what is chemical reaction in 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 .

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.

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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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Photosynthetic Cells

Cells get nutrients from their environment, but where do those nutrients come from? Virtually all organic material on Earth has been produced by cells that convert energy from the Sun into energy-containing macromolecules. This process, called photosynthesis, is essential to the global carbon cycle and organisms that conduct photosynthesis represent the lowest level in most food chains (Figure 1).

What Is Photosynthesis? Why Is it Important?

Most living things depend on photosynthetic cells to manufacture the complex organic molecules they require as a source of energy. Photosynthetic cells are quite diverse and include cells found in green plants, phytoplankton, and cyanobacteria. During the process of photosynthesis, cells use carbon dioxide and energy from the Sun to make sugar molecules and oxygen. These sugar molecules are the basis for more complex molecules made by the photosynthetic cell, such as glucose. Then, via respiration processes, cells use oxygen and glucose to synthesize energy-rich carrier molecules, such as ATP, and carbon dioxide is produced as a waste product. Therefore, the synthesis of glucose and its breakdown by cells are opposing processes.

However, photosynthesis doesn't just drive the carbon cycle — it also creates the oxygen necessary for respiring organisms. Interestingly, although green plants contribute much of the oxygen in the air we breathe, phytoplankton and cyanobacteria in the world's oceans are thought to produce between one-third and one-half of atmospheric oxygen on Earth.

What Cells and Organelles Are Involved in Photosynthesis?

Chlorophyll A is the major pigment used in photosynthesis, but there are several types of chlorophyll and numerous other pigments that respond to light, including red, brown, and blue pigments. These other pigments may help channel light energy to chlorophyll A or protect the cell from photo-damage. For example, the photosynthetic protists called dinoflagellates, which are responsible for the "red tides" that often prompt warnings against eating shellfish, contain a variety of light-sensitive pigments, including both chlorophyll and the red pigments responsible for their dramatic coloration.

What Are the Steps of Photosynthesis?

Photosynthesis consists of both light-dependent reactions and light-independent reactions . In plants, the so-called "light" reactions occur within the chloroplast thylakoids, where the aforementioned chlorophyll pigments reside. When light energy reaches the pigment molecules, it energizes the electrons within them, and these electrons are shunted to an electron transport chain in the thylakoid membrane. Every step in the electron transport chain then brings each electron to a lower energy state and harnesses its energy by producing ATP and NADPH. Meanwhile, each chlorophyll molecule replaces its lost electron with an electron from water; this process essentially splits water molecules to produce oxygen (Figure 5).

Once the light reactions have occurred, the light-independent or "dark" reactions take place in the chloroplast stroma. During this process, also known as carbon fixation, energy from the ATP and NADPH molecules generated by the light reactions drives a chemical pathway that uses the carbon in carbon dioxide (from the atmosphere) to build a three-carbon sugar called glyceraldehyde-3-phosphate (G3P). Cells then use G3P to build a wide variety of other sugars (such as glucose) and organic molecules. Many of these interconversions occur outside the chloroplast, following the transport of G3P from the stroma. The products of these reactions are then transported to other parts of the cell, including the mitochondria, where they are broken down to make more energy carrier molecules to satisfy the metabolic demands of the cell. In plants, some sugar molecules are stored as sucrose or starch.

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Photosynthesis – Equation, Formula & Products

Core concepts.

In this tutorial, you will learn all about photosynthesis . We begin with an introduction to photosynthesis and its balanced chemical equation. Then, we analyze the two key stages involved in this process and take a look at the final products. Lastly, we consider the different types of photosynthesis.

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Introduction to Photosynthesis

The process by which plants and other organisms convert light energy (sunlight) into chemical energy (glucose) is known as photosynthesis. Sunlight powers a series of reactions that use water and carbon dioxide to synthesize glucose and release oxygen as a byproduct. Energy is stored in the chemical bonds of glucose and can be later harvested to fuel the organism’s activities through cellular respiration or fermentation .

Photosynthesis is an endergonic process because it requires an input of energy from the surroundings in order for a chemical change to take place. Furthermore, photosynthesis is a reduction-oxidation (redox) reaction , meaning that it involves the transfer of electrons between chemical species. During the process, carbon dioxide is reduced (i.e., gains electrons) to form glucose, and water is oxidized (i.e., loses electrons) to form molecular oxygen.

The complex process of photosynthesis takes place in chloroplasts (i.e., membrane-bound organelles in plant and algal cells). Chloroplasts have an outer membrane and an inner membrane. The stroma is the fluid-filled space within the inner membrane; it surrounds flattened sac-like structures known as thylakoids. Thylakoids consist of a thylakoid space (lumen) surrounded by a thylakoid membrane. The thylakoid membrane contains photosystems, which are large complexes of proteins and pigments. There are two types of photosystems: photosystem I (PSI) and photosystem II (PSII).

Chemical Equation for Photosynthesis

The overall balanced equation for photosynthesis is commonly written as 6 CO 2 + 6 H 2 O → C 6 H 12 O 6 + 6 O 2 (shown below). In other words, six molecules of carbon dioxide and six molecules of water react in the presence of sunlight to produce one molecule of glucose (a six-carbon sugar) and six molecules of oxygen.

Stages of Photosynthesis

There are two main stages of photosynthesis: the light-dependent reactions and the Calvin cycle.

Light-Dependent Reactions

The light-dependent reactions use light energy to make ATP (an energy-carrying molecule) and NADPH (an electron carrier) for use in the Calvin cycle. In addition, oxygen is released as a result of the oxidation of water. In plants and algae, the light-dependent reactions take place in the thylakoid membrane of chloroplasts. The most common form of the light-dependent reactions is a process known as non-cyclic photophosphorylation. This process involves two key steps: ATP synthesis (via photosystem II) and NADPH synthesis (via photosystem I).

Step 1 (ATP Synthesis): Pigments in photosystem II (such as chlorophylls) absorb light and energize electrons. A proton gradient is formed as these excited electrons travel down an electron transport chain and release energy that pumps hydrogen ions from the stroma to the thylakoid lumen. The splitting of water molecules through photolysis produces hydrogen ions (as well as oxygen molecules) that further contribute to this electrochemical gradient. As hydrogen ions flow down their gradient (i.e., back across the thylakoid membrane and into the stroma), they travel through an enzyme known as ATP synthase. ATP synthase catalyzes the formation of adenosine triphosphate (ATP) using ADP (adenosine diphosphate) and inorganic phosphate (P i ).
Step 2 (NADPH Synthesis): Electrons are transferred to photosystem I and energized by the light absorbed by PSI pigments. The electrons reach the end of the electron transport chain and are passed to an enzyme known as ferredoxin-NADP + reductase (FNR). FNR catalyzes the reaction by which NADP + is reduced to NADPH.

Calvin Cycle

The Calvin cycle (also referred to as the light-independent reactions) takes place in the stroma of chloroplasts and is not directly dependent on sunlight. Instead, this stage utilizes the products of the light-dependent reactions (ATP and NADPH), along with carbon dioxide, to synthesize glucose. The Calvin cycle consists of three basic steps: carbon fixation, reduction, and regeneration.

Step 1 (Carbon Fixation): RuBisCO (the most abundant enzyme on Earth) catalyzes the carboxylation of ribulose-1,5-biphosphate (RuBP) by carbon dioxide to produce an unstable six-carbon compound. This six-carbon compound is then readily converted into two molecules of 3-phosphoglyceric acid (3-PGA).
Step 2 (Reduction): An enzyme known as phosphoglycerate kinase catalyzes the phosphorylation of 3-PGA by ATP to produce 1,3-biphosphoglyceric acid (1,3-BPG) and ADP. Next, another enzyme (glyceraldehyde 3-phosphate dehydrogenase) catalyzes the reduction of 1,3-BPG by NADPH to produce glyceraldehyde 3-phosphate (G3P) and NADP + .
Step 3 (Regeneration): Every turn of the Calvin cycle produces two molecules of G3P. Therefore, six turns of the cycle produce twelve molecules of G3P. Two of these G3P molecules exit the cycle and are used to synthesize one molecule of glucose. Meanwhile, the other ten molecules of G3P remain in the cycle and are used to regenerate six RuBP molecules. The regeneration of RuBP requires ATP, but it allows the cycle to continue.

Products of Photosynthesis

The major product of photosynthesis is glucose, a simple sugar with the molecular formula C 6 H 12 O 6 . Plants and other photosynthetic organisms use glucose for numerous functions, including those listed below.

Cellular Respiration: Glucose is broken down in order to produce ATP (which can be used to fuel other cellular activities) through a process known as cellular respiration.
Biosynthesis of Starch and Cellulose: Glucose molecules can be linked together to form complex carbohydrates such as starch and cellulose. Plants and other organisms use starch to store energy and cellulose to support/rigidify their cell walls.
Protein Synthesis: Glucose can be combined with nitrates (from the soil) to produce amino acids, which can then be used to build proteins.

In addition, oxygen is released into the atmosphere during the process of photosynthesis. Plants (along with many other organisms) use oxygen to carry out aerobic respiration.

Types of Photosynthesis

There are three main types of photosynthesis: C3, C4, and CAM (crassulacean acid metabolism). They differ in the way that they manage photorespiration, a wasteful process that occurs when the enzyme rubisco acts on oxygen instead of carbon dioxide. Photorespiration competes with the Calvin cycle and decreases the efficiency of photosynthesis (by wasting energy and using up fixed carbon).

C3 Photosynthesis

The majority of plants use C3 photosynthesis, a process in which no special features or adaptations are used to combat photorespiration. Hot, dry climates are not ideal for C3 plants (e.g., rice, wheat, and barley) because of the increased rate of photorespiration, which is due to the buildup of oxygen that occurs when plants close their stomata (leaf pores) in order to prevent water loss.

C4 Photosynthesis

C4 photosynthesis reduces photorespiration by performing the initial carbon dioxide fixation and Calvin cycle in two different cell types. This process utilizes an additional enzyme known as phosphoenolpyruvate (PEP) carboxylase. PEP carboxylase does not react with oxygen (unlike rubisco) and is able to catalyze a reaction between carbon dioxide and PEP in the mesophyll cells to produce the intermediate four-carbon compound oxaloacetate. Oxaloacetate is then reduced to malate and transported to bundle sheath cells. In these cells, malate undergoes decarboxylation, forming a special compartment for the concentration of carbon dioxide around rubisco.

As a result, the Calvin cycle can proceed as normal, and an opportunity for rubisco to bind to oxygen is prevented. C4 plants (e.g., maize and sugarcane) have a competitive advantage over C3 plants in hot, dry environments where the benefits of reduced photorespiration outweigh the additional energy costs associated with C4 photosynthesis.

CAM Photosynthesis

Crassulacean acid metabolism, also known as CAM photosynthesis, reduces photorespiration by performing the initial carbon dioxide fixation and Calvin cycle at separate times. CAM plants (e.g., cactus and pineapple) open their stomata at night, allowing carbon dioxide to enter the leaf. The carbon dioxide is converted to oxaloacetate by PEP carboxylase, the same enzyme used in C4 photosynthesis. Oxaloacetate is subsequently reduced to malate, which is stored as malic acid in vacuoles .

During the day (when light is readily available), CAM plants close their stomata and prepare for the Calvin cycle. Malate is transported into chloroplasts and broken down to release carbon dioxide, which is heavily concentrated around the enzyme rubisco. Similar to C4 photosynthesis, crassulacean acid metabolism is an energetically expensive process. However, it is quite useful for plants in hot, arid climates that need to minimize photorespiration and conserve water.

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What is photosynthesis?

Photosynthesis is the process plants, algae and some bacteria use to turn sunlight, carbon dioxide and water into sugar and oxygen.

Photosynthetic processes
Photosynthesis equation
The carbon exchange
How do plants absorb sunlight?
Location of photosynthesis

Light-dependent reactions

The Calvin cycle

Types of photosynthesis

Additional resources.

Photosynthesis is the process used by plants, algae and some bacteria to turn sunlight into energy. The process chemically converts carbon dioxide (CO2) and water into food (sugars) and oxygen . The chemical reaction often relies on a pigment called chlorophyll, which gives plants their green color. Photosynthesis is also the reason our planet is blanketed in an oxygen-rich atmosphere.

Types of photosynthetic processes

There are two types of photosynthesis: oxygenic and anoxygenic. They both follow very similar principles, but the former is the most common and is seen in plants, algae and cyanobacteria.

During oxygenic photosynthesis, light energy transfers electrons from water (H2O) taken up by plant roots to CO2 to produce carbohydrates . In this transfer, the CO2 is "reduced," or receives electrons, and the water is "oxidized," or loses electrons. Oxygen is produced along with carbohydrates.

This process creates a balance on Earth, in which the carbon dioxide produced by breathing organisms as they consume oxygen in respiration is converted back into oxygen by plants, algae and bacteria.

Anoxygenic photosynthesis, meanwhile, uses electron donors that are not water and the process does not generate oxygen, according to "Anoxygenic Photosynthetic Bacteria" by LibreTexts . The process typically occurs in bacteria such as green sulfur bacteria and phototrophic purple bacteria.

The Photosynthesis equation

Though both types of photosynthesis are complex, multistep affairs, the overall process can be neatly summarized as a chemical equation.

The oxygenic photosynthesis equation is:

6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O

Here, six molecules of carbon dioxide (CO2) combine with 12 molecules of water (H2O) using light energy. The end result is the formation of a single carbohydrate molecule (C6H12O6, or glucose) along with six molecules each of oxygen and water.

Similarly, the various anoxygenic photosynthesis reactions can be represented as a single generalized formula:

CO2 + 2H2A + Light Energy → [CH2O] + 2A + H2O

The letter A in the equation is a variable, and H2A represents the potential electron donor. For example, "A" may represent sulfur in the electron donor hydrogen sulfide (H2S), according to medical and life sciences news site News Medical Life Sciences .

How is carbon dioxide and oxygen exchanged?

stomata are the gatekeepers of the leaf, allowing gas exchange between the leaf and surrounding air.

Plants absorb CO2 from the surrounding air and release water and oxygen via microscopic pores on their leaves called stomata.

When stomata open, they let in CO2; however, while open, the stomata release oxygen and let water vapor escape. Stomata close to prevent water loss, but that means the plant can no longer gain CO2 for photosynthesis. This tradeoff between CO2 gain and water loss is a particular problem for plants growing in hot, dry environments.

How do plants absorb sunlight for photosynthesis?

Plants contain special pigments that absorb the light energy needed for photosynthesis.

Chlorophyll is the primary pigment used for photosynthesis and gives plants their green color, according to science education site Nature Education . Chlorophyll absorbs red and blue light and reflects green light. Chlorophyll is a large molecule and takes a lot of resources to make; as such, it breaks down towards the end of the leaf's life, and most of the pigment's nitrogen (one of the building blocks of chlorophyll) is resorbed back into the plant, When leaves lose their chlorophyll in the fall, other leaf pigments such as carotenoids and anthocyanins begin to show. While carotenoids primarily absorb blue light and reflect yellow, anthocyanins absorb blue-green light and reflect red light, according to Harvard University's The Harvard Forest .

Pigment molecules are associated with proteins, which allow them the flexibility to move toward light and toward one another. A large collection of 100 to 5,000 pigment molecules constitutes an "antenna," according to an article by Wim Vermaas , a professor at Arizona State University. These structures effectively capture light energy from the sun, in the form of photons.

The situation is a little different for bacteria. While cyanobacteria contain chlorophyll, other bacteria, for example, purple bacteria and green sulfur bacteria, contain bacteriochlorophyll to absorb light for anoxygenic photosynthesis, according to " Microbiology for Dummies " (For Dummies, 2019).

Related: What if humans had photosynthetic skin?

Where in the plant does photosynthesis take place?

Plants need energy from sunlight for photosynthesis to occur.

Photosynthesis occurs in chloroplasts, a type of plastid (an organelle with a membrane) that contains chlorophyll and is primarily found in plant leaves.

Chloroplasts are similar to mitochondria , the energy powerhouses of cells, in that they have their own genome, or collection of genes, contained within circular DNA. These genes encode proteins that are essential to the organelle and to photosynthesis.

Inside chloroplasts are plate-shaped structures called thylakoids that are responsible for harvesting photons of light for photosynthesis, according to the biology terminology website Biology Online . The thylakoids are stacked on top of each other in columns known as grana. In between the grana is the stroma — a fluid containing enzymes, molecules and ions, where sugar formation takes place.

Ultimately, light energy must be transferred to a pigment-protein complex that can convert it to chemical energy, in the form of electrons. In plants, light energy is transferred to chlorophyll pigments. The conversion to chemical energy is accomplished when a chlorophyll pigment expels an electron, which can then move on to an appropriate recipient.

The pigments and proteins that convert light energy to chemical energy and begin the process of electron transfer are known as reaction centers.

When a photon of light hits the reaction center, a pigment molecule such as chlorophyll releases an electron.

The released electron escapes through a series of protein complexes linked together, known as an electron transport chain. As it moves through the chain, it generates the energy to produce ATP (adenosine triphosphate, a source of chemical energy for cells) and NADPH — both of which are required in the next stage of photosynthesis in the Calvin cycle. The "electron hole" in the original chlorophyll pigment is filled by taking an electron from water. This splitting of water molecules releases oxygen into the atmosphere.

Light-independent reactions: The Calvin cycle

Photosynthesis involves a process called the Calvin cycle to use energy stored from the light-dependent reactions to fix CO2 into sugars needed for plant growth.

The Calvin cycle is the three-step process that generates sugars for the plant, and is named after Melvin Calvin , the Nobel Prize -winning scientist who discovered it decades ago. The Calvin cycle uses the ATP and NADPH produced in chlorophyll to generate carbohydrates. It takes plate in the plant stroma, the inner space in chloroplasts.

In the first step of this cycle, called carbon fixation, an enzyme called RuBP carboxylase/oxygenase, also known as rubiso, helps incorporate CO2 into an organic molecule called 3-phosphoglyceric acid (3-PGA). In the process, it breaks off a phosphate group on six ATP molecules to convert them to ADP, releasing energy in the process, according to LibreTexts.

In the second step, 3-PGA is reduced, meaning it takes electrons from six NADPH molecules and produces two glyceraldehyde 3-phosphate (G3P) molecules.

One of these G3P molecules leaves the Calvin cycle to do other things in the plant. The remaining G3P molecules go into the third step, which is regenerating rubisco. In between these steps, the plant produces glucose, or sugar.

Three CO2 molecules are needed to produce six G3P molecules, and it takes six turns around the Calvin cycle to make one molecule of carbohydrate, according to educational website Khan Academy.

There are three main types of photosynthetic pathways: C3, C4 and CAM. They all produce sugars from CO2 using the Calvin cycle, but each pathway is slightly different.

The three main types of photosynthetic pathways are C3, C4 and CAM.

C3 photosynthesis

Most plants use C3 photosynthesis, according to the photosynthesis research project Realizing Increased Photosynthetic Efficiency (RIPE) . C3 plants include cereals (wheat and rice), cotton, potatoes and soybeans. This process is named for the three-carbon compound 3-PGA that it uses during the Calvin cycle.

C4 photosynthesis

Plants such as maize and sugarcane use C4 photosynthesis. This process uses a four-carbon compound intermediate (called oxaloacetate) which is converted to malate , according to Biology Online. Malate is then transported into the bundle sheath where it breaks down and releases CO2, which is then fixed by rubisco and made into sugars in the Calvin cycle (just like C3 photosynthesis). C4 plants are better adapted to hot, dry environments and can continue to fix carbon even when their stomata are closed (as they have a clever storage solution), according to Biology Online.

CAM photosynthesis

Crassulacean acid metabolism (CAM) is found in plants adapted to very hot and dry environments, such as cacti and pineapples, according to the Khan Academy. When stomata open to take in CO2, they risk losing water to the external environment. Because of this, plants in very arid and hot environments have adapted. One adaptation is CAM, whereby plants open stomata at night (when temperatures are lower and water loss is less of a risk). According to the Khan Academy, CO2 enters the plants via the stomata and is fixed into oxaloacetate and converted into malate or another organic acid (like in the C4 pathway). The CO2 is then available for light-dependent reactions in the daytime, and stomata close, reducing the risk of water loss.

Discover more facts about photosynthesis with the educational science website sciencing.com . Explore how leaf structure affects photosynthesis with The University of Arizona . Learn about the different ways photosynthesis can be measured with the educational science website Science & Plants for Schools .

This article was updated by Live Science managing editor Tia Ghose on Nov. 3, 2022.

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Daisy Dobrijevic joined Space.com in February 2022 as a reference writer having previously worked for our sister publication All About Space magazine as a staff writer. Before joining us, Daisy completed an editorial internship with the BBC Sky at Night Magazine and worked at the National Space Centre in Leicester, U.K., where she enjoyed communicating space science to the public. In 2021, Daisy completed a PhD in plant physiology and also holds a Master's in Environmental Science, she is currently based in Nottingham, U.K.

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The Balanced Chemical Equation for Photosynthesis

Photosynthesis Overall Chemical Reaction

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Photosynthesis is the process in plants and certain other organisms that uses the energy from the sun to convert carbon dioxide and water into glucose (a sugar) and oxygen.

Here is the balanced equation for the overall reaction:

6 CO 2 + 6 H 2 O → C 6 H 12 O 6 + 6 O 2

Where: CO 2 = carbon dioxide H 2 O = water light is required C 6 H 12 O 6 = glucose O 2 = oxygen

Explanation

In words, the equation may be stated as: Six carbon dioxide molecules and six water molecules react to produce one glucose molecule and six oxygen molecules .

The reaction requires energy in the form of light to overcome the activation energy needed for the reaction to proceed. Carbon dioxide and water don't spontaneously convert into glucose and oxygen .

What Are the Products of Photosynthesis?
Chlorophyll Definition and Role in Photosynthesis
Photosynthesis Vocabulary Terms and Definitions
Examples of Chemical Reactions in Everyday Life
10 Fascinating Photosynthesis Facts
Calvin Cycle Steps and Diagram
Examples of 10 Balanced Chemical Equations
Simple Chemical Reactions
Photosynthesis Basics - Study Guide
What Is the Primary Function of the Calvin Cycle?
The Photosynthesis Formula: Turning Sunlight into Energy
Chemosynthesis Definition and Examples
Synthesis Reaction Definition and Examples
Equation for the Reaction Between Baking Soda and Vinegar
Chloroplast Function in Photosynthesis
Understanding Endothermic and Exothermic Reactions

5.2 The Light-Dependent Reactions of Photosynthesis

Learning objectives.

Explain how plants absorb energy from sunlight
Describe how the wavelength of light affects its energy and color
Describe how and where photosynthesis takes place within a plant

How can light be used to make food? It is easy to think of light as something that exists and allows living organisms, such as humans, to see, but light is a form of energy. Like all energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is transformed into chemical energy, which autotrophs use to build carbohydrate molecules. However, autotrophs only use a specific component of sunlight ( Figure 5.8 ).

Link to Learning

Watch the process of photosynthesis within a leaf in this video.

What Is Light Energy?

The sun emits an enormous amount of electromagnetic radiation (solar energy). Humans can see only a fraction of this energy, which is referred to as “visible light.” The manner in which solar energy travels can be described and measured as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength , the distance between two consecutive, similar points in a series of waves, such as from crest to crest or trough to trough ( Figure 5.9 ).

Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun. The electromagnetic spectrum is the range of all possible wavelengths of radiation ( Figure 5.10 ). Each wavelength corresponds to a different amount of energy carried.

Each type of electromagnetic radiation has a characteristic range of wavelengths. The longer the wavelength (or the more stretched out it appears), the less energy is carried. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.

The sun emits ( Figure 5.10 ) a broad range of electromagnetic radiation, including X-rays and ultraviolet (UV) rays. The higher-energy waves are dangerous to living things; for example, X-rays and UV rays can be harmful to humans.

Absorption of Light

Light energy enters the process of photosynthesis when pigments absorb the light. In plants, pigment molecules absorb only visible light for photosynthesis. The visible light seen by humans as white light actually exists in a rainbow of colors. Certain objects, such as a prism or a drop of water, disperse white light to reveal these colors to the human eye. The visible light portion of the electromagnetic spectrum is perceived by the human eye as a rainbow of colors, with violet and blue having shorter wavelengths and, therefore, higher energy. At the other end of the spectrum toward red, the wavelengths are longer and have lower energy.

Understanding Pigments

Different kinds of pigments exist, and each absorbs only certain wavelengths (colors) of visible light. Pigments reflect the color of the wavelengths that they cannot absorb.

All photosynthetic organisms contain a pigment called chlorophyll a , which humans see as the common green color associated with plants. Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not from green. Because green is reflected, chlorophyll appears green.

Other pigment types include chlorophyll b (which absorbs blue and red-orange light) and the carotenoids. Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light, which is its absorption spectrum .

Many photosynthetic organisms have a mixture of pigments; between them, the organism can absorb energy from a wider range of visible-light wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity decreases with depth, and certain wavelengths are absorbed by the water. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees block most of the sunlight ( Figure 5.11 ).

How Light-Dependent Reactions Work

The overall purpose of the light-dependent reactions is to convert light energy into chemical energy. This chemical energy will be used by the Calvin cycle to fuel the assembly of sugar molecules.

The light-dependent reactions begin in a grouping of pigment molecules and proteins called a photosystem . Photosystems exist in the membranes of thylakoids. A pigment molecule in the photosystem absorbs one photon , a quantity or “packet” of light energy, at a time.

A photon of light energy travels until it reaches a molecule of chlorophyll. The photon causes an electron in the chlorophyll to become “excited.” The energy given to the electron allows it to break free from an atom of the chlorophyll molecule. Chlorophyll is therefore said to “donate” an electron ( Figure 5.12 ).

To replace the electron in the chlorophyll, a molecule of water is split. This splitting releases an electron and results in the formation of oxygen (O 2 ) and hydrogen ions (H + ) in the thylakoid space. Technically, each breaking of a water molecule releases a pair of electrons, and therefore can replace two donated electrons.

The replacing of the electron enables chlorophyll to respond to another photon. The oxygen molecules produced as byproducts find their way to the surrounding environment. The hydrogen ions play critical roles in the remainder of the light-dependent reactions.

Keep in mind that the purpose of the light-dependent reactions is to convert solar energy into chemical carriers that will be used in the Calvin cycle. In eukaryotes and some prokaryotes, two photosystems exist. The first is called photosystem II, which was named for the order of its discovery rather than for the order of the function.

After the photon hits, photosystem II transfers the free electron to the first in a series of proteins inside the thylakoid membrane called the electron transport chain. As the electron passes along these proteins, energy from the electron fuels membrane pumps that actively move hydrogen ions against their concentration gradient from the stroma into the thylakoid space. This is quite analogous to the process that occurs in the mitochondrion in which an electron transport chain pumps hydrogen ions from the mitochondrial stroma across the inner membrane and into the intermembrane space, creating an electrochemical gradient. After the energy is used, the electron is accepted by a pigment molecule in the next photosystem, which is called photosystem I ( Figure 5.13 ).

Generating an Energy Carrier: ATP

In the light-dependent reactions, energy absorbed by sunlight is stored by two types of energy-carrier molecules: ATP and NADPH. The energy that these molecules carry is stored in a bond that holds a single atom or group of atoms to the molecule. For ATP, it is a phosphate group, and for NADPH, it is a hydrogen atom. Recall that NADH was a similar molecule that carried energy in the mitochondrion from the citric acid cycle to the electron transport chain. When these molecules release energy into the Calvin cycle, they each lose either atoms or groups of atoms to become the lower-energy molecules ADP and NADP + .

The buildup of hydrogen ions in the thylakoid space forms an electrochemical gradient because of the difference in the concentration of protons (H + ) and the difference in the charge across the membrane that they create. This potential energy is harvested and stored as chemical energy in ATP through chemiosmosis, the movement of hydrogen ions down their electrochemical gradient through the transmembrane enzyme ATP synthase, just as in the mitochondrion.

The hydrogen ions are allowed to pass through the thylakoid membrane through an embedded protein complex called ATP synthase. This same protein generated ATP from ADP in the mitochondrion. The energy generated by the hydrogen ion stream allows ATP synthase to attach a third phosphate to ADP, which forms a molecule of ATP in a process called photophosphorylation. The flow of hydrogen ions through ATP synthase is called chemiosmosis, because the ions move from an area of high to low concentration through a semi-permeable structure.

Generating Another Energy Carrier: NADPH

The remaining function of the light-dependent reaction is to generate the other energy-carrier molecule, NADPH. As the electron from the electron transport chain arrives at photosystem I, it is re-energized with another photon captured by chlorophyll. The energy from this electron drives the formation of NADPH from NADP + and a hydrogen ion (H + ). Now that the solar energy is stored in energy carriers, it can be used to make a sugar molecule.

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What is Photosynthesis Reaction?

Photosynthesis Reaction

It is the most important chemical reaction for life on the earth. What do pizza, campfires, automobiles dolphins and glaciers have common between them? They all rely on photosynthesis reaction. Some rely on many ways.

Similarly, we also consider that photosynthesis is the most important chemical reaction for life on the planet earth. Let us delve into the working of this process of photosynthesis reaction. And also find out why we are so indebted to it.

Process of Photosynthesis Reaction

Photosynthesis reaction involves a series of chemical reactions. Each of which converts one substance to the other substance. When taking this reaction as a whole we can summarize it in a single symbolic representation. As in the chemical reaction mentioned below.

6CO2 + 6H2O + light C6H12O6 + 6O2

We can substitute words for chemical symbols. Then the equation appears as follows:

Carbon Dioxide + Water + Light Energy Glucose + Oxygen Gas

Likewise, the other chemical equations, this equation for photosynthesis process shows that the reactants have a connection through plus signs. Moreover, on the left and products, also have a connection by the plus signs, on the right. Further, an arrow that that indicates the process or the chemical range leads from the reactants to the products.

And conditions are necessary for the chemical reaction are written over the top of the arrow. Similarly, the same kinds of atoms and the number of atoms are present on both of the sides of the equation. But, the types of compounds they form a change.

Application of Photosynthesis Reaction

We use chemical reactions every time we are cooking or baking. After that, we add the ingredients together (the reactants), place them in a suitable specific condition (often heat), and then enjoy the result that it gives (the products). The photosynthesis reaction is shown below:

6CO2 + 6H2O + Light C6H12O6 + 6O2

The above equation shows that the ingredients for the photosynthesis process are carbon dioxide, light energy, and water. Algae, plants and photosynthetic bacteria take in light through the sun, water molecules from their environment and carbon dioxide from the air and combine these reactants to produce the food i.e. glucose.

Of course, light, water and carbon dioxide mix up in the air even without the plants. However, they do not change chemically for making food without very specific necessary conditions. Similarly, these are found only in the cells of photosynthetic organisms. The necessary situation for this includes:

Enzymes – proteins that speed up the chemical reactions.

Chlorophyll – a pigment within the cells of the plants that absorbs the light.

Chloroplasts – organelles whose membranes embed accessory pigments, chlorophyll, and enzymes in patterns that maximize the photosynthesis process.

Within cells of the plants or algal cells, chloroplasts organize enzymes, accessory pigment molecules and chlorophyll and that are necessary for the photosynthesis process.

Efficiency of Photosynthesis

Further, the photosynthetic efficiency is the fraction responsible for converting into chemical energy in the duration of the photosynthesis method. Moreover, it occurs in plants and algae. Photosynthesis can be described by the simple chemical reaction given below:

6H2O + 6CO2 + energy → C6H12O6 + 6O2

Algae and Other Monocellular Organisms

From a study of 2010 by the University of Maryland. Moreover, photosynthesizing Cyanobacteria have been shown to be a particular species in the global carbon cycle. Accounting for 20-30% of Earth’s photosynthetic productivity and then converts the solar energy. Therefore, this results in the formation of biomass-stored chemical energy at the rate of ~450 TW.

Solved Question For You

Ques. Name the physiochemical process in which chemical energy is produced by light energy with the help of a photosynthetic organism?

(A). Photosynthesis. (B). Respiration. (C). Oxidative decarboxylation. (D). Oxidative phosphorylation.

Ans . (A). Photosynthesis.

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8.6: The Light-Dependent Reactions of Photosynthesis - Processes of the Light-Dependent Reactions

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Learning Objectives

Describe how light energy is converted into ATP and NADPH.

How Light-Dependent Reactions Work

The overall function of light-dependent reactions, the first stage of photosynthesis, is to convert solar energy into chemical energy in the form of NADPH and ATP, which are used in light-independent reactions and fuel the assembly of sugar molecules. Protein complexes and pigment molecules work together to produce NADPH and ATP.

Producing Chemical Energy

Light energy is converted into chemical energy in a multiprotein complex called a photosystem. Two types of photosystems, photosystem I (PSI) and photosystem II (PSII), are found in the thylakoid membrane inside the chloroplast. Each photosystem consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll a and b molecules, as well as other pigments like carotenoids. Cytochrome b6f complex and ATP synthase are also major protein complexes in the thylakoid membrane that work with the photosystems to create ATP and NADPH.

The two photosystems absorb light energy through proteins containing pigments, such as chlorophyll. The light-dependent reactions begin in photosystem II. In PSII, energy from sunlight is used to split water, which releases two electrons, two hydrogen atoms, and one oxygen atom. When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, the electron in this molecule attains a higher energy level. Because this state of an electron is very unstable, the electron is transferred to another molecule creating a chain of redox reactions called an electron transport chain (ETC). The electron flow goes from PSII to cytochrome b6f to PSI; as electrons move between these two photosystems, they lose energy. Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by PSI. Therefore, another photon is absorbed by the PSI antenna. That energy is transmitted to the PSI reaction center. This reaction center, known as P700, is oxidized and sends a high-energy electron to reduce NADP+ to NADPH. This process illustrates oxygenic photosynthesis, wherein the first electron donor is water and oxygen is created as a waste product.

Cytochrome b6f and ATP synthase work together to create ATP. This process, called photophosphorylation, occurs in two different ways. In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump hydrogen ions from the lumen (an area of high concentration) to the stroma (an area of low concentration). The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms ATP. This flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure. In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from both PSII and PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to maintain the right proportions of NADPH and ATP, which will carry out light-independent reactions later on.

The net-reaction of all light-dependent reactions in oxygenic photosynthesis is: 2H 2 O + 2NADP+ + 3ADP + 3Pi → O 2 + 2NADPH + 3ATP

Light energy splits water and extracts electrons in photosystem II (PSII); then electrons are moved from PSII to cytochrome b6f to photosystem I (PSI) and reduce in energy.
Electrons are re-energized in PSI and those high energy electrons reduce NADP + to NADPH.
In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump hydrogen ions from the lumen to the stroma; this energy allows ATP synthase to attach a third phosphate group to ADP, which forms ATP.
In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from both PSII and PSI to create more ATP and to stop the production of NADPH, maintaining the right proportions of NADPH and ATP.
photosystem : Either of two biochemical systems, active in chloroplasts, that are part of photosynthesis.
photophosphorylation : The addition of a phosphate (PO43-) group to a protein or other organic molecule by photosynthesis.
chemiosmosis : The movement of ions across a selectively permeable membrane, down their electrochemical gradient.

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What Kind of Reaction Is Photosynthesis?

Enzyme Activity in Photosynthesis

Without the series of chemical reactions collectively known as photosynthesis, you wouldn't be here and neither would anyone else you know. This might strike you as an odd claim if you happen to know that photosynthesis is exclusive to plants and a few micro-organisms, and that not a single cell in your body or that of any animal has the apparatus to carry out this elegant assortment of reactions. What gives?

Put simply, plant life and animal life are almost perfectly symbiotic, meaning that the way that plants go about fulfilling their metabolic needs is of supreme benefit to animals and vice versa. In simplest terms, animals take in oxygen gas (O 2 ) to derive energy from non-gaseous carbon sources and excrete carbon dioxide gas (CO 2 ) and water (H 2 O) in the process, while plants use CO 2 and H 2 O to make food and release O 2 to the environment. In addition, about 87 percent of the world's energy is presently derived from the burning of fossil fuels, which are ultimately products of photosynthesis as well.

It is sometimes said that "photosynthesis is to plants what respiration is to animals," but this is a flawed analogy because plants make use of both, while animals only use respiration. Think of photosynthesis as the way plants consume and digest carbon, relying on light rather than locomotion and the act of eating to put carbon in a form that tiny cellular machines can put to use.

A Quick Overview of Photosynthesis

Photosynthesis, despite not being used directly by a significant fraction of living things, can be reasonably viewed as the one chemical process responsible for ensuring the ongoing existence of life on Earth itself. Photosynthetic cells take CO 2 and H 2 O gathered by the organism from the environment and use the energy from sunlight to power the synthesis of glucose (C 6 H 12 O 6 ), releasing O 2 as a waste product. This glucose is then processed by different cells in the plant in the same way glucose is used by animal cells: It undergoes respiration to release energy in the form of adenosine triphosphate (ATP) and releases CO 2 as a waste product. (Phytoplankton and cyanobacteria also make use of photosynthesis, but for purposes of this discussion, organisms containing photosynthetic cells are referred to generically as "plants.")

Organisms that use photosynthesis to make glucose are called autotrophs, which translates loosely from Greek to "self food." That is, plants do not rely on other organisms directly for food. Animals, on the other hand, are heterotrophs ("other food") because they have to ingest carbon from other living sources in order to grow and remain alive.

What Type of Reaction Is Photosynthesis?

Photosynthesis is considered a redox reaction. Redox is short for "reduction-oxidation," which describes what occurs at the atomic level in the various biochemical reactions. The complete, balanced formula for the series of reactions called photosynthesis – the components of which will be explored shortly – is:

6H 2 O + light + 6CO 2 → C 6 H 12 O 6 + 6O 2

You can verify for yourself that the number of each type of atom is the same on each side of the arrow: Six carbon atoms, 12 hydrogen atoms and 18 oxygen atoms.

Reduction is the removal of electrons from an atom or molecule, while oxidation is the gaining of electrons. Correspondingly, compounds that readily yield electrons to other compounds are called oxidizing agents, while those that tend to gain electrons are called reducing agents. Redox reactions usually involve the addition of hydrogen to the compound being reduced.

The Structures of Photosynthesis

The first step in photosynthesis might be summed up as "let there be light." Sunlight strikes the surface of plants, setting the whole process in motion. You might already suspect why many plants look the way they do: A great deal of surface area in the form of leaves and the branches that support them that appears unnecessary (albeit attractive) if you don't know why these organisms are structured this way. The "goal" of the plant is to expose as much of itself to sunlight as it can – making the shortest, smallest plants in any ecosystem rather like the runts of an animal litter in that they both struggle to obtain enough energy. Leaves, not surprisingly, are extremely dense in photosynthetic cells.

These cells are rich in organisms called chloroplasts, which is where the work of photosynthesis is done, just like mitochondria are the organelles in which respiration occurs. In fact, chloroplasts and mitochondria are structurally quite similar, a fact that, like practically everything in the world of biology, can be traced to the marvels of evolution.) Chloroplasts contain specialized pigments that optimally absorb light energy rather than reflecting it. That which is reflected rather than absorbed happens to be in a range of wavelengths that is interpreted by the human eye and brain as being a particular color (hint: It starts with "g"). The main pigment used for this purpose is known as chlorophyll.

Chloroplasts are surrounded by a double plasma membrane, as is the case with all living cells as well as the organelles they contain. In plants, however, a third membrane exists internal to the plasma bilayer, called a thylakoid membrane. This membrane is folded very extensively so that disclike structures stacked atop each other result, not unlike a package of breath mints. These thylakoid structures contain chlorophyll. The space between the inner chloroplast membrane and the thylakoid membrane is called the stroma.

The Mechanism of Photosynthesis

Photosynthesis is divided into a set of light-dependent and light-independent reactions, usually called the light and dark reactions and described in detail later. As you may have concluded, the light reactions occur first.

When light from the sun strikes the chlorophyll and other pigments inside the thylakoids, it essentially blasts loose electrons and protons from the atoms in chlorophyll and elevates them to a higher energy level, making them freer to migrate. The electrons are diverted into the electron transport chain reactions that unfold on the thylakoid membrane itself. Here, electron acceptors such as NADP receive some of these electrons, which are also used to drive the synthesis of ATP. ATP is essentially to cells what dollars are to the U.S. financial system: It is "the energy currency" using which virtually all metabolic processes are ultimately carried out.

While this is happening, the sun-bathing chlorophyll molecules have suddenly found themselves short of electrons. This is where water enters the fray and contributes replacement electrons in the form of hydrogen, thereby reducing the chlorophyll. With its hydrogen gone missing, what was once water is now molecular oxygen – O 2 . This oxygen diffuses out of the cell and out of the plant entirely, and some of it has managed to find its way into your own lungs at precisely this second.

Is Photosynthesis Endergonic?

Photosynthesis is termed an endergonic reaction because it requires an input of energy in order to proceed. The sun is the ultimate source of all energy on the planet (a fact perhaps understood at some level by the various cultures of antiquity that considered the sun a deity in its own right) and plants are the first to intercept it for productive use. Without this energy, there would be no way for carbon dioxide, a small, simple molecule, to be converted to glucose, a considerably larger and more complex molecule. Imagine yourself walking up a flight of stairs while somehow not expending any energy, and you can see the problem faced by plants.

In arithmetic terms, endergonic reactions are those in which the products have a higher energy level than the reactants do. The opposite of these reactions, energetically speaking, are called exergonic, in which the products have lower energy than the reactions and energy is thereby liberated during the reaction. (This is often in the form of heat – again, do you become warmer or do you grow colder with exercise?) This is expressed in terms of the free energy ΔG° of the reaction, which for photosynthesis is +479 kJ ⋅ mol -1 or 479 joules of energy per mole. The positive sign indicates an endothermic reaction, while a negative sign indicates an exothermic process.

The Light and Dark Reactions of Photosynthesis

In the light reactions, water is broken apart by sunlight, while in the dark reactions, the protons (H + ) and electrons (e − ) freed in the light reactions are used to assemble glucose and other carbohydrates from CO 2 .

The light reactions are given by the formula:

2H 2 O + light → O 2 + 4H + + 4e − (ΔG° = +317 kJ ⋅ mol −1 )

and the dark reactions are given by:

CO 2 + 4H + + 4e − → CH 2 O + H 2 O (ΔG° = +162 kJ ⋅ mol −1 )

Overall, this yields the complete equation revealed above:

H 2 O + light + CO 2 → CH 2 O + O 2 (ΔG° = +479 kJ ⋅ mol −1 )

You can see that both sets of reactions are endergonic, the light reactions more strongly so.

What Is Energy Coupling?

Energy coupling in living systems means using energy made available from one process to drive other processes that would otherwise not take place. Society itself sort of works this way: Businesses often have to borrow large sums of money up front in order to get off the ground, but ultimately some of these businesses become highly profitable and can make funds available for other start-up companies.

Photosynthesis represents a good example of energy coupling, as energy from sunlight is coupled to reactions in chloroplasts so that the reactions can unfold. The plant eventually rewards the global carbon cycle by synthesizing glucose and other carbon compounds that can be coupled to other reactions, immediately or in the future. For example, wheat plants produce starch, used the world over as a main source of foods for humans and other animals. But not all of the glucose made by plants is stored; some of it proceeds to different parts of plant cells, where the energy liberated in glycolysis is ultimately coupled to reactions in the plant mitochondria that result in the formation of ATP. While plants represent the bottom of the food chain and are widely viewed as passive energy and oxygen donors, they do have metabolic needs of their own, having to grow larger and reproduce just like other organisms.

Why Can't Subscripts Be Changed?

As an aside, students often have trouble learning to balance chemical reactions if these are not provided in balanced form. As a result, in their tinkering, students may be tempted to change the values of the subscripts in molecules in the reaction in order to achieve a balanced result. This confusion may stem from knowing that it is permissible to change the numbers in front of the molecules in order to balance reactions. Changing the subscript of any molecule turns that molecule into a different molecule altogether. For example, changing O 2 to O 3 does not merely add 50 percent more oxygen in terms of mass; it changes oxygen gas into ozone, which would not participate in the reaction under study in a remotely similar way.

Components of photosynthesis, what happens in the light reaction of photosynthesis, what is produced as a result of photosynthesis, what is reduced & oxidized in photosynthesis, what is the photosynthesis equation, describe what a photosystem does for photosynthesis, what happens to carbon dioxide during photosynthesis, what provides electrons for the light reactions, 10 facts on photosynthesis, what is the waste product of photosynthesis, what are light dependent reactions, importance of pigments in photosynthesis, how does darkness affect plant growth, steps to photosynthesis for middle school science, materials needed for photosynthesis, what is the end product of photosynthesis, cellular respiration: definition, equation & steps, what is the role of carotenoids in photosynthesis.

Scitable by Nature Education: Photosynthetic Cells
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About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.

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Chemical Communications

Recent development of photocatalytic production of hydrogen peroxide.

Hydrogen peroxide (H2O2), an environmentally friendly strong oxidant and energy carrier, has attracted widespread attention in photocatalysis. Artificial photosynthesis of H2O2 using water and oxygen as raw materials, solar energy as an energy source, and semiconductor materials as catalysts is considered a promising technology. In the past decade, encouraging progress has been made in the photocatalytic production of H2O2. Therefore, we summarize the research achievements in this field in recent years. This review first briefly introduces the reaction pathway, detection techniques and evaluation metrics. Then, the recent advances in photocatalysts are highlighted. Furthermore, the existing challenges and possible solutions in this field are presented. At last, we look forward to the future development direction of this field. This review provides valuable insights and guidance for efficient photocatalytic H2O2 production.

This article is part of the themed collections: Chemical Communications HOT Articles 2024 and CC 60th Anniversary Authors Collection

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X. Fang, X. Huang, Q. Hu, B. Li, C. Hu, B. Ma and Y. Ding, Chem. Commun. , 2024, Accepted Manuscript , DOI: 10.1039/D4CC01577K

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