesis is an important one because it provides the energy to make sugar. How does a plant take carbon dioxide and water molecules and make a food molecule? The Sun! The energy from light causes a chemical reaction that breaks down the molecules of carbon dioxide and water and reorganizes them to make the sugar (glucose) and oxygen gas. After the sugar is produced, it is then broken down by the mitochondria into energy that can be used for growth and repair. The oxygen that is produced is released from the same tiny holes through which the carbon dioxide entered. Even the oxygen that is released serves another purpose. Other organisms, such as animals, use oxygen to aid in their survival. 

If we were to write a formula for photosynthesis, it would look like this: 

6CO 2 + 6H 2 O + Light energy → C 6 H 12 O 6 (sugar) + 6O 2 

The whole process of photosynthesis is a transfer of energy from the Sun to a plant. In each sugar molecule created, there is a little bit of the energy from the Sun, which the plant can either use or store for later. 

Imagine a pea plant. If that pea plant is forming new pods, it requires a large amount of sugar energy to grow larger. This is similar to how you eat food to grow taller and stronger. But rather than going to the store and buying groceries, the pea plant will use sunlight to obtain the energy to build sugar. When the pea pods are fully grown, the plant may no longer need as much sugar and will store it in its cells. A hungry rabbit comes along and decides to eat some of the plant, which provides the energy that allows the rabbit to hop back to its home. Where did the rabbit’s energy come from? Consider the process of photosynthesis. With the help of carbon dioxide and water, the pea pod used the energy from sunlight to construct the sugar molecules. When the rabbit ate the pea pod, it indirectly received energy from sunlight, which was stored in the sugar molecules in the plant. 

Humans, other animals, fungi, and some microorganisms cannot make food in their own bodies like autotrophs, but they still rely on photosynthesis. Through the transfer of energy from the Sun to plants, plants build sugars that humans consume to drive our daily activities. Even when we eat things like chicken or fish, we are transferring energy from the Sun into our bodies because, at some point, one organism consumed a photosynthetic organism (e.g., the fish ate algae). So the next time you grab a snack to replenish your energy, thank the Sun for it! 

This is an excerpt from the  Structure and Function  unit of our curriculum product line, Science and Technology Concepts TM  (STC). Please visit our publisher,  Carolina Biological , to learn more. 

[BONUS FOR TEACHERS] Watch "Photosynthesis: Blinded by the Light" to explore student misconceptions about matter and energy in photosynthesis and strategies for eliciting student ideas to address or build on them.

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8.1: Overview of Photosynthesis - The Purpose and Process of Photosynthesis

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Learning Objectives

• Describe the process of photosynthesis

The Importance of Photosynthesis

The processes of all organisms—from bacteria to humans—require energy. To get this energy, many organisms access stored energy by eating food. Carnivores eat other animals and herbivores eat plants. But where does the stored energy in food originate? All of this energy can be traced back to the process of photosynthesis and light energy from the sun.

Photosynthesis is essential to all life on earth. It is the only biological process that captures energy from outer space (sunlight) and converts it into chemical energy in the form of G3P ( Glyceraldehyde 3-phosphate) which in turn can be made into sugars and other molecular compounds. Plants use these compounds in all of their metabolic processes; plants do not need to consume other organisms for food because they build all the molecules they need. Unlike plants, animals need to consume other organisms to consume the molecules they need for their metabolic processes.

The Process of Photosynthesis

During photosynthesis, molecules in leaves capture sunlight and energize electrons, which are then stored in the covalent bonds of carbohydrate molecules. That energy within those covalent bonds will be released when they are broken during cell respiration. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis almost 200 million years ago.

Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performing photosynthesis. Because they use light to manufacture their own food, they are called photoautotrophs (“self-feeders using light”). Other organisms, such as animals, fungi, and most other bacteria, are termed heterotrophs (“other feeders”) because they must rely on the sugars produced by photosynthetic organisms for their energy needs. A third very interesting group of bacteria synthesize sugars, not by using sunlight’s energy, but by extracting energy from inorganic chemical compounds; hence, they are referred to as chemoautotrophs.

The importance of photosynthesis is not just that it can capture sunlight’s energy. A lizard sunning itself on a cold day can use the sun’s energy to warm up. Photosynthesis is vital because it evolved as a way to store the energy in solar radiation (the “photo-” part) as high-energy electrons in the carbon-carbon bonds of carbohydrate molecules (the “-synthesis” part). Those carbohydrates are the energy source that heterotrophs use to power the synthesis of ATP via respiration. Therefore, photosynthesis powers 99 percent of Earth’s ecosystems. When a top predator, such as a wolf, preys on a deer, the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to light, to photosynthesis, to vegetation, to deer, and finally to wolf.

• Photosynthesis evolved as a way to store the energy in solar radiation as high-energy electrons in carbohydrate molecules.
• Plants, algae, and cyanobacteria, known as photoautotrophs, are the only organisms capable of performing photosynthesis.
• Heterotrophs, unable to produce their own food, rely on the carbohydrates produced by photosynthetic organisms for their energy needs.
• photosynthesis : the process by which plants and other photoautotrophs generate carbohydrates and oxygen from carbon dioxide, water, and light energy in chloroplasts
• photoautotroph : an organism that can synthesize its own food by using light as a source of energy
• chemoautotroph : a simple organism, such as a protozoan, that derives its energy from chemical processes rather than photosynthesis

• Biology Article

Photosynthesis

Photosynthesis is a process by which phototrophs convert light energy into chemical energy, which is later used to fuel cellular activities. The chemical energy is stored in the form of sugars, which are created from water and carbon dioxide.

Table of Contents

• What is Photosynthesis?
• Site of photosynthesis

Photosynthesis definition states that the process exclusively takes place in the chloroplasts through photosynthetic pigments such as chlorophyll a, chlorophyll b, carotene and xanthophyll. All green plants and a few other autotrophic organisms utilize photosynthesis to synthesize nutrients by using carbon dioxide, water and sunlight. The by-product of the photosynthesis process is oxygen.Let us have a detailed look at the process, reaction and importance of photosynthesis.

What Is Photosynthesis in Biology?

The word “ photosynthesis ” is derived from the Greek words  phōs  (pronounced: “fos”) and σύνθεσις (pronounced: “synthesis “) Phōs means “light” and σύνθεσις   means, “combining together.” This means “ combining together with the help of light .”

Photosynthesis also applies to other organisms besides green plants. These include several prokaryotes such as cyanobacteria, purple bacteria and green sulfur bacteria. These organisms exhibit photosynthesis just like green plants.The glucose produced during photosynthesis is then used to fuel various cellular activities. The by-product of this physio-chemical process is oxygen.

A visual representation of the photosynthesis reaction

• Photosynthesis is also used by algae to convert solar energy into chemical energy. Oxygen is liberated as a by-product and light is considered as a major factor to complete the process of photosynthesis.
• Photosynthesis occurs when plants use light energy to convert carbon dioxide and water into glucose and oxygen. Leaves contain microscopic cellular organelles known as chloroplasts.
• Each chloroplast contains a green-coloured pigment called chlorophyll. Light energy is absorbed by chlorophyll molecules whereas carbon dioxide and oxygen enter through the tiny pores of stomata located in the epidermis of leaves.
• Another by-product of photosynthesis is sugars such as glucose and fructose.
• These sugars are then sent to the roots, stems, leaves, fruits, flowers and seeds. In other words, these sugars are used by the plants as an energy source, which helps them to grow. These sugar molecules then combine with each other to form more complex carbohydrates like cellulose and starch. The cellulose is considered as the structural material that is used in plant cell walls.

Where Does This Process Occur?

Chloroplasts are the sites of photosynthesis in plants and blue-green algae.  All green parts of a plant, including the green stems, green leaves,  and sepals – floral parts comprise of chloroplasts – green colour plastids. These cell organelles are present only in plant cells and are located within the mesophyll cells of leaves.

Also Read:  Photosynthesis Early Experiments

Photosynthesis Equation

Photosynthesis reaction involves two reactants, carbon dioxide and water. These two reactants yield two products, namely, oxygen and glucose. Hence, the photosynthesis reaction is considered to be an endothermic reaction. Following is the photosynthesis formula:

Unlike plants, certain bacteria that perform photosynthesis do not produce oxygen as the by-product of photosynthesis. Such bacteria are called anoxygenic photosynthetic bacteria. The bacteria that do produce oxygen as a by-product of photosynthesis are called oxygenic photosynthetic bacteria.

Structure Of Chlorophyll

The structure of Chlorophyll consists of 4 nitrogen atoms that surround a magnesium atom. A hydrocarbon tail is also present. Pictured above is chlorophyll- f,  which is more effective in near-infrared light than chlorophyll- a

Chlorophyll is a green pigment found in the chloroplasts of the  plant cell   and in the mesosomes of cyanobacteria. This green colour pigment plays a vital role in the process of photosynthesis by permitting plants to absorb energy from sunlight. Chlorophyll is a mixture of chlorophyll- a  and chlorophyll- b .Besides green plants, other organisms that perform photosynthesis contain various other forms of chlorophyll such as chlorophyll- c1 ,  chlorophyll- c2 ,  chlorophyll- d and chlorophyll- f .

Also Read:   Biological Pigments

Process Of Photosynthesis

At the cellular level,  the photosynthesis process takes place in cell organelles called chloroplasts. These organelles contain a green-coloured pigment called chlorophyll, which is responsible for the characteristic green colouration of the leaves.

As already stated, photosynthesis occurs in the leaves and the specialized cell organelles responsible for this process is called the chloroplast. Structurally, a leaf comprises a petiole, epidermis and a lamina. The lamina is used for absorption of sunlight and carbon dioxide during photosynthesis.

Structure of Chloroplast. Note the presence of the thylakoid

“Photosynthesis Steps:”

• During the process of photosynthesis, carbon dioxide enters through the stomata, water is absorbed by the root hairs from the soil and is carried to the leaves through the xylem vessels. Chlorophyll absorbs the light energy from the sun to split water molecules into hydrogen and oxygen.
• The hydrogen from water molecules and carbon dioxide absorbed from the air are used in the production of glucose. Furthermore, oxygen is liberated out into the atmosphere through the leaves as a waste product.
• Glucose is a source of food for plants that provide energy for  growth and development , while the rest is stored in the roots, leaves and fruits, for their later use.
• Pigments are other fundamental cellular components of photosynthesis. They are the molecules that impart colour and they absorb light at some specific wavelength and reflect back the unabsorbed light. All green plants mainly contain chlorophyll a, chlorophyll b and carotenoids which are present in the thylakoids of chloroplasts. It is primarily used to capture light energy. Chlorophyll-a is the main pigment.

The process of photosynthesis occurs in two stages:

• Light-dependent reaction or light reaction
• Light independent reaction or dark reaction

Stages of Photosynthesis in Plants depicting the two phases – Light reaction and Dark reaction

Light Reaction of Photosynthesis (or) Light-dependent Reaction

• Photosynthesis begins with the light reaction which is carried out only during the day in the presence of sunlight. In plants, the light-dependent reaction takes place in the thylakoid membranes of chloroplasts.
• The Grana, membrane-bound sacs like structures present inside the thylakoid functions by gathering light and is called photosystems.
• These photosystems have large complexes of pigment and proteins molecules present within the plant cells, which play the primary role during the process of light reactions of photosynthesis.
• There are two types of photosystems: photosystem I and photosystem II.
• Under the light-dependent reactions, the light energy is converted to ATP and NADPH, which are used in the second phase of photosynthesis.
• During the light reactions, ATP and NADPH are generated by two electron-transport chains, water is used and oxygen is produced.

The chemical equation in the light reaction of photosynthesis can be reduced to:

2H 2 O + 2NADP+ + 3ADP + 3Pi → O 2 + 2NADPH + 3ATP

Dark Reaction of Photosynthesis (or) Light-independent Reaction

• Dark reaction is also called carbon-fixing reaction.
• It is a light-independent process in which sugar molecules are formed from the water and carbon dioxide molecules.
• The dark reaction occurs in the stroma of the chloroplast where they utilize the NADPH and ATP products of the light reaction.
• Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin photosynthesis cycle.
• In the Calvin cycle , the ATP and NADPH formed during light reaction drive the reaction and convert 6 molecules of carbon dioxide into one sugar molecule or glucose.

The chemical equation for the dark reaction can be reduced to:

3CO 2 + 6 NADPH + 5H 2 O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi

* G3P – glyceraldehyde-3-phosphate

Calvin photosynthesis Cycle (Dark Reaction)

Also Read:  Cyclic And Non-Cyclic Photophosphorylation

Importance of Photosynthesis

• Photosynthesis is essential for the existence of all life on earth. It serves a crucial role in the food chain – the plants create their food using this process, thereby, forming the primary producers.
• Photosynthesis is also responsible for the production of oxygen – which is needed by most organisms for their survival.

Frequently Asked Questions

1. what is photosynthesis explain the process of photosynthesis., 2. what is the significance of photosynthesis, 3. list out the factors influencing photosynthesis., 4. what are the different stages of photosynthesis, 5. what is the calvin cycle, 6. write down the photosynthesis equation..

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Please What Is Meant By 300-400 PPM

PPM stands for Parts-Per-Million. It corresponds to saying that 300 PPM of carbon dioxide indicates that if one million gas molecules are counted, 300 out of them would be carbon dioxide. The remaining nine hundred ninety-nine thousand seven hundred are other gas molecules.

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What is Photosynthesis?

This essay will explain the process of photosynthesis, detailing how plants convert light energy into chemical energy. It will discuss the role of chlorophyll, the light-dependent and light-independent reactions, and the importance of photosynthesis in the Earth’s ecosystems. The piece will also consider the impact of photosynthesis on the environment and its significance in scientific research and sustainable development. At PapersOwl too, you can discover numerous free essay illustrations related to Cellular Respiration.

How it works

Photosynthesis is the process that transforms organisms from light energy into chemical energy. In order for photosynthesis to take place, it needs these three things: Water, carbon dioxide, and sunlight. As humans, in order to live plants, must take in gases. Plants are known as “”autotrophs, which means organisms that can make their own food.

The process of photosynthesis was created and developed Jan Ingenhousz, a British physician and scientist. Joseph Priestley was another scientist who contributed to the discovery of photosynthesis Jan also discovered that plants have cellular respiration just like animals.

Priestley discovered that plants absorb and make gases, he also identified “”gas like oxygen. Another fact that he discovered was, oxygen gives off more light than carbon dioxide that is given off in the dark.

Photosynthesis has two major stages, the light stage, and the dark stage. The light stage is used to make NADPH and ATP. NADPH gives the electrons to fix the carbon dioxide into carbohydrates. Dark reactions will not continue if plants are taken away from light for too long. “”Plants get carbon dioxide through their leaves and water from the ground through their roots, plants also make their own food from sunlight. During photosynthesis, plants make their own foods. The formula for photosynthesis is known as carbon dioxide + water + light energy oxygen + glucose.

Gases get gas through the process called respiration. Respiration is the process that cells use oxygen to break down sugar and absorb energy, it is the opposite of photosynthesis. During respiration, oxygen is joined with hydrogen to form water for plants. This process involves using the sugars that were made during photosynthesis to produce energy for the growth of a plant. During respiration, plants take up nutrients just to keep the plant cells from dying. The formula for respiration is known as, oxygen + glucose carbon dioxide + water + heat energy.

If there was no photosynthesis, we would not be alive today because there would be no oxygen or food for us to live. When it is daytime photosynthesis creates glucose and oxygen more quickly than respiration utilizes it. Photosynthesis occurs in the cells of plant leaves, it takes place in the chloroplasts. The cells in the plant take in light from the sun through chlorophyll. The stem and leaves of a plant have little holes called the stomata, this is where carbon dioxide enters the plant. When the carbon dioxide is sucked up by the leaves, water goes into the plant through the plant’s roots. When sunlight goes onto the leaves of a plant the chlorophyll traps the energy into the leaves and stores the energy.

The Calvin cycle is apart of photosynthesis, it is another word for light-independent reactions. The Calvin cycle is where chemical reactions occur in chloroplasts while photosynthesis is occurring. This cycle takes place in the stroma, this doesn’t need direct sunlight, the Calvin cycle also takes place at night time. ATP and NADPH are products of light reactions, ATP is present to be broken down to have energy released. NADPH is present to transform carbon dioxide molecules into glucose (sugars).

During darkness hours plants can’t perform photosynthesis so instead they do cellular respiration, which occurs in the mitochondria. During the dark reaction plants use carbon dioxide from the light-dependent reactions in order to produce glucose.

There are steps to photosynthesis in a plant of course but, what are these steps? You might ask. The first step is, the plant pulls up minerals and water from the ground’s roots, the second step is, leaves take in carbon dioxide, they do this to set free oxygen. The third step is the sunlight releases energy to the chloroplasts to create glucose which is sugar, the food. The last step is especially important because it provides energy for the plant so that it can produce sugars (glucose). When the sugar is made, it gets broken down by the mitochondria (used to produce energy in a cell) to produce energy for the growth of the plant.

Light-dependent reactions make molecules needed for the next stage of photosynthesis by using light energy, light energy is kinetic energy. Oxygen that is produced during the light-dependent reactions is released into the atmosphere. As said in paragraph 5, ATP and NADPH are products of light reactions moreover, are compounds and are made by sunlight being absorbed and converted into chemical energy. Light energy travels in waves and is a form of electromagnetic radiation. Pigments absorb the light that is used in the photosynthesis process.

In the 20th century, correlations between the photosynthetic procedures in green plants and in certain photosynthetic sulfur, microscopic organisms gave important data about the photosynthetic component. Sulfur microbes use hydrogen sulfide as a source of hydrogen and deliver sulfur rather than oxygen when photosynthesis is occurring. In the 1930’s, a Dutch biologist named Cornelis Van Niel recognized that the use of carbon dioxide in organic compounds in both photosynthetic organisms was similar. The biologist suggested that there were differences in the light-dependent stage and was used as a source for hydrogen atoms, Niel suggested that hydrogen should be transferred from hydrogen sulfide or into the water.

Overall, photosynthesis is a very complex process with steps involved. Photosynthesis is a very important part of life because it provides oxygen to all living things and living things need oxygen in order to survive. All organisms must grow and reproduce energy and without photosynthesis that wouldn’t be possible. The energy derived from the sun would be wasted without photosynthesis and would leave behind lifeless plants that we need on earth. Plants need photosynthesis and humans need plants, so where exactly would we be without photosynthesis?

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What is photosynthesis?

Photosynthesis is arguably the most important biological process on earth. By liberating oxygen and consuming carbon dioxide, it has transformed the world into the hospitable environment we know today. Directly or indirectly, photosynthesis fills all of our food requirements and many of our needs for fiber and building materials. The energy stored in petroleum, natural gas and coal all came from the sun via photosynthesis, as does the energy in firewood, which is a major fuel in many parts of the world. This being the case, scientific research into photosynthesis is vitally important. If we can understand and control the intricacies of the photosynthetic process, we can learn how to increase crop yields of food, fiber, wood, and fuel, and how to better use our lands. The energy-harvesting secrets of plants can be adapted to man-made systems which provide new, efficient ways to collect and use solar energy. These same natural "technologies" can help point the way to the design of new, faster, and more compact computers, and even to new medical breakthroughs. Because photosynthesis helps control the makeup of our atmosphere, understanding photosynthesis is crucial to understanding how carbon dioxide and other "greenhouse gases" affect the global climate. In this document, we will briefly explore each of the areas mentioned above, and illustrate how photosynthesis research is critical to maintaining and improving our quality of life.

Photosynthesis and food. All of our biological energy needs are met by the plant kingdom, either directly or through herbivorous animals. Plants in turn obtain the energy to synthesize foodstuffs via photosynthesis. Although plants draw necessary materials from the soil and water and carbon dioxide from the air, the energy needs of the plant are filled by sunlight. Sunlight is pure energy. However, sunlight itself is not a very useful form of energy; it cannot be eaten, it cannot turn dynamos, and it cannot be stored. To be beneficial, the energy in sunlight must be converted to other forms. This is what photosynthesis is all about. It is the process by which plants change the energy in sunlight to kinds of energy that can be stored for later use. Plants carry out this process in photosynthetic reaction centers. These tiny units are found in leaves, and convert light energy to chemical energy, which is the form used by all living organisms. One of the major energy-harvesting processes in plants involves using the energy of sunlight to convert carbon dioxide from the air into sugars, starches, and other high-energy carbohydrates. Oxygen is released in the process. Later, when the plant needs food, it draws upon the energy stored in these carbohydrates. We do the same. When we eat a plate of spaghetti, our bodies oxidize or "burn" the starch by allowing it to combine with oxygen from the air. This produces carbon dioxide, which we exhale, and the energy we need to survive. Thus, if there is no photosynthesis, there is no food. Indeed, one widely accepted theory explaining the extinction of the dinosaurs suggests that a comet, meteor, or volcano ejected so much material into the atmosphere that the amount of sunlight reaching the earth was severely reduced. This in turn caused the death of many plants and the creatures that depended upon them for energy.

Photosynthesis and energy. One of the carbohydrates resulting from photosynthesis is cellulose, which makes up the bulk of dry wood and other plant material. When we burn wood, we convert the cellulose back to carbon dioxide and release the stored energy as heat. Burning fuel is basically the same oxidation process that occurs in our bodies; it liberates the energy of "stored sunlight" in a useful form, and returns carbon dioxide to the atmosphere. Energy from burning "biomass" is important in many parts of the world. In developing countries, firewood continues to be critical to survival. Ethanol (grain alcohol) produced from sugars and starches by fermentation is a major automobile fuel in Brazil, and is added to gasoline in some parts of the United States to help reduce emissions of harmful pollutants. Ethanol is also readily converted to ethylene, which serves as a feedstock to a large part of the petrochemical industry. It is possible to convert cellulose to sugar, and then into ethanol; various microorganisms carry out this process. It could be commercially important one day.

Our major sources of energy, of course, are coal, oil and natural gas. These materials are all derived from ancient plants and animals, and the energy stored within them is chemical energy that originally came from sunlight through photosynthesis. Thus, most of the energy we use today was originally solar energy!

Photosynthesis, fiber, and materials. Wood, of course, is not only burned, but is an important material for building and many other purposes. Paper, for example, is nearly pure photosynthetically produced cellulose, as is cotton and many other natural fibers. Even wool production depends on photosynthetically-derived energy. In fact, all plant and animal products including many medicines and drugs require energy to produce, and that energy comes ultimately from sunlight via photosynthesis. Many of our other materials needs are filled by plastics and synthetic fibers which are produced from petroleum, and are thus also photosynthetic in origin. Even much of our metal refining depends ultimately on coal or other photosynthetic products. Indeed, it is difficult to name an economically important material or substance whose existence and usefulness is not in some way tied to photosynthesis.

Photosynthesis and the environment. Currently, there is a lot of discussion concerning the possible effects of carbon dioxide and other "greenhouse gases" on the environment. As mentioned above, photosynthesis converts carbon dioxide from the air to carbohydrates and other kinds of "fixed" carbon and releases oxygen to the atmosphere. When we burn firewood, ethanol, or coal, oil and other fossil fuels, oxygen is consumed, and carbon dioxide is released back to the atmosphere. Thus, carbon dioxide which was removed from the atmosphere over millions of years is being replaced very quickly through our consumption of these fuels. The increase in carbon dioxide and related gases is bound to affect our atmosphere. Will this change be large or small, and will it be harmful or beneficial? These questions are being actively studied by many scientists today. The answers will depend strongly on the effect of photosynthesis carried out by land and sea organisms. As photosynthesis consumes carbon dioxide and releases oxygen, it helps counteract the effect of combustion of fossil fuels. The burning of fossil fuels releases not only carbon dioxide, but also hydrocarbons, nitrogen oxides, and other trace materials that pollute the atmosphere and contribute to long-term health and environmental problems. These problems are a consequence of the fact that nature has chosen to implement photosynthesis through conversion of carbon dioxide to energy-rich materials such as carbohydrates. Can the principles of photosynthetic solar energy harvesting be used in some way to produce non-polluting fuels or energy sources? The answer, as we shall see, is yes.

Why study photosynthesis?

Because our quality of life, and indeed our very existence, depends on photosynthesis, it is essential that we understand it. Through understanding, we can avoid adversely affecting the process and precipitating environmental or ecological disasters. Through understanding, we can also learn to control photosynthesis, and thus enhance production of food, fiber and energy. Understanding the natural process, which has been developed by plants over several billion years, will also allow us to use the basic chemistry and physics of photosynthesis for other purposes, such as solar energy conversion, the design of electronic circuits, and the development of medicines and drugs. Some examples follow.

Photosynthesis and agriculture. Although photosynthesis has interested mankind for eons, rapid progress in understanding the process has come in the last few years. One of the things we have learned is that overall, photosynthesis is relatively inefficient. For example, based on the amount of carbon fixed by a field of corn during a typical growing season, only about 1 - 2% of the solar energy falling on the field is recovered as new photosynthetic products. The efficiency of uncultivated plant life is only about 0.2%. In sugar cane, which is one of the most efficient plants, about 8% of the light absorbed by the plant is preserved as chemical energy. Many plants, especially those that originate in the temperate zones such as most of the United States, undergo a process called photorespiration. This is a kind of "short circuit" of photosynthesis that wastes much of the plants' photosynthetic energy. The phenomenon of photorespiration including its function, if any, is only one of many riddles facing the photosynthesis researcher.

If we can fully understand processes like photorespiration, we will have the ability to alter them. Thus, more efficient plants can be designed. Although new varieties of plants have been developed for centuries through selective breeding, the techniques of modern molecular biology have speeded up the process tremendously. Photosynthesis research can show us how to produce new crop strains that will make much better use of the sunlight they absorb. Research along these lines is critical, as recent studies show that agricultural production is leveling off at a time when demand for food and other agricultural products is increasing rapidly.

Because plants depend upon photosynthesis for their survival, interfering with photosynthesis can kill the plant. This is the basis of several important herbicides, which act by preventing certain important steps of photosynthesis. Understanding the details of photosynthesis can lead to the design of new, extremely selective herbicides and plant growth regulators that have the potential of being environmentally safe (especially to animal life, which does not carry out photosynthesis). Indeed, it is possible to develop new crop plants that are immune to specific herbicides, and to thus achieve weed control specific to one crop species.

Photosynthesis and energy production. As described above, most of our current energy needs are met by photosynthesis, ancient or modern. Increasing the efficiency of natural photosynthesis can also increase production of ethanol and other fuels derived from agriculture. However, knowledge gained from photosynthesis research can also be used to enhance energy production in a much more direct way. Although the overall photosynthesis process is relatively wasteful, the early steps in the conversion of sunlight to chemical energy are quite efficient. Why not learn to understand the basic chemistry and physics of photosynthesis, and use these same principles to build man-made solar energy harvesting devices? This has been a dream of chemists for years, but is now close to becoming a reality. In the laboratory, scientists can now synthesize artificial photosynthetic reaction centers which rival the natural ones in terms of the amount of sunlight stored as chemical or electrical energy. More research will lead to the development of new, efficient solar energy harvesting technologies based on the natural process.

The role of photosynthesis in control of the environment. How does photosynthesis in temperate and tropical forests and in the sea affect the quantity of greenhouse gases in the atmosphere? This is an important and controversial issue today. As mentioned above, photosynthesis by plants removes carbon dioxide from the atmosphere and replaces it with oxygen. Thus, it would tend to ameliorate the effects of carbon dioxide released by the burning of fossil fuels. However, the question is complicated by the fact that plants themselves react to the amount of carbon dioxide in the atmosphere. Some plants, appear to grow more rapidly in an atmosphere rich in carbon dioxide, but this may not be true of all species. Understanding the effect of greenhouse gases requires a much better knowledge of the interaction of the plant kingdom with carbon dioxide than we have today. Burning plants and plant products such as petroleum releases carbon dioxide and other byproducts such as hydrocarbons and nitrogen oxides. However, the pollution caused by such materials is not a necessary product of solar energy utilization. The artificial photosynthetic reaction centers discussed above produce energy without releasing any byproducts other than heat. They hold the promise of producing clean energy in the form of electricity or hydrogen fuel without pollution. Implementation of such solar energy harvesting devices would prevent pollution at the source, which is certainly the most efficient approach to control.

Photosynthesis and electronics. At first glance, photosynthesis would seem to have no association with the design of computers and other electronic devices. However, there is potentially a very strong connection. A goal of modern electronics research is to make transistors and other circuit components as small as possible. Small devices and short connections between them make computers faster and more compact. The smallest possible unit of a material is a molecule (made up of atoms of various types). Thus, the smallest conceivable transistor is a single molecule (or atom). Many researchers today are investigating the intriguing possibility of making electronic components from single molecules or small groups of molecules. Another very active area of research is computers that use light, rather than electrons, as the medium for carrying information. In principle, light-based computers have several advantages over traditional designs, and indeed many of our telephone transmission and switching networks already operate through fiber optics. What does this have to do with photosynthesis? It turns out that photosynthetic reaction centers are natural photochemical switches of molecular dimensions. Learning how plants absorb light, control the movement of the resulting energy to reaction centers, and convert the light energy to electrical, and finally chemical energy can help us understand how to make molecular-scale computers. In fact, several molecular electronic logic elements based on artificial photosynthetic reaction centers have already been reported in the scientific literature.

Photosynthesis and medicine. Light has a very high energy content, and when it is absorbed by a substance this energy is converted to other forms. When the energy ends up in the wrong place, it can cause serious damage to living organisms. Aging of the skin and skin cancer are only two of many deleterious effects of light on humans and animals. Because plants and other photosynthetic species have been dealing with light for eons, they have had to develop photoprotective mechanisms to limit light damage. Learning about the causes of light- induced tissue damage and the details of the natural photoprotective mechanisms can help us can find ways to adapt these processes for the benefit of humanity in areas far removed from photosynthesis itself. For example, the mechanism by which sunlight absorbed by photosynthetic chlorophyll causes tissue damage in plants has been harnessed for medical purposes. Substances related to chlorophyll localize naturally in cancerous tumor tissue. Illumination of the tumors with light then leads to photochemical damage which can kill the tumor while leaving surrounding tissue unharmed. Another medical application involves using similar chlorophyll relatives to localize in tumor tissue, and thus act as dyes which clearly delineate the boundary between cancerous and healthy tissue. This diagnostic aid does not cause photochemical damage to normal tissue because the principles of photosynthesis have been used to endow it with protective agents that harmlessly convert the absorbed light to heat.

Conclusions

The above examples illustrate the importance of photosynthesis as a natural process and the impact that it has on all of our lives. Research into the nature of photosynthesis is crucial because only by understanding photosynthesis can we control it, and harness its principles for the betterment of mankind. Science has only recently developed the basic tools and techniques needed to investigate the intricate details of photosynthesis. It is now time to apply these tools and techniques to the problem, and to begin to reap the benefits of this research.

Written by and Copyright ©1996 Devens Gust Professor of Chemistry and Biochemistry, Arizona State University

A  translation  of this article into Belorussian by Martha Ruszkowski is available

Learn more about Devens Gust

Home — Essay Samples — Science — Photosynthesis — The Process of Photosynthesis within a Carbon Cycle

The Process of Photosynthesis Within a Carbon Cycle

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Published: Aug 10, 2018

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