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11.1: Electron Transport Chains - Biology

11.1: Electron Transport Chains - Biology


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Electron Transport Chains

An electron transport chain, or ETC, is composed of a group of protein complexes in and around a membrane that help energetically couple a series of exergonic/spontaneous red/ox reactions to the endergonic pumping of protons across the membrane to generate an electrochemical gradient. This electrochemical gradient creates a free energy potential that is termed a proton motive force whose energetically "downhill" exergonic flow can later be coupled to a variety of cellular processes.

ETC overview

Step 1: Electrons enter the ETC from an electron donor, such as NADH or FADH2, which are generated during a variety of catabolic reactions, including those associated glucose oxidation. Depending on the number and types of electron carriers of the ETC being used by an organism, electrons can enter at a variety of places in the electron transport chain. Entry of electrons at a specific "spot" in the ETC depends upon the respective reduction potentials of the electron donors and acceptors.


Step 2: After the first red/ox reaction, the initial electron donor will become oxidized and the electron acceptor will become reduced. The difference in red/ox potential between the electron acceptor and donor is related to ΔG by the relationship ΔG = -nFΔE, where n = the number of electrons transferred and F = Faraday's constant. The larger a positive ΔE, the more exergonic the red/ox reaction is.


Step 3: If sufficient energy is transferred during an exergonic red/ox step, the electron carrier may couple this negative change in free energy to the endergonic process of transporting a proton from one side of the membrane to the other.


Step 4: After usually multiple red/ox transfers, the electron is delivered to a molecule known as the terminal electron acceptor. In the case of humans, the terminal electron acceptor is oxygen. However, there are many, many, many, other possible electron acceptors in nature; see below.

Note: possible discussion

Electrons entering the ETC do not have to come from NADH or FADH2. Many other compounds can serve as electron donors; the only requirements are (1) that there exists an enzyme that can oxidize the electron donor and then reduce another compound, and (2) that the ∆E0' is positive (e.g., ΔG<0). Even a small amounts of free energy transfers can add up. For example, there are bacteria that use H2 as an electron donor. This is not too difficult to believe because the half reaction 2H+ + 2 e-/H2 has a reduction potential (E0') of -0.42 V. If these electrons are eventually delivered to oxygen, then the ΔE0' of the reaction is 1.24 V, which corresponds to a large negative ΔG (-ΔG). Alternatively, there are some bacteria that can oxidize iron, Fe2+ at pH 7 to Fe3+ with a reduction potential (E0') of + 0.2 V. These bacteria use oxygen as their terminal electron acceptor, and, in this case, the ΔE0' of the reaction is approximately 0.62 V. This still produces a -ΔG. The bottom line is that, depending on the electron donor and acceptor that the organism uses, a little or a lot of energy can be transferred and used by the cell per electrons donated to the electron transport chain.

What are the complexes of the ETC?

ETCs are made up of a series (at least one) of membrane-associated red/ox proteins or (some are integral) protein complexes (complex = more than one protein arranged in a quaternary structure) that move electrons from a donor source, such as NADH, to a final terminal electron acceptor, such as oxygen. This particular donor/terminal acceptor pair is the primary one used in human mitochondria. Each electron transfer in the ETC requires a reduced substrate as an electron donor and an oxidized substrate as the electron acceptor. In most cases, the electron acceptor is a member of the enzyme complex itsef. Once the complex is reduced, the complex can serve as an electron donor for the next reaction.

How do ETC complexes transfer electrons?

As previously mentioned, the ETC is composed of a series of protein complexes that undergo a series of linked red/ox reactions. These complexes are in fact multi-protein enzyme complexes referred to as oxidoreductases or simply, reductases. The one exception to this naming convention is the terminal complex in aerobic respiration that uses molecular oxygen as the terminal electron acceptor. That enzyme complex is referred to as an oxidase. Red/ox reactions in these complexes are typically carried out by a non-protein moiety called a prosthetic group. The prosthetic groups are directly involved in the red/ox reactions being catalyzed by their associated oxidoreductases. In general, these prosthetic groups can be divided into two general types: those that carry both electrons and protons and those that only carry electrons.

Note

This use of prosthetic groups by members of ETC is true for all of the electron carriers with the exception of quinones, which are a class of lipids that can directly be reduced or oxidized by the oxidoreductases. Both the Quinone(red) and the Quinone(ox) forms of these lipids are soluble within the membrane and can move from complex to complex to shuttle electrons.

The electron and proton carriers

  • Flavoproteins (Fp), these proteins contain an organic prosthetic group called a flavin, which is the actual moiety that undergoes the oxidation/reduction reaction. FADH2 is an example of an Fp.
  • Quinones are a family of lipids, which means they are soluble within the membrane.
  • It should also be noted that NADH and NADPH are considered electron (2e-) and proton (2 H+) carriers.

Electron carriers

  • Cytochromes are proteins that contain a heme prosthetic group. The heme is capable of carrying a single electron.
  • Iron-Sulfur proteins contain a nonheme iron-sulfur cluster that can carry an electron. The prosthetic group is often abbreviated as Fe-S

Aerobic versus anaerobic respiration

We humans use oxygen as the terminal electron acceptor for the ETCs in our cells. This is also the case for many of the organisms we intentionally and frequently interact with (e.g. our classmates, pets, food animals, etc). We breath in oxygen; our cells take it up and transport it into the mitochondria where it is used as the final acceptor of electrons from our electron transport chains. That process - because oxygen is used as the terminal electron acceptor - is called aerobic respiration.

While we may use oxygen as the terminal electron acceptor for our respiratory chains, this is not the only mode of respiration on the planet. Indeed, the more general processes of respiration evolved at a time when oxygen was not a major component of the atmosphere. As a consequence, many organisms can use a variety of compounds including nitrate (NO3-), nitrite (NO2-), even iron (Fe3+) as terminal electron acceptors. When oxygen is NOT the terminal electron acceptor, the process is referred to as anaerobic respiration. Therefore, respiration or oxidative phosphorylation does not require oxygen at all; it simply requires a compound with a high enough reduction potential to act as a terminal electron acceptor, accepting electrons from one of the complexes within the ETC.

The ability of some organisms to vary their terminal electron acceptor provides metabolic flexibility and can ensure better survival if any given terminal acceptor is in limited supply. Think about this: in the absence of oxygen, we die; but other organisms can use a different terminal electron acceptor when conditions change in order to survive.

A generic example: A simple, two-complex ETC

The figure below depicts a generic electron transport chain, composed of two integral membrane complexes; Complex I(ox) and Complex II(ox). A reduced electron donor, designated DH (such as NADH or FADH2) reduces Complex I(ox), giving rise to the oxidized form D (such as NAD+ or FAD+). Simultaneously, a prosthetic group within Complex I is now reduced (accepts the electrons). In this example, the red/ox reaction is exergonic and the free energy difference is coupled by the enzymes in Complex I to the endergonic translocation of a proton from one side of the membrane to the other. The net result is that one surface of the membrane becomes more negatively charged, due to an excess of hydroxyl ions (OH-), and the other side becomes positively charged due to an increase in protons on the other side. Complex I(red) can now reduce a mobile electron carrier Q, which will then move through the membrane and transfer the electron(s) to the prosthetic group of Complex II(red). Electrons pass from Complex I to Q then from Q to Complex II via thermodynamically spontaneous red/ox reactions, regenerating Complex I(ox), which can repeat the previous process. Complex II(red) then reduces A, the terminal electron acceptor to regenerate Complex II(ox) and create the reduced form of the terminal electron acceptor, AH. In this specific example, Complex II can also translocate a proton during the process. If A is molecular oxygen, AH represents water and the process would be considered to be a model of an aerobic ETC. By contrast, if A is nitrate, NO3-, then AH represents NO2- (nitrite) and this would be an example of an anaerobic ETC.

Figure 1. Generic 2 complex electron transport chain. In the figure, DH is the electron donor (donor reduced), and D is the donor oxidized. A is the oxidized terminal electron acceptor, and AH is the final product, the reduced form of the acceptor. As DH is oxidized to D, protons are translocated across the membrane, leaving an excess of hydroxyl ions (negatively charged) on one side of the membrane and protons (positively charged) on the other side of the membrane. The same reaction occurs in Complex II as the terminal electron acceptor is reduced to AH.

Attribution: Marc T. Facciotti (original work)

Exercise 1

Thought question

Based on the figure above, use an electron tower to figure out the difference in the electrical potential if (a) DH is NADH and A is O2, and (b) DH is NADH and A is NO3-. Which pairs of electron donor and terminal electron acceptor (a) or (b) "extract" the greatest amount of free energy?

Detailed look at aerobic respiration

The eukaryotic mitochondria has evolved a very efficient ETC. There are four complexes composed of proteins, labeled I through IV depicted in the figure below. The aggregation of these four complexes, together with associated mobile, accessory electron carriers, is called also called an electron transport chain. This type of electron transport chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes.

Figure 2. The electron transport chain is a series of electron transporters embedded in the inner mitochondrial membrane that shuttles electrons from NADH and FADH2 to molecular oxygen. In the process, protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water.

Complex I

To start, two electrons are carried to the first protein complex aboard NADH. This complex, labeled I in Figure 2, includes flavin mononucleotide (FMN) and iron-sulfur (Fe-S)-containing proteins. FMN, which is derived from vitamin B2, also called riboflavin, is one of several prosthetic groups or cofactors in the electron transport chain. Prosthetic groups are organic or inorganic, nonpeptide molecules bound to a protein that facilitate its function; prosthetic groups include coenzymes, which are the prosthetic groups of enzymes. The enzyme in Complex I is also called NADH dehydrogenase and is a very large protein containing 45 individual polypeptide chains. Complex I can pump four hydrogen ions across the membrane from the matrix into the intermembrane space thereby helping to generate and maintain a hydrogen ion gradient between the two compartments separated by the inner mitochondrial membrane.

Q and Complex II

Complex II directly receives FADH2, which does not pass through Complex I. The compound connecting the first and second complexes to the third is ubiquinone (Q). The Q molecule is lipid soluble and freely moves through the hydrophobic core of the membrane. Once it is reduced, (QH2), ubiquinone delivers its electrons to the next complex in the electron transport chain. Q receives the electrons derived from NADH from Complex I and the electrons derived from FADH2 from Complex II, succinate dehydrogenase. Since these electrons bypass and thus do not energize the proton pump in the first complex, fewer ATP molecules are made from the FADH2 electrons. As we will see in the following section, the number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane.

Complex III

The third complex is composed of cytochrome b, another Fe-S protein, Rieske center (2Fe-2S center), and cytochrome c proteins; this complex is also called cytochrome oxidoreductase. Cytochrome proteins have a prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. As a result, the iron ion at its core is reduced and oxidized as it passes the electrons, fluctuating between different oxidation states: Fe2+ (reduced) and Fe3+ (oxidized). The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, giving slightly different characteristics to each complex. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes (cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time).

Complex IV

The fourth complex is composed of cytochrome proteins c, a, and a3. This complex contains two heme groups (one in each of the two Cytochromes, a, and a3) and three copper ions (a pair of CuA and one CuB in Cytochrome a3). The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up two hydrogen ions from the surrounding medium to make water (H2O). The removal of the hydrogen ions from the system contributes to the ion gradient used in the process of chemiosmosis.

Chemiosmosis

In chemiosmosis, the free energy from the series of red/ox reactions just described is used to pump protons across the membrane. The uneven distribution of H+ ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient), owing to the proton's positive charge and their aggregation on one side of the membrane.

If the membrane were open to diffusion by protons, the ions would tend to diffuse back across into the matrix, driven by their electrochemical gradient. Ions, however, cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, protons in the intermembrane space can only traverse the inner mitochondrial membrane through an integral membrane protein called ATP synthase (depicted below). This complex protein acts as a tiny generator, turned by transfer of energy mediated by protons moving down their electrochemical gradient. The movement of this molecular machine (enzyme) serves to lower the activation energy of reaction and couples the exergonic transfer of energy associated with the movement of protons down their electrochemical gradient to the endergonic addition of a phosphate to ADP, forming ATP.

Figure 3. ATP synthase is a complex, molecular machine that uses a proton (H+) gradient to form ATP from ADP and inorganic phosphate (Pi).

Credit: modification of work by Klaus Hoffmeier

Note: possible discussion

Dinitrophenol (DNP) is a small chemical that serves to uncouple the flow of protons across the inner mitochondrial membrane to the ATP synthase, and thus the synthesis of ATP. DNP makes the membrane leaky to protons. It was used until 1938 as a weight-loss drug. What effect would you expect DNP to have on the difference in pH across both sides of the inner mitochondrial membrane? Why do you think this might be an effective weight-loss drug? Why might it be dangerous?

In healthy cells, chemiosmosis (depicted below) is used to generate 90 percent of the ATP made during aerobic glucose catabolism; it is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation. Recall that the production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation and that a similar process can occur in the membranes of bacterial and archaeal cells. The overall result of these reactions is the production of ATP from the energy of the electrons removed originally from a reduced organic molecule like glucose. In the aerobic example, these electrons ultimatel reduce oxygen and thereby create water.

Figure 4. In oxidative phosphorylation, the pH gradient formed by the electron transport chain is used by ATP synthase to form ATP in a Gram-bacteria.

Helpful link: How ATP is made from ATP synthase

Note: possible discussion

Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning occurs, would you expect the pH of the intermembrane space to increase or decrease? What effect would cyanide have on ATP synthesis?


OCIAD1 Controls Electron Transport Chain Complex I Activity to Regulate Energy Metabolism in Human Pluripotent Stem Cells

Pluripotent stem cells (PSCs) derive energy predominantly from glycolysis and not the energy-efficient oxidative phosphorylation (OXPHOS). Differentiation is initiated with energy metabolic shift from glycolysis to OXPHOS. We investigated the role of mitochondrial energy metabolism in human PSCs using molecular, biochemical, genetic, and pharmacological approaches. We show that the carcinoma protein OCIAD1 interacts with and regulates mitochondrial complex I activity. Energy metabolic assays on live pluripotent cells showed that OCIAD1-depleted cells have increased OXPHOS and may be poised for differentiation. OCIAD1 maintains human embryonic stem cells, and its depletion by CRISPR/Cas9-mediated knockout leads to rapid and increased differentiation upon induction, whereas OCIAD1 overexpression has the opposite effect. Pharmacological alteration of complex I activity was able to rescue the defects of OCIAD1 modulation. Thus, hPSCs can exist in energy metabolic substates. OCIAD1 provides a target to screen for additional modulators of mitochondrial activity to promote transient multipotent precursor expansion or enhance differentiation.

Keywords: early mesodermal progenitors (EMPs) energy metabolic sub-states of pluripotency human embryonic stem cells (hESCs) idebenone mitochondrial complex I mitochondrial complex activity mitochondrial morphology in live cells ovarian carcinoma immunoreactive antigen domain containing-1 (OCIAD1) oxidative phosphorylation (OXPHOS) transient multipotent precursor expansion.

Copyright © 2018 The Authors. Published by Elsevier Inc. All rights reserved.


Photosynthesis – Electron Transport Chain and Calvin Cycle

Light Dependent Reactions , or Electron Transport Chain
– Occurs in chloroplasts’ thylakoids, only operates under exposure to sunlight
– Thylakoids are flat, thus aiming for maximum surface area in order to capture most sunlight
* During Electron Transport Chain, the aim is the create ATP

Procedures:
1.1. Sunlight energy (photons) hits the reaction center of the chlorophyll Photosystem II, it excites electron and thus it moves to electron-carrier protein, while Water-Splitting Enzyme splits the water (2 H2O -> 4H+ + O2 + 4e-), using the electrons from the split water to replace the excited electron in Photosystem II the oxygen diffuses out of cell
2. Moving along the electron-carrier protein, it reaches the first proton pump and releases energy to pump a hydrogen ion into the thylakoid, then the electron moves into another electron-carrier protein
3. Here, the electron from first Photosystem II replaces the electron that Photosystem I releases after hit by photons, to which Photosystem I’s electron moves into another electron-carrier protein and bonds to NADP and a hydrogen ion in NADP Reductase enzyme.
NADP + H+ + e- -> NADPH, energy rich molecule
1.2. Meanwhile NADPH’s line of reactions occurs, the hydrogen ions from splitting water and from proton pump will stack up inside the thylakoid, thus building potential energy inside and thus, utilizing the potential energy, ATP Synthase enzyme allows hydrogen ions to exist while using their energy to combine ADP + P -> ATP

H+ concentration change:
– Increases inside thylakoid with Photosystem II & proton pump
– Decreases outside thylakoid with proton pump and NADP Reductase (it uses H+ from outside)

Light Independent Reactions , or Calvin Cycle
– Occurs in chloroplasts’ storma
– Series of reactions to take energy from ATP and NADPH, transfer them into sugar, carbohydrates, etc.

Procedures (counting with 3 RuBp)
1. Rubisco (enzyme) fixes 3 CO2’s carbon onto 3 RuBp (5 carbon molecule) to form an unstable 6-carbon molecule
2. Then 6-carbon molecules (3 in quantity) breaks down into 6 G3P (3 carbon molecule) while 6 ATP and 6 NADPH are used on G3P (now has 1 phosphate group attached) here 1 G3P leaves the cycle
3. The rest (5) G3P gets recycled to make 3 RuBp using 3 ATP
2 G3P (modified by ATP and NADPH) are required to make one glucose (6 carbon) molecule

Total of 9 ATP and 6 NADPH used for one cycle starting with 3 RuBp, ADPs and NADPs goes back to Light Dependent Reaction


Formation of Reactive Oxygen Species and Cellular Damage

Reactive oxygen species (ROS) are molecules containing an oxygen atom with an unpaired electron in its outer shell. As ROS are formed, they become very unstable due to the unpaired electron now residing in the outermost shell. The unstable forms of oxygen are sometimes called free radicals.

How do ROS actually get generated in cells? One way is via cellular respiration driven by the electron transport chain in the mitochondria. The electron transport chain is responsible for generating ATP, the main source of energy for a cell to function. A key molecule that helps “jump start” the electron transport chain, is NADH (or nicotinamide adenine dinucleotide), which serves as the electron donor (i.e., the H in the NADH). NADH is often referred to as a “coenzyme”, even though it is not an enzyme (a protein).

NADH is present in all cells–it is generated by many biochemical reactions. One way that NADH gets generated in large quantities is when alcohol is metabolized (or oxidized) to form acetaldehyde and then to acetic acid. During the metabolism of alcohol, the enzyme alcohol dehydrogenase (ADH) and NAD + convert alcohol to acetylaldehyde, generating NADH. A second enzyme, aldehyde dehydrogenase (ALDH) and NAD + convert acetaldehyde to acetic acid, generating even more NADH. In these reactions, the coenzyme NAD + is reduced to NADH (and alcohol and acetaldehyde are oxidized).

Review the oxidation of alcohol by alcohol dehydrogenase (ADH)

To learn more about the oxidation of alcohol by ADH, you can participate in a virtual reality game called “DiVE into Alcohol” at www.rise.duke.edu/dive-alcohol.

Now there is plenty of NADH available to “jump start” mitochondrial respiration. NADH moves from the cytosol into the mitochondria where it donates an electron to the electron transport chain. The electron transport chain consists of a group of proteins (and some lipids) that work together to pass electrons “down the line”. Finally in the presence of oxygen, ATP is formed, providing energy for many cellular functions.

However, some electrons can “escape” the electron transport chain and combine with oxygen to form a very unstable form of oxygen called a superoxide radical (O2•-). The superoxide radical is one of the reactive oxygen species (ROS).

The superoxide radical is a type of free radical. Free radicals have a lone electron in their outer electron orbital and they are very reactive molecules because they tend to donate single electrons (e-) or steal e- from other molecules. Free radicals can be destructive to cellular components. Free radicals often have a • shown to indicate the lone e-.

Our cells have ways to protect themselves from the damaging effects of these reactive molecules. For example, our cells are able to maintain low levels of the superoxide radicals with the help of the enzyme superoxide dismutase (SOD). SOD helps reduce superoxide to form hydrogen peroxide (H2O2), which is then converted (detoxified) by the enzyme catalase to water and O2.

However, sometimes the levels of superoxide rise, for example after alcohol exposure (which generates a lot of NADH). Thus, more hydrogen peroxide is formed and can’t be detoxified by the limited amount of catalase. Instead hydrogen peroxide becomes reduced by iron (Fe 2+ ) (normally present in cells), which donates an electron to produce the hydroxyl radical (•OH), a very nasty molecule. It is extremely reactive, and it’s a great oxidizing agent. The hydroxyl radical oxidizes cellular components such as lipids, proteins, and DNA by literally stealing an e- (associated with an H atom) from them, damaging cells.

Figure: The metabolism (i.e., oxidation) of alcohol produces NADH, which acts as an electron donor for the electron transport chain (molecules designated with roman numerals). Electrons (e-) that “leak out” of the electron transport chain (stars at I and III) combine with oxygen to produce superoxide radicals (O2•-). Through a series of reactions the superoxide radicals generate hydroxyl (OH•) radicals. Oxygen radicals are circled in red.


The Electron Transport Chain

This activity provides students an interactive demonstration of the electron transport chain and chemiosmosis during aerobic respiration. Students use simple, everyday objects as hydrogen ions and electrons and play the roles of the various proteins embedded in the inner mitochondrial membrane to show how this specific process in cellular respiration produces ATP. The activity works best as a supplement after you have already discussed the electron transport chain in lecture but can be used prior to instruction to help students visualize the processes that occur. This demonstration was designed for general college biology for majors at a community college, but it could be used in any introductory college-level or advanced placement biology course.

Journal

The American Biology Teacher &ndash University of California Press

Published: Sep 1, 2014

Keywords: Key Words: Cellular respiration chemiosmosis electron transport chain metabolism ATP NADH oxidation reduction mitochondrion general biology


Glycolysis, Krebs Cycle, and Electron Transport System - PowerPoint PPT Presentation

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What is the electron transport chain and what is its function?

The electron transport chain is a series of four protein complexes, along with accessory electron carriers, embedded in the inner mitochondrial membrane of mitochondria. The enzyme ATP synthase is closely associated with the electron transport chain. There are multiple copies of the electron transport chain found in the inner mitochondrial membrane of every mitochondrion. Electrons from electron carriers that have been reduced in either glycolysis, the link reaction or the Krebs Cycle are donated to either Complex I or Complex II.The function of the electron transport chain is to ultimately produce adenosine triphosphate (ATP) which is the source of energy for the majority of cellular processes. The production of ATP is driven by the generation of a H+ ion gradient between the inter-membrane space (between the inner and outer mitochondrial matrix) and the mitochondrial matrix. The H+ gradient is formed as each successive component of the electron transport chain has a greater affinity for electrons. This means energy is released as electrons are travel through the electron transport chain. This energy is then used by the complexes to pump H+ into the inter-membrane space. Subsequently, the H+ ions reenter the matrix through a pore in ATP synthase. The falling of these H+ ions from a high concentration in the inter-membrane space to a low concentration in the mitochondrial matrix leads to a further release of energy. This process is called chemiosmosis. The energy released then leads to a change in structure of ATP synthase and allows it to catalyse the formation of ATP from adenosine diphosphate (ADP) and inorganic phosphate. The metabolic pathway leading to the production of ATP in this manner is called oxidative phosphorylation as electron donors lose electrons (become oxidised) and ADP becomes phosphorylated.


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Roughly, around 30-32 ATP is produced from one molecule of glucose in cellular respiration. However, the number of ATP molecules generated from the breakdown of glucose varies between species. The number of H + ions that the electron transport chain pumps differ within them.

From a single molecule of glucose producing two ATP molecules in glycolysis and another two in the citric acid cycle, all other ATPs are produced through oxidative phosphorylation. Based on the experiment, it is obtained that four H + ions flow back through ATP synthase to produce a single molecule of ATP. After moving through the electron transport chain, each NADH yields 2.5 ATP, whereas each FADH2 yields 1.5 ATP.

Given below is a table showing the breakdown of ATP formation from one molecule of glucose through the electron transport chain:

Name of the PathwayNet Yield of ATP
Glycolysis2 ATP (direct) + 3-5 ATP (from 2 NADH)
Oxidation of Pyruvate5 ATP (from 2 NADH)
Citric Acid Cycle2 ATP (from 2 GTP), 15 ATP (from 6 NADH) + 3 ATP (from 2 FADH2)
Total32 ATP

As given in the table, the ATP yield from NADH made in glycolysis is not precise. The reason is that glycolysis occurs in the cytosol, which needs to cross the mitochondrial membrane to participate in the electron transport chain. Cells with a shuttle system to transfer electrons to the transport chain via FADH2 are found to produce 3 ATP from 2 NADH. In others, the delivery of electrons is done through NADH, where they produce 5 ATP molecules.


Photosynthesis

The reaction centre in PS II absorbs 680 nm wavelength of red light. This causes electrons to become excited and jump into an orbit farther from the atomic nucleus. These electrons are picked up by an electron acceptor and sent to an electrons transport system consisting of cytochromes.

In terms of an oxidation-reduction potential scale, this movement of electrons is downhill. The electrons are not used up as they pass through the electron transport chain. They are passed on to the pigments of PS I.

At the same time, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700 nm. They are then transferred to another acceptor molecule which has a greater redox potential.

Z Scheme: These electrons are then moved downhill again, to a molecule of energy-rich NADP + . The addition of these electrons reduces NADP + to NADPH + H + . This whole transfer of electrons from PS II to the acceptor, to PS I, to another acceptor and finally to NADP+ is called the Z scheme, because of its characteristic shape.

Splitting of Water

Water is split into H + , [O] and electrons. The splitting of water is associated with PS II. This creates oxygen. Photosystem II provides replacement for electrons removed from PS I.

Cyclic and Non-cyclic Photo-phosphorylation

  • Synthesis of ATP from ADP and inorganic phosphate in the presence of light is called photophosphorylaton.
  • When the two photosystems work in a series first PS II and then the PS I a process called non-cyclic photophosphorylation occurs.
  • When only PS I is functional, the electron is circulated within the photosystem and the cyclic flow of electrons leads to phosphorylation. The stroma lamellae are the possible location of phosphorylation. The stroma lamellae lack PS II and NADP reductase enzyme. The excited electron does not pass on to NADP + but is cycled back to the PS I complex. Hence, the cyclic flow results only in the synthesis of ATP but not of NADPH + H + . Cyclic photophsophorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation.

Chemiosmotic Hypothesis

Synthesis of ATP in chloroplast can be explained by chemiosmotic hypothesis. The way it happens in respiration, ATP synthesis during photosynthesis happens because of development of a proton gradient across a membrane, i.e. membrane of the thylakoid. The following steps are involved in development of proton gradient across the thylakoid membrane.

When the electrons move through the photosystems, protons are transported across the membrane. The primary acceptor of electron is located towards the outer side of the membrane. It transfers its electrons not to an electron carrier but to an H carrier. Due to this, it removes a proton from the stroma while transporting an electron. When electron is passed to the electron carrier on the inner side of the membrane, the proton is released into the inner side or the lumen side of the membrane.

The NADP reductase enzyme is located on the stroma side of the membrane. Protons are also necessary for the reduction of NADP + to NADPH + H + . These protons are also removed from the stroma.

Thus, protons in the stroma decrease in number and accumulate in the lumen. This results in development of a proton gradient across the thylakoid membrane. Additionally, there is a measurable decrease in pH in the lumen.

The breakdown of this gradient leads to release of energy. The movement of protons across the membrane to the stroma results in breakdown of this gradient. The movement of protons takes place through the transmembrane channel of the F0 of the ATPase.

The ATPase enzyme consists of two parts. One part is called the F0 and is embedded in the membrane. This forms a transmembrane channel which carries out facilitated diffusion of protons across the membrane. The other portion is called F1. It protrudes on the outer surface of thylakoid membrane on the lumen side.

The breakdown of the gradient provides enough energy to cause a change in the F1 particle of the ATPase which results in synthesis of several molecules of energy-packed ATP.

To summarise, it can be said that chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase. Energy is utilised to pump protons across a membrane, to create a gradient of protons within the thylakoid membrane. ATPase has a channel. This channel allows diffusion of protons back across the membrane. The diffusion of protons releases enough energy to activate ATPase enzyme. The ATPase enzyme catalyses the formation of ATP. NADPH and the ATP are used in the biosynthetic reaction which takes place in the stroma. This reaction is responsible for fixing CO2 and for synthesis of sugars.


Watch the video: First Year. Ch 11. Bioenergetics. Part 24. Electron transport chain. Oxidative phosphorylation (May 2022).