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18.3D: Electron Transport Chain and Chemisomosis

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• Page ID 3414

• Gary Kaiser
• Community College of Baltimore Country (Cantonsville)

Learning Objectives

• Briefly describethe function of the electron transport chain during aerobic respiration.
• Briefly describethe chemiosmotic theory of generation of ATP as a result of an electron transport chain.
• Compare where the electron transport chain occurs in prokaryotic cells and in eukaryotic cells.
• State what is meant by proton motive force.
• State the function of ATP synthases in chemiosmosis.
• State the final electron acceptor and the end product formed at the end of aerobic respiration.

During various steps in glycolysis and the citric acid cycle, the oxidation of certain intermediate precursor molecules causes the reduction of NAD + to NADH + H + and FAD to FADH 2 . NADH and FADH 2 then transfer protons and electrons to the electron transport chain to produce additional ATPs by oxidative phosphorylation .

As mentioned in the previous section on energy, during the process of aerobic respiration, coupled oxidation-reduction reactions and electron carriers are often part of what is called an electron transport chain , a series of electron carriers that eventually transfers electrons from NADH and FADH 2 to oxygen. The diffusible electron carriers NADH and FADH 2 carry hydrogen atoms (protons and electrons) from substrates in exergonic catabolic pathways such as glycolysis and the citric acid cycle to other electron carriers that are embedded in membranes. These membrane-associated electron carriers include flavoproteins, iron-sulfur proteins, quinones, and cytochromes. The last electron carrier in the electron transport chain transfers the electrons to the terminal electron acceptor, oxygen.

The chemiosmotic theory explains the functioning of electron transport chains. According to this theory, the transfer of electrons down an electron transport system through a series of oxidation-reduction reactions releases energy (Figure \(\PageIndex{1}\)). This energy allows certain carriers in the chain to transport hydrogen ions (H + or protons) across a membrane.

Depending on the type of cell, the electron transport chain may be found in the cytoplasmic membrane or the inner membrane of mitochondria.

• In prokaryotic cells, the protons are transported from the cytoplasm of the bacterium across the cytoplasmic membrane to the periplasmic space located between the cytoplasmic membrane and the cell wall .
• In eukaryotic cells, protons are transported from the matrix of the mitochondria across the inner mitochondrial membrane to the intermembrane space located between the inner and outer mitochondrial membranes (Figure \(\PageIndex{2}\)).

As the hydrogen ions accumulate on one side of a membrane, the concentration of hydrogen ions creates an electrochemical gradient or potential difference (voltage) across the membrane. (The fluid on the side of the membrane where the protons accumulate acquires a positive charge; the fluid on the opposite side of the membrane is left with a negative charge.) The energized state of the membrane as a result of this charge separation is called proton motive force or PMF.

This proton motive force provides the energy necessary for enzymes called ATP synthases (see Figure \(\PageIndex{3}\)), also located in the membranes mentioned above, to catalyze the synthesis of ATP from ADP and phosphate. This generation of ATP occurs as the protons cross the membrane through the ATP synthase complexes and re-enter either the bacterial cytoplasm (Figure \(\PageIndex{4}\)) or the matrix of the mitochondria. As the protons move down the concentration gradient through the ATP synthase, the energy released causes the rotor and rod of the ATP synthase to rotate. The mechanical energy from this rotation is converted into chemical energy as phosphate is added to ADP to form ATP.

Proton motive force is also used to transport substances across membranes during active transport and to rotate bacterial flagella.

At the end of the electron transport chain involved in aerobic respiration, the last electron carrier in the membrane transfers 2 electrons to half an oxygen molecule (an oxygen atom) that simultaneously combines with 2 protons from the surrounding medium to produce water as an end product (Figure \(\PageIndex{5}\)).

Movie illustrating the electron transport system in the mitochondria of eukaryotic cells.

• Aerobic respiration involves four stages: glycolysis, a transition reaction that forms acetyl coenzyme A, the citric acid (Krebs) cycle, and an electron transport chain and chemiosmosis.
• During various steps in glycolysis and the citric acid cycle, the oxidation of certain intermediate precursor molecules causes the reduction of NAD + to NADH + H + and FAD to FADH 2 . NADH and FADH 2 then transfer protons and electrons to the electron transport chain to produce additional ATPs by oxidative phosphorylation.
• The electron transport chain consists of a series of electron carriers that eventually transfer electrons from NADH and FADH 2 to oxygen.
• The chemiosmotic theory states that the transfer of electrons down an electron transport system through a series of oxidation-reduction reactions releases energy. This energy allows certain carriers in the chain to transport hydrogen ions (H + or protons) across a membrane.
• As the hydrogen ions accumulate on one side of a membrane, the concentration of hydrogen ions creates an electrochemical gradient or potential difference (voltage) across the membrane called proton motive force.
• This proton motive force provides the energy necessary for enzymes called ATP synthases, also located in the membranes mentioned above, to catalyze the synthesis of ATP from ADP and phosphate.
• During aerobic respiration, the last electron carrier in the membrane transfers 2 electrons to half an oxygen molecule (an oxygen atom) that simultaneously combines with 2 protons from the surrounding medium to produce water as an end product.

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From carbon dioxide to oxygen and back - Class 11

Course: from carbon dioxide to oxygen and back - class 11   >   unit 2.

• Electron transport chain

Oxidative phosphorylation and chemiosmosis

• Calculating ATP produced in cellular respiration
• Electron transport chain and oxidative phosphorylation 1
• Electron transport chain and oxidative phosphorylation 2

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A perspective on Peter Mitchell and the chemiosmotic theory

• Published: 10 October 2008
• Volume 40 , pages 407–410, ( 2008 )

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In 1991 Peter Mitchell wrote a last article that summarised his views on the origin, development and current status of his chemiosmotic ideas. I here review some of his views of that time on structures and mechanisms of several key bioenergetic components in relation to the subsequent advances that have been made.

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Chemistry: From Matter to Life

Deciphering Life Sciences with “Live” Chemistry—The 2022 Nobel Prize in Chemistry

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Rich, P.R. A perspective on Peter Mitchell and the chemiosmotic theory. J Bioenerg Biomembr 40 , 407–410 (2008). https://doi.org/10.1007/s10863-008-9173-7

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Received : 11 July 2008

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Published : 10 October 2008

Issue Date : October 2008

DOI : https://doi.org/10.1007/s10863-008-9173-7

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Chemiosmotic hypothesis

Table of Contents

Introduction

The chemiosmotic hypothesis is a widely accepted model. It describes how energy from electron transfer reactions is converted into ATP synthesis in living organisms. The hypothesis was first proposed by Peter Mitchell in 1961. It revolutionized our understanding of how cells generate ATP, the universal energy currency of living systems. This study note will explore the chemiosmotic hypothesis in detail.

Historical context

Before the chemiosmotic hypothesis, the prevailing theory for ATP synthesis was known as the “substrate-level phosphorylation” model. This model suggested that ATP was produced by directly transferring a phosphate group from a substrate molecule to ADP. However, experimental evidence did not fully support this model, and many scientists recognized that there must be an alternative mechanism for ATP synthesis.

In 1961, Peter Mitchell proposed the chemiosmotic hypothesis, which provided a new explanation for how ATP is synthesized in living systems. The hypothesis was initially met with skepticism, but as more evidence was accumulated, it gradually became accepted as a fundamental principle of bioenergetics.

Key components of the chemiosmotic hypothesis

The chemiosmotic hypothesis involves several key components, including electron transport chains, proton gradients, ATP synthase, and the mitochondrial inner membrane.

Electron transport chains

Electron transport chains are a series of membrane-bound proteins that transfer electrons from one molecule to another. In eukaryotic cells, electron transport chains are found in the inner mitochondrial membrane. In prokaryotic cells, they are located in the plasma membrane.

During electron transport, electrons are passed from one electron carrier to the next, releasing energy at each step. This energy is used to pump protons (H+) across the membrane, creating a proton gradient.

Proton gradients

Proton gradients are created when protons are pumped across a membrane, generating a difference in proton concentration (pH) and charge (electric potential) across the membrane. The chemiosmotic hypothesis proposes that this gradient can be used to generate ATP.

ATP synthase

ATP synthase is an enzyme complex that spans the mitochondrial inner membrane in eukaryotic cells. It consists of two major components: a proton channel, which allows protons to flow down their electrochemical gradient, and a catalytic domain, which synthesizes ATP from ADP and inorganic phosphate (Pi) using the energy from the proton gradient.

Mitochondrial inner membrane

The mitochondrial inner membrane is a highly impermeable membrane that separates the mitochondrial matrix from the intermembrane space. It contains electron transport chains and ATP synthase, which are the key components involved in the chemiosmotic hypothesis.

Mechanism of ATP synthesis

The chemiosmotic hypothesis proposes that ATP is synthesized when protons flow back across the mitochondrial inner membrane through the ATP synthase complex. This process is known as oxidative phosphorylation and can be divided into two main stages: electron transport and ATP synthesis.

Electron transport

During electron transport, electrons are passed from one electron carrier to the next, releasing energy at each step. This energy is used to pump protons across the mitochondrial inner membrane, creating a proton gradient. The proton gradient consists of a difference in proton concentration (pH) and charge (electric potential) across the membrane.

ATP synthesis

In the second stage of oxidative phosphorylation, protons flow back across the mitochondrial inner membrane through the ATP synthase complex. As the protons flow through the channel, they release energy. That energy is used to drive the synthesis of ATP from ADP and Pi. This process is known as chemiosmotic coupling, as it links the flow of protons down their electrochemical gradient to the synthesis of ATP.

Importance of the chemiosmotic hypothesis in cellular metabolism

The chemiosmotic hypothesis is a fundamental principle of bioenergetics and plays a crucial role in cellular metabolism. It explains how the energy stored in electron transfer reactions can be harnessed to generate ATP, the universal energy currency of living systems.

The chemiosmotic hypothesis is also relevant in the context of mitochondrial diseases. Mitochondria are the powerhouses of the cell. Disruptions to their function can lead to a range of diseases, including neurological disorders and muscle wasting. Many mitochondrial diseases are caused by defects in the electron transport chain or ATP synthase. And these highlight the importance of the chemiosmotic hypothesis in understanding these conditions.

The chemiosmotic hypothesis is a widely accepted model that describes how energy from electron transfer reactions is converted into ATP synthesis in living organisms. The hypothesis was first proposed by Peter Mitchell in 1961 and has since become a fundamental principle of bioenergetics. It involves several key components, including electron transport chains, proton gradients, ATP synthase, and the mitochondrial inner membrane. The chemiosmotic hypothesis is crucial for understanding how cells generate energy and how disruptions to this process can lead to disease.

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• Published: 14 January 1967

Chemiosmotic Hypothesis of Oxidative Phosphorylation

• PETER MITCHELL 1 &
• JENNIFER MOYLE 1  

Nature volume  213 ,  pages 137–139 ( 1967 ) Cite this article

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Dr. Moyle and Dr. Mitchell answer criticisms of their interpretation of tests of the hypothesis proposed by Dr. Mitchell in 1961 to explain ATP synthesis in the inner membrane of mitochondria and of chloroplasts by a fuel-cell type of mechanism

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Mitchell, P., and Moyle, J., in Colloquium on the Biochemistry of Mitochondria, Third Meet. Fed. Europ. Biochem. Socs ., 53 (Academic Press and P. W. N. London and Warsaw,1967).

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Mitchell, P., Chemiosmotic Coupling in Oxidative and Photosynthetic Phosphorylation (Glynn Research Ltd. Bodmin, 1966); Biol. Rev. , 41 , 445 (1966).

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MITCHELL, P., MOYLE, J. Chemiosmotic Hypothesis of Oxidative Phosphorylation. Nature 213 , 137–139 (1967). https://doi.org/10.1038/213137a0

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Chemiosmotic Hypothesis: The chemiosmotic hypothesis is a biological mechanism proposed in 1961 by a British biochemist  named Peter Dennis Mitchell.

• It is the mechanism by which  ATP  molecules are synthesised by the activity of  ATP  synthase.
• It is a process that describes how  ATP  molecules or energy molecules are formed as a result of the process of photosynthesis.
• The Nobel Prize in Chemistry was granted to the biochemist for his important contributions to the discipline of Biology, since his study gave a clearer understanding of the complete process of the Chemiosmotic hypothesis.

The Process of Chemiosmotic Hypothesis

The  proton  gradient that prevails across the thylakoid membrane causes  ATP: Adenosine triphosphates  to be generated throughout this process. Proton gradient, ATP synthase, and proton pump are three essential components for the chemiosmosis process. ATP synthase is the  enzyme  that is needed for the formation of ATP molecules.

F0 and F1 are the 2 subunits of the  ATP  synthase enzyme. The F0 subunit is implicated in proton transport across the membrane, which results in changes in F1 structure and  enzyme  activation. The enzyme phosphorylates ADP (adding a phosphate group) and transforms ADP to ATP. ATP synthase is primarily driven by the proton gradient that develops throughout the membrane.

Photosystems assist  chlorophyll  absorb  light  during the light response phase of photosynthesis. This causes  hydrolysis , in which water molecules are torn apart, releasing  electrons  and protons. The liberated electrons are energised and proceed to a higher energy level, where the electron transport system gives them away.

Meanwhile, the stroma’s discharged protons begin to accumulate within the membrane. As a result, a proton gradient is created, which is a product of the electron transport chain. Photosystem I use a little amount of the resulting protons to convert NADP+ to NADPH using electrons obtained from water photolysis. The proton gradient eventually collapses, releasing energy and protons that are transported back to the stroma via ATP synthase F0. The energy released causes changes in F1 conformation, which initiates the ATP synthase, which transforms ADP to ATP.

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Chemiosmotic hypothesis

Definition noun A theory postulated by the biochemist Peter Mitchell in 1961 to describe ATP synthesis by way of a proton electrochemical coupling . Accordingly, hydrogen ion s ( proton s) are pumped from the mitochondrial matrix to the intermembrane space via the hydrogen carrier protein s while the electrons are transferred along the electron transport chain in the mitochondrial inner membrane. As the hydrogen ions accumulate in the intermembrane space, an energy-rich proton gradient is established. As the proton gradient becomes sufficiently intense the hydrogen ion s tend to diffuse back to the matrix (where hydrogen ion s are less) via the ATP synthase (a transport protein). As the hydrogen ion s diffuse (through the ATP synthase ) energy is released which is then used to drive the conversion of ADP to ATP (by phosphorylation ). Supplement This theory was not previously well accepted until a great deal of evidence for proton pumping by the complexes of the electron transfer chain emerged. This began to favor the chemiosmotic hypothesis, and in 1978, Peter Mitchell was awarded the Nobel Prize in Chemistry. Also called: chemiosmotic theory See also: chemiosmosis

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Setting the Record Straight: A New Twist on the Chemiosmotic Mechanism of Oxidative Phosphorylation

Magdalena juhaszova.

Laboratory of Cardiovascular Science, National Institute on Aging, NIH, Baltimore, MD 21224, USA

Evgeny Kobrinsky

Dmitry b zorov.

A.N. Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow 119992, Russia

Miguel A Aon

Translational Gerontology Branch, National Institute on Aging, NIH, Baltimore, MD 21224, USA

Sonia Cortassa

Steven j sollott.

Mitchell's chemiosmotic theory has been the cornerstone of the mechanistic understanding of mitochondrial energy metabolism for the past 7 decades. 1 , 2 Although aspects have remained controversial (in the minds of some) since its original description, many of its features and predictions have withstood the test of time and it has gained widespread acceptance; yet there have been certain areas that it clearly does not seem to adequately describe. 3–8 The original Mitchell view posited that the respiratory chain, harnessing redox energy, in turn pumps protons out of the matrix setting up an inward-directed H + gradient with energy components stored as ΔΨ m and ΔpH. These energies, particularly that stored as ΔΨ m , are harnessed by driving H + back into the matrix, exerting work by its specific translocation path in turning the F o motor in ATP synthase, and in a mechanochemical energy transduction with the F 1 motor of ATP synthase, produces ATP from ADP + Pi. 9 This constitutes the (electro)chemical energy transduction part of “chemiosmosis.” A particular area that probably isn't adequately accounted for by Mitchell's theory as originally described 10 is the “osmotic” part of chemiosmosis (see below). Historically, this osmotic aspect has been considered to be a key mechanistic insight. 11 , 12

What we have discovered 13 , 14 is entirely compatible with Mitchell, but also builds upon this foundation and extends these mechanisms by revealing an additional “K + uniport circuit” through ATP synthase (beyond the presently accepted H + uniport circuit) whereby cytosolic K + is also able to be driven by ΔΨ m and, in an analogous manner as H + , transported without being exchanged by ATP synthase (and turning its F o motor, etc.) to make ATP ( Fig. 1A ). In short, the electrochemical uniport of K + in addition to that of H + (in approximately the ratio of 3 K + for every H + ), to make ATP, and its accumulation in the matrix in proportion to workload, is a new discovery that advances the understanding of not only the electrochemical , but also the osmotic part of chemiosmosis. Specifically, the electrogenic uniport of K + by ATP synthase that substantially contributes to the synthesis of ATP, accumulates matrix K + at levels producing much higher concentrations (by many orders) than that of ATP synthase-uniported H + levels, so that these K + levels and, importantly, their changes in direct proportion to that of workload , produce relevant osmotic-drive signals that control matrix volume changes, which feeds back to cause changes in the activation state of the respiratory chain (by the so-called “volume-activation of respiration” mechanism), serving to facilitate matching energy supply with demand. The resulting matrix-accumulated K + is then extruded via a separate K/H exchanger, which is (likely) kinetically phase-lagged, and this enables the dynamics of the process to produce transient/temporary, controlled “up” and “down” matrix-volume signals to new work-level-related physiological steady-state levels (i.e., by this process, preventing excess volume expansion or contraction). All the input energy involved in K + extrusion still derives from the original respiratory chain-generated H + gradient (Δ µ H ), and thus the entire process is again entirely consistent with Mitchell.

Comparison and functional implications of alternate models of ion-directed energy coupling in ATP synthase. (A) Chemiosmotic theory extended by the capacity of ATP synthase to utilize K + , H + (and Na + ) electrogenic uniport to drive ATP synthesis. The respiratory chain pumps protons out of the matrix setting up an inward-directed H + gradient with energy components stored as ΔΨ m and ΔpH. These energies (mainly ΔΨ m ) are used by driving H + , K + (and Na + ) back into the matrix via an electrogenic translocation path enabling them to perform work (by exerting torque on the F o   c -ring) and turning the F o rotary motor in ATP synthase, and in a mechanochemical energy transduction with the F 1 catalytic complex of ATP synthase, produces ATP from ADP + Pi in the mitochondrial matrix. K + and H + each occupy (but physically separately) the same ion-binding locations on the c -ring of the F o motor in ATP synthase. In the standard “H + -only”-running model of chemiosmotic theory, all eight positions on the c -ring (in the mammalian F o ) would be H + (shown as green balls, as in Panel B). In our model shown here, of the eight c -ring positions, on average approximately six will be occupied by K + (red balls) and two will be occupied by H + (in a net ratio of ∼3:1) in a random ordering, to produce three ATP for a full c -ring turn with the translocation of those eight ions (N.B., for clarity, 5 K + and 3 H + are shown on the c -ring in the illustration; since only ∼1 Na + is translocated for every 24 cations driving ATP synthase, it is not shown; the direction of ion movement shown here in F o is for convenient illustration purposes only—the actual direction of rotation is clockwise for ATP synthesis, with the ion entrance and exit “hemichannels” arranged respectively). The resulting matrix-accumulated K + is then extruded via a separate (kinetically phase-lagged) K + /H + exchanger. The electrogenic uniport of K + by ATP synthase is adaptive and workload dependent, producing relevant osmotic-driven signals that control matrix volume changes that in turn result in activation of the respiratory chain (volume-activation of respiration mechanism), serving as feedback for matching energy supply with demand. See text and accompanying references for details. (B) Nath's electroneutral “two-ion” theory of energy coupling. According to Nath's electroneutral “two-ion” theory the primary redox energy-harnessing step at the respiratory chain is the net electroneutral pumping of H + out of the matrix in exchange for K + , which secondarily generates ΔpK on the matrix side (together with ΔpH on the intermembrane space side). Then, harnessing the newly formed ΔpK, K + is translocated first from the matrix down its concentration gradient by a postulated K/H antiport mechanism residing within the ATP synthase. The resulting K + movement-related local charge separation generates a temporary, localized ΔΨ m (within the ATP synthase complex itself) that electrostatically attracts H + , enabling translocation of H + and binding to the c-ring (producing torque, etc.), and ultimately (together with ΔpH) driving H + back into the matrix. Except for H + using a putative, coupled ion-antiport/translocation activity (structurally uncorroborated and unverified by any physical measurement technique) hypothetically occurring at the a - c -subunits’ interface of F o to access the c -ring (Panel B), rather than using the accepted, structurally identified a -subunit aqueous access half-channels (as depicted in Panel A; e.g., for bovine structure, see 16 , and references therein for concordant structural evidence across taxonomic domains), the subsequent processes apparently involve the standard “H + -only”-running model to drive ATP synthase, with all positions on the F o   c -ring bound exclusively by H + (shown as green balls). While this latter K/H-exchanged energy (together with that of ΔpH) transferred back to H + is finally used to drive ATP synthesis, another important distinction in this model (vs that depicted in Panel A) is that K +   neither binds to, nor has its energy directly harnessed to turn the c -ring. In fact, the vector direction of this latter ATP synthase-antiported K + movement (resulting from the putative K/H exchange activity) is exactly the opposite of that depicted in the model shown in Panel A. This process would produce apparently paradoxically maladaptive reciprocal changes in matrix K + and workload , by retaining higher matrix K + , volumes and respiratory activities at low workloads , and in the opposite case, causing a lower matrix K + , volumes and respiratory activities at high workloads , which would serve the opposite purposes required for appropriate energy supply-demand matching. Because all these ion exchange processes are net electroneutral, the bulk phase ΔΨ m is functionally irrelevant in this model.

Prof. Sunil Nath has written an Opinion article related to our work in the previous issue of Function 15 that we feel needs a response. From his published work, Prof. Nath is clearly a skilled and dedicated scientist. We appreciate his enthusiasm in pushing the scientific envelope, a maverick trait that we appreciate as kindred spirits in the pursuit of scientific understanding. Prof. Nath's current conjecture in his Opinion article 15 suggests that the compendium of our 2-decades of research that we have put forth in the two-part series in the current issue of Function 13 , 14 is somehow the same idea and mechanism as his “two-ion theory of energy coupling” of ATP synthase. We respectfully point out that these 2 scientific stories, which might appear to be superficially similar because they happen to involve the same two cations (K + and H + ) in mitochondrial energy metabolism, are nevertheless actually entirely distinct and even incompatible models with each other regarding the way Nature works. In the spirit of collegiality, we infer this to be a simple misunderstanding on the part of Prof. Nath, as we will explain below.

We describe a process that involves the movement of univalent, alkali metal cations (K + and Na + ) in addition to H + , all in the same direction from the intermembrane space into the matrix, travelling along the same path in ATP synthase and occupying the same location on the c -ring (i.e., the monovalent cation-conducting, rotary component of ATP synthase's F o motor). The electrochemical potential of each ion (K + , Na + or H + ) including ΔΨ m is harnessed as it moves across the inner mitochondrial membrane to make ATP 13 , 14 , 17 ( Fig. 1A ). This is emphatically not a charge-neutral process, and intrinsically dissipates ΔΨ m (and the respective ion-gradients, if present).

In stark contrast, Nath's “two-ion theory of energy coupling” model 15 proposes an electroneutral H + /K +   anti port within the F 1 F o -ATP synthase itself to maintain electroneutrality 15 ( Fig. 1B ). As recognized by Bertero and Maack 18 , our model 13 , 14 , 19 defines electrogenic “ uni port” of H + and K + in the same direction via the ATP synthase, where K + extrusion is accounted for by a separate molecularly distinct K + /H + exchanger ( Fig. 1A ).

The mechanism we describe is not only compatible but extends Mitchell's chemiosmotic theory: an electrogenic, ΔΨ m (Δ µ H )-driven one-ion-uniport mechanism of H + , K + (and some Na + ) in ATP synthase to make ATP, explicitly leads to the adaptive workload-dependent changes in matrix K + controlling osmotic drive and changes in matrix volume in direct proportion to regulate respiratory chain activity. Both H + and K + (as well as a lesser amount of Na + ) are directly involved in making ATP by turning the F o motor. Because of the electrogenic nature of the main processes described here, an obligatory feature is that the development of even small additional electrical charge imbalances due to the primary ion movements (for a sufficient time lag before the extrusion process “catches up” 13 , 14 , 19 ) will be accompanied by the movement of appropriate charge-balancing counterions. This produces the ability to develop changes in osmotic driving gradients that can regulate water and matrix volume dynamics, and in turn, the function of the respiratory chain ( Fig. 1A ).

In contrast, Nath's mechanism is completely contrary to this Mitchellian mechanism, as Nath has unambiguously stated. 20 Specifically, his mechanism describes the redox-driven electroneutral extrusion of matrix H + by respiratory complexes with an obligatory, associated K/H exchange activity producing a “secondary translocation” of K + at the respiratory complexes producing a matrix K + gradient without changing bulk phase ΔΨ m . This outwardly directed matrix K + gradient (Δ[K + ]) then apparently drives K + back across the inner mitochondrial membrane through an entirely hypothetical path in mammalian ATP synthase to power an obligatory, net- electroneutral exchange process (putatively inside ATP synthase) of this K + with H + , using the energy transferred and acquired in that exchange process to drive that H + back through the usual path that turns the F o motor enabling ATP synthase to make ATP. Only H + is directly making ATP by turning the F o motor ( Fig. 1B ).

So, the Nath mechanism is an electro neutral, ΔΨ m -independent (ΔpH → ΔpK → ΔpH)-driven two -ion- anti port mechanism. It involves the two -ion anti port of K + with H + inside the ATP synthase molecule to make ATP, without a clear ability to develop significant, positively adaptive changes in osmotic drive ( Fig. 1B ). Even if , for some reason, an intrinsic K + -related osmotic change mechanism would yet be asserted for the Nath hypothesis, it would still seem to produce apparently paradoxically maladaptive reciprocal changes in matrix K + and workload : it would seem this mechanism would serve the opposite purposes required for energy supply-demand matching by retaining higher matrix K + , volumes and respiratory activities at low workloads , and in the opposite case, causing a lower matrix K + , volumes and respiratory activities at high workloads .

Finally, and paradoxically, there are at least four lines of definitive and independent experimental evidence in our published work 13 , 14 that was cited by Nath as “confirming” or “revalidating” his theory 15 , each based on different and distinct experimental techniques, that instead entirely contradict and refute Nath's hypothesized mechanism. On the basis of our experimental evidence (elaborated below) ATP synthase behaves electrogenically , as demonstrated by, (1) the voltage-dependence of ion channel currents produced by ATP synthase, (2) the inability of single ATP synthase molecules—that are demonstrably competent to utilize a K + gradient to synthesize ATP under certain defined conditions—to also achieve ΔpK-driven ATP synthesis at the system reversal potential, E rev (i.e., the specific potential where the net flow of charge across the membrane is zero), (3) F 1 F o -ATP synthase reconstituted proteoliposomes in a K + gradient (without an H + gradient) can only make ATP when an exogenous protonophore is provided, and (4) a large K + gradient creates a large, stable membrane (K + diffusion) potential in F 1 F o -ATP synthase reconstituted proteoliposomes in the absence of an added protonophore. 13

Importantly, regarding the first and second points raised above, when ATP synthase is examined in isolation, it behaves electrogenically —showing macroscopic currents that vary by voltage—rather than in an electro neutral fashion ( 13 see Fig. 1D-G therein). Furthermore, these macroscopic currents in ATP synthase can be manifest by a K + gradient itself in the absence of a H + gradient at a holding potential of 0 mV ( 13 see Fig. 4 therein). Under these latter conditions, these are pure K + currents (driven by ΔpK), and when the vector of their charge movement is properly directed (with respect to the orientation of ATP synthase), they drive and enable ATP synthase to produce ATP. On the contrary, these same K + gradient conditions (absent a H + gradient) when set at a holding potential where there are no macroscopic currents (i.e., at the system's reversal potential, E rev ) result in that same molecule of ATP synthase producing no ATP (i.e., in the same experiments) ( 13 see Fig. 4B therein). These are primary proof-of-principle results in the present research 13 . In contrast, if ATP synthase could function to synthesize ATP in an electroneutral fashion (i.e., driven by the K + gradient and putatively enabled by an internal 1:1 exchange with H + contained within ATP synthase, as Nath proposes), ATP synthesis would have occurred regardless of voltage in these experiments, but this does not happen 13 , and thus the Nath hypothesis is experimentally refuted .

Two additional, key experiments also contradict and refute the Nath hypothesis of an intrinsic K + /H + exchange mechanism in ATP synthase to maintain electroneutrality: (1) F 1 F o -ATP synthase reconstituted proteoliposomes in a K + gradient (without an H + gradient) can only make ATP when a protonophore, FCCP, is provided, to enable the exchange of transported K + with a positive counterion, H + (i.e., in this case, together with the movement of K + through ATP synthase to make ATP, the counter-movement of an equal amount of H + constitutes a “K/H exchanger activity” and enables ATP synthesis) ( 13 see Fig. 2 therein). Importantly, without adding FCCP (to allow a H + leak pathway), there is no ATP synthesis in a K + gradient, so there must be no functional K/H exchange activity present inside ATP synthase such as is central to the proposed Nath mechanism. (2) The membrane potential (K + diffusion-potential) generated in F 1 F o -ATP synthase reconstituted proteoliposomes by a large K + gradient is extremely stable (>> minutes) unless FCCP is added to enable the counter-movement of an equal amount of charge as H + constituting a synthetic “K/H exchanger activity.” ( 13 see Fig. 1A, C therein). In short, all of this evidence definitively refutes Nath.

In conclusion, our experiments 13 , 14   most certainly did not validate the Nath “Two-ion Theory of Energy Coupling and ATP Synthesis” hypothesis 15 . On the other hand, the ability of mitochondria to have a K/H (Na/H) exchanger activity separate from the ATP synthase (as we propose 13 , 14 , 19 ) produces a situation whereby even small kinetic differences between those cation fluxes, which are physically different and (potentially) spatially/diffusively separate entities (e.g., the effects of those component processes having different rise- and relaxation-times to a driving step-function) can yield small but important, regulatory accumulations or reductions in matrix K + and Na + . This, in turn, controls changes in osmotic drive, matrix volume, 13 , 14 Ca 2+ levels and redox status, 21 with consequent actions on respiratory chain function, energy supply-demand matching and cardioprotection signaling. These are some of the important implications of our findings. Notably, while our principles, experimental findings and conclusions are fully consistent with Mitchell's chemiosmotic mechanism 13 , 14 , 19 , Nath's “torsional mechanism of energy transduction” and ATP synthesis is stated to be incompatible with Mitchell's chemiosmotic theory and cannot be explained by it. 15 , 20

We confidently feel that the theoretical subject matter referenced in Prof. Nath's Opinion Article, 15 while interesting, is not fundamentally or substantially relevant to, or consistent with, our novel body of experimental work. 13 , 14 , 19

ACKNOWLEDGEMENTS

This work was supported by the Intramural Research Program, National Institute on Aging, NIH.

Contributor Information

Magdalena Juhaszova, Laboratory of Cardiovascular Science, National Institute on Aging, NIH, Baltimore, MD 21224, USA.

Evgeny Kobrinsky, Laboratory of Cardiovascular Science, National Institute on Aging, NIH, Baltimore, MD 21224, USA.

Dmitry B Zorov, Laboratory of Cardiovascular Science, National Institute on Aging, NIH, Baltimore, MD 21224, USA. A.N. Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow 119992, Russia.

Miguel A Aon, Laboratory of Cardiovascular Science, National Institute on Aging, NIH, Baltimore, MD 21224, USA. Translational Gerontology Branch, National Institute on Aging, NIH, Baltimore, MD 21224, USA.

Sonia Cortassa, Laboratory of Cardiovascular Science, National Institute on Aging, NIH, Baltimore, MD 21224, USA.

Steven J Sollott, Laboratory of Cardiovascular Science, National Institute on Aging, NIH, Baltimore, MD 21224, USA.

Conflict of interest

The authors have declared that no conflict of interest exists.

• Biology Article

Chemiosmotic Hypothesis

In 1961, Peter Mitchell postulated the Chemiosmotic hypothesis. It explains the mechanism of ATP synthesis within chloroplast during photosynthesis. During the photochemical phase or light reaction, ATP and NADP are generated. These are the key components and used in the dark reaction for the production of the final product of photosynthesis i.e. sugar molecules. Let us see how this ATP and NADPH are generated during the light reaction.

As per the chemiosmotic hypothesis, ATP production is the outcome of the proton gradient established across the membrane of thylakoids. The required components for chemiosmosis are proton gradient, proton pump, and ATP synthase. ATP synthase is an enzyme aiding in bringing about ATP synthesis. The enzyme has two portions -F 0 and F 1 . F0 is a transmembrane channel while configuration changes in F1 activate the enzymes . They phosphorylate ADP. One of the driving factors of ATP Synthase is the protein gradient that is developed across a membrane. 

In plants, during the light reaction, photosystems help chlorophyll to absorb light. As a result, hydrolysis (splitting of water) takes place and releases electrons and protons. The electrons get excited to higher energy level and are transported by the electron transport system while protons (hydrogen ions) from the stroma starts to accumulate inside the membrane. This creates a proton gradient. Some protons are used by photosystem I for reduction   to NADPH. When the proton gradient is collapsed, it releases energy and protons are carried out back to stroma via F 0 , the transmembrane channel of ATP synthase. This released energy causes changes in F­ 1 configuration and triggers the ATP synthase to convert ADP to ATP.

The Chemiosmotic Theory

According to this theory, molecules like glucose are metabolized to develop acetyl CoA in the form of an intermediate that is energy-rich. The proper oxidation of acetyl CoA occurs in the mitochondrial matrix and is combined to the reduced form of a carrier molecule namely FAD and NAD. The carriers then supply electrons to the transport chain of the electron in the inner membrane of mitochondria , which further supply them to different other proteins present in the ETC. The energy present in the electrons is basically used to pump out protons from the matrix in the inner mitochondrial membrane. It is used for energy storage in the form of a transmembrane electrochemical gradient.

The protons return to the inner membrane by the ATP enzyme synthase. The proton-flow travels into the matrix of mitochondria through ATP synthase, which gets a good amount of energy for the ADP to integrate with inorganic phosphate to produce ATP.

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• Biotechnology
• Biochemistry
• Microbiology
• Cell Biology
• Cell Signaling
• Diversity in Life Form
• Molecular Biology
• CBSE Class 11 Biology Notes

Chapter 1: The Living World

• Diversity In The Living World
• Binomial Nomenclature - Definition, Rules, Classification and Examples
• Taxonomic Hierarchy In Biological Classification
• Genus and Family
• Difference Between Phylum and Class
• Taxonomical Aids
• Botanical Gardens
• The Living World - Introduction, Classification, Characteristics, FAQs

Chapter 2: Biological Classification

• Biological Classification
• Kingdom Monera - Definition, Classification, Characteristics, Examples
• Archaebacteria
• Eubacteria - Structure, Characteristics, Classification, and Types
• Kingdom Protista
• Chrysophytes
• Dinoflagellates
• Slime Moulds
• Protozoans - Structure, Classification, Characteristics, Examples
• Kingdom Fungi
• Phycomycetes
• Ascomycetes - Introduction, Characteristics, Reproduction, Importance
• Basidiomycetes
• Deuteromycetes
• Kingdom Plantae - Class 11 Biology
• Kingdom Animalia - Definition, Classification, Characteristics

Chapter 3: Plant Kingdom

• What is Plant Kingdom?
• Algae - Definition, Characteristics, Types and Examples
• Chlorophyceae
• Phaeophyceae - Overview, Characteristics, Importance, Examples
• Rhodophyceae
• Bryophytes | Class 11 Biology
• Pteridophytes
• Gymnosperms - Definition, Characteristics, Uses and Examples
• Angiosperms
• Difference between Angiosperms and Gymnosperms

Chapter 4: Animal Kingdom

• Animal Kingdom
• Classification of Animal Kingdom
• Levels of Organization in Animals
• Symmetry in Animals - Definition, Types and Importance
• Diploblastic And Triploblastic Organization
• Classification of Animals
• Phylum Porifera
• Phylum Coelenterata | Class 11 Biology
• Phylum Ctenophora
• Platyhelminthes
• Phylum Aschelminthes
• Phylum Annelida
• Phylum Arthropoda
• Phylum Mollusca
• Phylum Echinodermata
• Phylum Hemichordata
• Phylum Chordata

Chapter 5: Morphology of Flowering Plants

• Morphology of Flowering Plants - Flower, Fruit, Seed, Roots
• Root System in Plants - Types and Functions of Root
• Stem - Characteristics and Functions
• Inflorescence
• Morphology of Flower - Definition, Structure, Parts, Examples
• Parts of a Flower and their Functions
• Androecium - Definition, Components, Structure, Functions
• Gynoecium - Definition, Concept, Parts, Functions
• What is a Fruit?
• Structure Of A Dicotyledonous Seed
• Structure Of Monocotyledonous Seed
• Semi Technical Description of a Flowering Plant - Class 11 Biology
• Fabaceae - Overview, Characteristics, Classification, Importance
• Solanaceae - Characteristics, Importance, Examples

Chapter 6: Anatomy of Flowering Plants

• Meristematic Tissues | Class 11 Biology
• Permanent Tissues
• Why are Xylem and Phloem called Complex Tissues?
• Epidermal Tissue System: Its Functions and Tissue in Plant
• Difference between Dicot and Monocot Root
• Monocot and Dicot Stems - Definition, Structure, Characteristics, Examples
• Describe the Internal Structure of a Dorsiventral Leaf
• Isobilateral (Monocotyledonous) Leaf - Definition, Features, Structure, Examples
• Secondary Growth
• Cork Cambium

Chapter 7: Structural Organization In Animals

• NCERT Notes of Class 11 Biology Chapter 7 Structural Organisation in Animals
• Structural Organization in Animals
• Epithelial Tissue - Introduction, Characteristics, Types, Importance
• Connective Tissue - Definition, Functions, Types, Examples
• Organ System
• Morphology of Earthworm - Definition, Classification, Diagram and Examples
• Earthworm Anatomy
• Morphology of Cockroach
• Anatomy of Cockroach
• Morphology and Anatomy of Frogs

Chapter 8: Cell-The Unit of Life

• Cell the Unit of Life Class 11 Notes CBSE Biology Chapter 8
• Prokaryotic Cells
• Cell Envelope - Definition, Classification, Types, Functions
• Ribosomes and Inclusion Bodies
• Eukaryotic Cells
• Cell Membrane
• Endomembrane System - Overview, Structure, and Functions
• Mitochondria
• Golgi Apparatus
• Plastids - Definition, Classification, Structure, Functions
• Ribosomes | Class 11 Biology
• Cytoskeleton - Definition, Structure, Components, Functions
• Cilia And Flagella - Definition, Structure, Functions and FAQs
• What is Nucleus? | Class 11 Biology

Chapter 9: Biomolecules

• Biomolecules - Definition, Structure, Classification, Examples
• How To Analyze Chemical Composition?
• What are Metabolites? - Primary and Secondary
• Biomacromolecules - Definition, Types, Functions, Significance
• Proteins - Definition, Structure, Significance, Examples
• Polysaccharides
• Nucleic Acid - Definition, Function, Structure, and Types
• Protein Structure - Primary, Secondary, Tertiary, Quaternary
• Metabolic Basis For Living | CBSE Class 11 Biology Chapter 9
• Enzymes - Definition, Structure, Classification, Examples
• How Do Enzymes Bring About Such High Rates Of Chemical Conversions?
• Nature of Enzyme Action
• Mechanism of Enzymes Action
• Factors Affecting Enzyme Activity
• Cofactors - Definition, Structure, Types, Examples

Chapter 10: Cell Cycle and Cell Division

• Cell Division: Mitosis & Meiosis, Different Phases of Cell Cycle
• Mitosis - Overview, Phases, & Significance Class Notes
• Mitosis Cell Division: Definition, Stages and Diagram
• Meiosis - Definition, Stages, Function and Purpose

Chapter 11: Photosynthesis in Higher Plants

• Where Does Photosynthesis Take Place?
• Photosynthetic Pigments
• What Is Light Dependent Reaction?
• Electron Transport System (ETS) And Oxidative Phosphorylation
• Cyclic and Non-cyclic Photo-phosphorylation

Chemiosmotic Hypothesis

• Where are the Atp and Nadph Used?
• Calvin Cycle Notes Class 11
• C4 Cycle of Photosynthesis
• Photorespiration - Definition, Diagram, Process, Significance
• Factors Affecting Photosynthesis

Chapter 12: Respiration in Plants

• Respiration In Plants Class 11 Notes
• Do Plants Breathe?
• What is Glycolysis ?
• Fermentation: Meaning, Process, Types and Importance
• Aerobic Respiration
• Tricarboxylic Acid Cycle - Overview, Stages, Roles, Significance
• Amphibolic Pathway
• Respiratory Quotient

Chapter 13: Plant-Growth and Development

• Plant Growth - Definition, Types, Factors Affecting, Examples
• Phases of Growth In Plants - Growth Rates
• Differentiation, Dedifferentiation and Redifferentiation in Plant Growth
• Plant Growth and Development
• Physiological Effects Of Plant Growth Regulators
• Vernalization
• Seed Dormancy

Chapter 14: Breathing and Exchange of Gases

• NCERT Notes Class 11 Biology Chapter 14 - Breathing and Exchange of Gases
• Human Respiratory System
• Inspiration and Expiration
• Lung Volumes And Capacities
• Transport of Oxygen
• Transport of Carbon Dioxide in the Blood
• Regulation Of Respiration
• Respiratory System Disorders - Definition, Causes, Types, Symptoms

Chapter 15: Body Fluids and Circulation

• Body Fluids and Circulation
• Plasma And Formed Elements
• Blood Groups - ABO Blood Group and Rh Group System
• Blood Coagulation
• Lymphatic System
• Circulatory Pathways - Anatomy and Functions
• Cardiac Cycle | Class 11 Biology
• Electrocardiogram (ECG)
• Double Circulation
• Regulation of Cardiac Activity
• Disorders of the Circulatory System

Chapter 16: Excretory Products and their Elimination

• Excretory Products and their Elimination
• Human Excretory System
• Mechanism of Urine Formation
• Functions of Renal Tubules
• Mechanism Of Concentration Of The Filtrate
• Micturition - The process of Urination
• Role of Other Organs In Excretion
• Disorders Of The Excretory System

Chapter 17: Locomotion and Movement

• NCERT Notes for Class 11 Biology Chapter 17: Locomotion and Movement
• Locomotion And Movement
• Muscular Tissue
• Muscular and Skeletal Disorders

Chapter 18: Neural Control and Coordination

• Class 11 Biology NCERT Notes Chapter 18 - Neural Control and Coordination
• Neural Tissue
• Generation And Conduction Of Nerve Impulse - NCERT Notes
• Transmission of Nerve Impulses
• Central Nervous System
• Reflex Action
• Anatomy and Physiology of Human Eye
• Anatomy and Physiology of Human Ear

Chapter 19: Chemical Coordination and Integration

• NCERT Notes for Class 11 Biology Chapter 19: Chemical Coordination and Integration
• Endocrine Glands
• Hypothalamus - Function, Hormones and Disorder
• Pituitary Gland
• Pineal Gland
• Thyroid Gland - Anatomy, Function and Clinical Aspects
• Parathyroid Gland - Functions and Disorders
• What is Thymus Gland?
• Adrenal Gland
• Testes - Anatomy and Functions
• Ovary - Female Reproductive System
• Hormones of Heart, Kidney And Gastrointestinal

NCERT Solution

• NCERT Solutions Class 11- Biology
• NCERT Solutions Class 11 Biology Chapter 1 Living World
• NCERT Solutions for Class 11 Biology Chapter 2 - Biological Classification
• NCERT Solutions for Class 11 Biology Chapter 3 - Plant Kingdom
• NCERT Solutions Class 11 Biology Chapter 4 Animal Kingdom
• NCERT Solutions for Class 11 Biology Chapter 5: Morphology of Flowering Plants
• NCERT Solutions for Class 11 Biology Chapter 7 Structural Organisation in Animals
• NCERT Solutions for Class 11 Biology Chapter 8 Cell The Unit of Life
• NCERT Solutions for Class 11 Biology Chapter 9 - Biomolecules
• NCERT Solutions Class 11 Biology Chapter 10 Cell Cycle and Cell Division
• NCERT Solutions Chapter 11 of Class 11 Biology - Photosynthesis in Higher Plants
• NCERT Solutions for Class 11 Biology Chapter 12 Respiration in Plants
• NCERT Solutions for Class 11 Biology Chapter 13 - Plant Growth and Development
• NCERT Solutions of Class 11 Chapter 14 Breathing and Exchange of Gases
• NCERT Solutions Class 11 Biology Chapter 15 Body Fluids and Circulation
• NCERT Solutions for Class 11 Biology Chapter 16 Excretory Products and Their Elimination
• NCERT Solutions for Class 11 Biology Chapter 17 Locomotion and Movement
• NCERT Solutions Class 11 Biology Chapter 18: Neural Control and Coordination
• NCERT Solutions for Class 11 Biology Chapter 19 - Chemical Coordination and Integration

The process through which a plant transforms light energy into chemical energy to produce food is known as photosynthesis . In the presence of chlorophyll, plants use water, carbon dioxide, and sunlight to make food or energy in the form of sugar, and as a by-product, they release oxygen. This suggests that light energy is used as a catalyst in chemical synthesis or reaction. Certain bacteria and prokaryotes also use this to prepare their food; it is not just for green plants. The chloroplast, a crucial organelle in green plants and algae that contains the pigment chlorophyll , is where synthesis occurs. The leaves, flowers, stems, sepals, and plastids all contain chlorophyll.

Chemiosmosis

Chemiosmosis refers to the process by which ions move over a semi-permeable membrane , such as the membrane within mitochondria . Molecules containing a net electric charge are called ions. Examples include the specialized usage of Na + , Cl – , and H + in chemiosmosis to generate energy. During chemiosmosis, ions move along an electrochemical gradient or a gradient of electrochemical potential (a form of potential energy). Chemiosmosis is a type of diffusion, and it causes ions to move from areas of high concentration to areas of low concentration across a membrane. Ions also move to balance the electric charge across a membrane.

The biological process by which ATP synthase produces ATP molecules is known as this process. An explanation of how energy molecules (ATP: Adenosine triphosphate) are produced during photosynthesis is provided by the Chemiosmotic theory, which was put forth by a British biochemist by the name of Peter Dennis Mitchell in 1961. Because of the improved understanding, it gave off the entire process of ATP generation within chloroplasts, his study was awarded the Nobel Prize. Nicotinamide adenine dinucleotide phosphate, often known as NADP or NADP + , is created with ATP during the light reaction or photochemical phase. These constitute the essential components of photosynthesis . They are used to produce sugar molecules throughout the dark reaction or Calvin cycle .

Chemiosmotic Hypothesis Process

The proton gradient that exists across the thylakoid membrane is what causes the ATP- Adenosine Triphosphates to be created in this process. The proton gradient, ATP synthase, and proton pump are important elements required for the chemiosmosis process. ATP synthase is the name of the enzyme that is necessary for the synthesis of ATP molecules. Two subunits designated F0 and F1, make up the enzyme ATP synthase. To move protons across the membrane, the F0 subunit is necessary. This alters the F1 subunit’s conformation, which activates enzymes . By adding a phosphate group to ADP, the enzyme phosphorylates it, turning it into ATP. Across the membrane, there is a proton gradient, which acts as ATP synthase’s main propulsion source.

Chlorophyll absorbs light with the aid of photosystems during the light response stage of photosynthesis. As a result, the water molecules split, releasing protons and electrons in the process. This is known as hydrolysis. The electron transport system carries the liberated electrons as they become energized and proceed to a higher energy level.

In the meantime, the stroma’s released protons start assembling inside the membrane. As a result, a proton gradient is produced, which is a by-product of the electron transport chain. Photosystem I use the few remaining protons to convert NADP+ to NADPH using electrons from the photolysis of water. The proton gradient eventually collapses, releasing energy and protons that are then transported back to the stroma by ATP synthase F0. ADP is converted to ATP by the ATP synthase when the F­1 conformation is altered by the resulting energy.

FAQs on Chemiosmosis Hypothesis

Question 1: What ion undergoes chemiosmosis as part of the production of ATP?

The process by which protons (H+) move down a proton gradient during cellular respiration is known as chemiosmosis. Adenosine diphosphate (ADP) and a phosphate group are joined by the enzyme ATP synthase as a result, creating ATP.

Question 2: What molecule is produced by chemiosmosis, and who proposed the chemiosmotic hypothesis?

ATP synthase’s role in the creation of ATP molecules naturally. The Chemiosmotic theory describes how energy molecules (ATP: adenosine triphosphate) are produced during photosynthesis and was put out by a British biochemist by the name of Peter Dennis Mitchell in 1961. He received the Nobel Prize for his contribution to the discovery of this idea.

Question 3: What cell organelle is capable of chemiosmosis?

During cellular respiration in the mitochondria and photosynthesis in the chloroplasts, chemiosmosis takes place. These two procedures both produce ATP.

Question 4: Exactly what does the chemiosmotic theory entail?

A British biochemist by the name of Peter Dennis Mitchell first put forth the chemiosmotic idea in 1961. The chemiosmotic theory proposes that energy is released through a series of oxidation-reduction events when electrons are transported through an electron transport system. This energy can be used by specific chain carriers to move hydrogen ions (H+ or protons) through a membrane.

Question 5: Where does chemiosmosis take place in the mitochondria?

Within the inner mitochondrial membrane. The efficiency of chemiosmosis is determined by the electron transport chain (ETC), which is situated in the inner mitochondrial membrane. A group of proteins known as the ETC cooperate and pass electrons back and forth like a hot potato. As ion pumps for hydrogen ions, the ETC contains three proteins.

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Chemiosmotic Hypothesis | Capsule course of Botany for NEET PDF Download

OXIDATIVE PHOSPHORYLATION

Chemiosmotic theory / Coupling theory :

During ETC of respiration CoQ & FMN can releases H+ ions in perimitochondrial space and leads to differenctial H + ion concetration across inner mitochondrial membrane. This differential H+ ion concentration across inner mitochondrial membrane leads to creation of proton gradiant (PH gradient) and Electrical potential (diffrence of charge). Both are collectively known as Proton motive force (PMF).

PMF do not allow stay of H + ions in Peri mitochondrial space (PMS) so they return towards the matrix through F0 particles selectively.

The passage of 3H + ions activate ATP synthase and gives rise to 1ATP from ADP & Pi.

Some physiologist believe that passage of 2H + ions through F0 particle or coupling factor or proton channel leads to synthesis of 1 ATP.

Bioenergetics of respiration – (1 mol. of glucose)

EMP-Pathway

(i) ATP formed at substrate level phosphorylation ⇒ 4.ATP

(ii) ATP produced via ETS (2NADH 2 ) ⇒ 6 ATP

(iii) ATP consumed in glycolysis ⇒ 2 ATP

10 ATP – 2 ATP = 8 ATP

Gross – Expenditure = Net or Total gain

Direct Gain = 2 ATP

(2) Link reaction or Gateway reaction –

2NADH 2 = 6 ATP (via ETS)

Kreb's Cycle – (i) ATP produced at substrate level phosphorylation = 2 GTP/2ATP

1 Sucrose = 80 ATP

 1 Fructose 1,6–Bisphosphate = 40 ATP

 1 Pyurvic acid = 15 ATP

 1 Acetyl Co-A or 1 TCA cycle = 12 ATP

PPP is also called as Warburg - Dickens pathway/HMP shunt/Phosphogluconolactone pathway/ Carbohydrate degradation without mitochondria/Cytosolic oxidative decarboxylation/Horecker -Racker Pathway

Glycolysis & TCA cycle is the main route of carbohydrate oxidation, but Warburg & Dickens (1935) discovered an alternative route of carbohydrate break down, existing in plants, some animal tissues (Mammary glands, adipose, liver & microbes).

HMP/PPP occurs when

(i) NADPH 2 requirement of cell increases during biosynthetic processes.

(ii) When EMP pathway blocked by iodoacetate, fluorides, arsenates.

(iii) When mitochondria is busy in other pathways.

Most of the intermediates are similar to Calvin cycle, but PPP is amphibolic and oxidative process.

One ATP is utilised in phosphorylation of glucose, so net gain equals to 35 ATP. (12 NADPH2) Significance of HMP shunt :-

(1) An intermediate erythrose-P (4C) of this pathway is precursor of shikimic acid, which goes to synthesis of aromatic compounds and amino acids.

(2) This cycle provides pentose sugars Ribose-p for synthesis of nucleotides, nucleosides, ATP and GTP.

(3) A five carbon intermediate Ribulose-5-phosphate may used as CO 2 acceptor in green cells.

(4) This pathway produces reducing power NADPH2 for the various biosynthetic pathways, other than photosynthesis like fats synthesis, starch synthesis, hormone synthesis and chlorophyll synthesis.

(5) Intermediates like PGAL and fructose-6-phosphate of this pathway may link with glycolytic reactions. b-Oxidation of Fatty acids

b-oxidation takes place mainly in perimitochondrial space but also in glyoxisome, peroxisome, cytosol.

Liberation of 2C segments from the fatty acid mol. in the form of acetyl Co-A is known as b-oxidation. These acetyl-CoA provides ATP after oxidation in krebs cycle.

Acetyl CoA is oxidised in TCA cycle to CO 2 & H 2 O with the production of 12 ATP molecules.

Glyoxylate Cycle

Discovered by Kornberg  &  Krebs ,during  germination of fatty seeds .

This cycle converts fats into sugars so it is an example of gluconeogenesis  in plants.

Glyoxylate cycle occurs in glyoxysomes, cytosol,  & mitochondria.

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