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Bis2A 5.6: Oxidative Phosphorylation
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and the Electron Transport Chain
The BIS2A Team
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0†
Abstract
This module will discuss the fate of NADH which was produced in glycolysis and the TCA cycle.
NADH donates high energy electrons to the electron transport chain to generate a proton motive force
which is then used by the cell to make ATP through oxidation phosphorylation.
1 Module Summary
The electron transport chain (ETC) is the portion of respiration that uses an external electron acceptor as
the nal/terminal acceptor for the electrons that were removed from the intermediate compounds in glucose
catabolism. In eukaryotic cells the ETC is composed of four large, multiprotein complexes embedded in the
inner mitochondrial membrane and two small diusible electron carriers shuttling electrons between them.
The electrons are passed from enzyme to enzyme through a series of redox reactions. These reactions are
couple the exergonic redox transfers to the endergonic transport of hydrogen ions across the membrane.
This process contributes to the creation of a transmembrane electrochemical gradient. The electrons passing
through the ETC gradually lose potential energy up until the point they are deposited on the terminal
electron acceptor. The free energy dierence of this multistep redox process is ∼ -60 kcal/mol when NADH
donates electrons or 45 kcal/mol when FADH donates, for organisms using oxygen as the nal electron
acceptor.
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Introduction to Red/Ox, oxidative phosphorylation and Electron Transport Chains
In modules 5.1, we discussed the general concept of Red/Ox reactions in biology and introduced the Electron
Tower, a tool to help you understand Red/Ox chemistry and to estimate the direction and magnitude of potential energy dierences for various Red/Ox couples. In modules 5.3 and 5.4 substrate level phosphorylation
and fermentation were discussed and we saw how exergonic Red/Ox reactions could be directly coupled by
enzymes to the endergonic synthesis of ATP. These processes are hypothesized to be one of the oldest forms
of energy production used by cells. In this section we discuss the next evolutionary advancement in cellular
energy metabolism, oxidative phosphorylation. First and foremost, oxidative phosphorylation does not
imply the use of oxygen, it can, but it does not have to use oxygen. It is called oxidative phosphorylation
because it relies on Red/Ox reactions to generate a electrochemical transmembrane potential that can
then be used by the cell to do work.
A quick summary of Electron Transport Chains
The ETC begins with the addition of electrons, donated from NADH, FADH or other reduced compounds.
These electrons move through a series of electron transporters, enzymes that are embedded in a membrane, or
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carriers that undergo Red/Ox reactions. The free energy transferred from these exergonic Red/Ox reactions
is coupled to the endergonic movement of protons across a membrane. This unequal accumulation of protons
on either side of the membrane "polarizes" or "charges" the membrane, with a net positive (protons) on one
side of the membrane and a negative charge on the other side of the membrane. The separation of charge
creates an electrical potential. In addition, the accumulation of protons also causes a pH gradient known
as a chemical potential across the membrane. Together these two gradients (electrical and chemical) are
called an electro-chemical gradient.
2 Review: The Electron Tower
Since Red/Ox chemistry is so central to the topic we begin with a quick review of the table of reduction
potential - sometimes called the "redox tower". As we discussed in Module 5.1, all kinds of compounds can
participate in biological Red/Ox reactions. Making sense of all of this information and ranking potential
Red/Ox pairs can be confusing. A tool has been developed to rate Red/Ox half reactions based on their
reduction potentials or E values. Whether a particular compound can act as an electron donor (reductant)
or electron acceptor (oxidant) depends on what other compound it is interacting with. The redox tower
ranks a variety of common compounds (their half reactions) from most negative E , compounds that readily
get rid of electrons, to the most positive E , compounds most likely to accept electrons. The tower organizes
these half reactions based on the ability of electrons to accept electrons. In addition, in many redox towers
each half reaction is written by convention with the oxidized form on the left followed by the reduced form to
its right. The two forms may be either separated by a slash, for example the half reaction for the reduction
of NAD to NADH is written: NAD /NADH + 2e , or by separate columns. An electron tower is shown
in gure 1 below.
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Figure 1: Common Red/ox tower
Use the redox tower above as a reference guide to orient you as to the reduction potential
of the various compounds in the ETC. Red/Ox reactions may be either exergonic or endergonic
depending on the relative Red/Ox potentials of the donor and acceptor. Also remember there are
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many dierent ways of looking at this conceptually; this type of redox tower is just one way.
In the Red/Ox table above some entries seem to be written in unconventional ways. For
instance Cytochrome c
. There only appears to be one form listed. Why? This is another
example of language shortcuts (likely because someone was too lazy to write cytochrome twice) that
can be confusing - particularly to students. The notation above could be rewritten as Cytochrome
c /Cytochrome c to indicate that the cytochrome c protein can exist in either and oxidized state
Cytochrome c or reduced state Cytochrome c .
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Review Red/Ox Tower video from Module 5.1
For a short video on how to use the redox tower in red/ox problems click here . This video was made by
Dr. Easlon for Bis2A students.
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2.1 Using the Red/Ox Tower: A tool to help understand electron transport chains
By convention the tower half reactions are written with the oxidized form of the compound on the left
and the reduced form on the right. Notice that compounds such as glucose and hydrogen gas are excellent
electron donors and have very low reduction potentials E . Compounds, such as oxygen and nitrite, whose
half reactions have relatively high positive reduction potentials (E ) generally make good electron acceptors
are found at the opposite end of the table.
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Menaquinone: an example
Let's look at menaquinone
. This compound sits in the middle of the redox tower with an half-reaction
E value of -0.074 eV. Menaquinone can spontaneously (∆G<0) accept electrons from reduced forms of
compounds with lower half-reaction E . Such transfers form menaquinone and the oxidized form of the
original electron donor. In the table above, examples of compounds that could act as electron donors to
menaquinone include FADH , an E value of -0.22, or NADH, with an E value of -0.32 eV. Remember the
reduced forms are on the right hand side of the red/ox pair.
Once menaquinone has been reduced, it can now spontaneously (∆G<0) donate electrons to any compound with a higher half-reaction E value. Possible electron acceptors include cytochrome b with an E
value of 0.035 eV; or ubiquinone with an E of 0.11 eV. Remember that the oxidized forms lie on the left
side of the half reaction.
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3 The Electron Transport Chain
The electron transport chain, or ETC, is composed of a group of protein complexes in and around
a membrane that help couple to energetically couple a series of exergonic/spontaneous red/ox reactions
to the endergonic pumping of protons across the membrane to generate a an electro-chemical gradient.
This electrochemical gradient creates a free energy potential that is termed a proton motive force whose
energetically "downhill" exergonic transfer can later be later coupled to a variety of cellular processes.
ETC Overview
Step 1. Electrons enter the ETC from a high energy electron donor, such as NADH or FADH , which are
generated during a variety of catabolic reactions like and including those associated glucose oxidation
(review modules 5.3-5.5). Depending on the complexity (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 - this depends upon the respective reduction potentials of the proposed electron donors and
acceptors.
Step 2. After the rst redox reaction, the initial electron donor will become oxidized and the electron acceptor
will become reduced. The dierence in redox potential between the electron acceptor an donor is
related to ∆G by the relationship ∆G = -nF∆E, where n = the number of electrons transferred and
F = Faraday's constant. The larger a positive ∆E the more exergonic a reaction.
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Step 3. If sucient energy transferred during an exergonic redox 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 multiple redox transfers, the electron is delivered to a molecule known as the terminal electron
acceptor. In the case of humans and plants, this is oxygen. However, there are many, many, many,
other possible electron acceptors, see below.
Electrons entering the ETC do not have to come from NADH or FADH . Many other
compounds can serve as electron donors, the only requirements are that there exists an enzyme that
can oxidize the electron donor and then reduce another compound and that the E be positive (e.g.
∆G<0). Even a small amounts of free energy transfers can add up. For example there are bacteria
that use H as an electron donor. This is not too dicult to believe because the half reaction 2H
+ 2 e /H has a reduction potential (E ) of -0.42 V. If these electrons are eventually delivered to
oxygen then the ∆E of the reaction is 1.24 V which corresponds to a large negative ∆G (-∆G).
Alternatively, there are some bacteria that can oxidize iron, Fe at pH 7 to Fe with a reduction
potential (E ) of +0.2 V. These bacteria use oxygen as their terminal electron acceptor and in
this case, the ∆E of the reaction is approximately 0.62 V. This still produces a -∆G. 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.
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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 nal terminal electron acceptor, such as oxygen - this
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. 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 multiprotein 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. This 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. In this case, both
the Quinone and the Quinone is soluble within the membrane and can move from complex to complex.
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.
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The Electron and Proton carriers
•
Flavoproteins (Fp), these proteins contain an organic prosthetic group called a avin, which is the
actual moiety that undergoes the oxidation/reduction reaction. FADH is an example of a Fp.
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• 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.
Electon carriers
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•
Cytochromes are proteins that contain a heme prosthetic group. The Heme is capable of carrying a
•
Iron-Sulfur proteins contain a non-heme iron-sulfur clusters that can carry an electron. The prosthetic group is often abbreviated as Fe-S
single electron.
Aerobic versus Anaerobic respiration
In the world we live in, most of the organisms we interact with breath air, which is approximately 20%
oxygen. Oxygen is our terminal electron acceptor. We call this process respiration, specically aerobic
respiration, we breath in oxygen, our cells take it up and transport it into the mitochondria where it is used
as the nal acceptor of electrons from our electron transport chains. That is aerobic respiration: the
process of using oxygen as a terminal electron acceptor in an electron transport chain.
While most of the organisms we interact with use oxygen as the terminal electron acceptor, this process
of respiration evolved at time when oxygen was not a major component of the atmosphere. Respiration or
oxidative phosphorylation does not require oxygen at all; it simply requires a compound with a high
reduction potential to act as a terminal electron acceptor; accept electrons from one of the complexes within
the ETC. Many organisms can use a variety of compounds including nitrate (NO ), nitrite (NO ), even
iron (Fe
) as terminal electron acceptors. When oxygen is NOT the terminal electron acceptor, the
process is referred to as anaerobic respiration. The ability of an organism to vary its terminal electron
acceptor provides metabolic exibility 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 an organism that can use a dierent
terminal electron acceptor can survive.
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A generic example of a simple, 2 complex ETC
Figure 2 shows a generic electron transport chain, composed of two integral membrane complexes; Complex
I and Complex II . A reduced electron donor, designated DH (such as NADH or FADH ) reduces Complex
1 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 redox reaction is exergonic and the
free energy dierence 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 can now reduce the prosthetic group
in Complex II while simultaneously oxidizing Complex I . Electrons pass from Complex I to Complex
II via thermodynamically spontaneous red/ox reactions, regenerating Complex I which can repeat the
previous process. Complex II reduces A, the terminal electron acceptor to regenerate Complex II and
create the reduced form of the terminal electron acceptor. In this case, Complex II can also translocate a
proton during the process. If A is molecular oxygen, water (AH) will be produced. This reaction would then
be considered a model of an aerobic ETC. However, if A is nitrate, NO then Nitrite, NO is produced
(AH) and this would be an example of an anaerobic ETC.
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Generic electron transport chain
Figure 2: Generic 2 complex electron transport chain. In the gure, DH is the electron donor (donor
reduced) and D is the donor oxidized. A is the oxidized terminal electron acceptor and AH is the nal
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.
Exercise 1: Thought question
(Solution on p. 15.)
Based on Figure 2 above and using the electron tower in Figure 1, what is the dierence in the
electrical potential if (A) DH is NADH and A is O and (B) DH is NADH and A is NO . Which
pairs (A or B) provide the most amount of usable energy?
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Detailed look at aerobic respiration
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The eukaryotic mitochondria has evolved a very ecient ETC. There are four complexes composed of proteins, labeled I through IV in Figure 3 (An Aerobic Electron Transport Chain), and the aggregation of these
four complexes, together with associated mobile, accessory electron carriers, is called the electron transport
chain. The electron transport chain is present in multiple copies in the inner mitochondrial membrane of
eukaryotes and the plasma membrane of bacteria and arechaea.
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An Aerobic Electron Transport Chain
Figure 3: 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.
3.1 Complex I
To start, two electrons are carried to the rst complex aboard NADH. This complex, labeled I, is composed
of avin mononucleotide (FMN) and an iron-sulfur (Fe-S)-containing protein. FMN, which is derived from
vitamin B also called riboavin, is one of several prosthetic groups or co-factors in the electron transport
chain. A prosthetic group is a non-protein molecule required for the activity of a protein. Prosthetic groups
are organic or inorganic, non-peptide molecules bound to a protein that facilitate its function; prosthetic
groups include co-enzymes, which are the prosthetic groups of enzymes. The enzyme in complex I is NADH
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dehydrogenase and is a very large protein, containing 45 amino acid chains. Complex I can pump four
hydrogen ions across the membrane from the matrix into the intermembrane space, and it is in this way that
the hydrogen ion gradient is established and maintained between the two compartments separated by the
inner mitochondrial membrane.
3.2 Q and Complex II
Complex II directly receives FADH , which does not pass through complex I. The compound connecting the
rst 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, (QH ), 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 FADH from complex II, including succinate dehydrogenase. This
enzyme and FADH form a small complex that delivers electrons directly to the electron transport chain,
bypassing the rst complex. Since these electrons bypass and thus do not energize the proton pump in the
rst complex, fewer ATP molecules are made from the FADH 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.
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3.3 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, uctuating
between dierent oxidation states: Fe
(reduced) and Fe
(oxidized). The heme molecules in the
cytochromes have slightly dierent characteristics due to the eects of the dierent proteins binding them,
giving slightly dierent 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).
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3.4 Complex IV
The fourth complex is composed of cytochrome proteins c, a, and a . This complex contains two heme
groups (one in each of the two cytochromes, a, and a ) and three copper ions (a pair of Cu and one Cu
in cytochrome a ). 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 (H O). The removal of the hydrogen ions from the system contributes
to the ion gradient used in the process of chemiosmosis.
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Helpful Links
• YouTubeElectron Transport Chain2
• YouTubeElectron Transport Chain #23
3.5 Chemiosmosis
In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen
ions (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 hydrogen ions'
positive charge and their aggregation on one side of the membrane.
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If the membrane were open to diusion by the hydrogen ions, the ions would tend to diuse back across
into the matrix, driven by their electrochemical gradient. Many ions cannot diuse through the nonpolar
regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix
space can only pass through the inner mitochondrial membrane through an integral membrane protein
called ATP synthase (Figure 4). 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.
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Figure 4: ATP synthase is a complex, molecular machine that uses a proton (H+ ) gradient to form
ATP from ADP and inorganic phosphate (Pi). (Credit: modication of work by Klaus Homeier)
Dinitrophenol (DNP) is a small chemical that serves to uncouple the ow 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 eect would you
expect DNP to have on the dierence in pH across both sides of the inner mitochondrial membrane?
Why do you think this might be an eective weight-loss drug? Why might it be dangerous?
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Chemiosmosis (Figure 5) 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
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mitochondria is called oxidative phosphorylation. The overall result of these reactions is the production of
ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a
glucose molecule. At the end of the pathway, the electrons are used to reduce an oxygen molecule to oxygen
ions. The extra electrons on the oxygen attract hydrogen ions (protons) from the surrounding medium, and
water is formed.
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How ATP is made from ATP synthase
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Figure 5: In oxidative phosphorylation, the pH gradient formed by the electron transport chain is used
by ATP synthase to form ATP in a Gram- bacteria.
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 eect would cyanide have on ATP synthesis?
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3.6 A Hypothesis as to how ETC may have evolved
A proposed link between SLP/Fermentation and the evolution of ETCs
When we last discussed energy metabolism, it was in context of substrate level phosphorylation (SLP) and
fermentation reactions. One of the questions in the Discussion points was what would be the consequences
of SLP, both short-term and long-term to the environment? We discussed how cells would need to co-evolve
mechanisms to remove protons from the cytosol (interior of the cell), which lead to the evolution of the
F F ATPase, a multi-subunit enzyme that translocates protons from the inside of the cell to the outside of
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the cell by hydrolyzing ATP as shown in gure 6 below. This arrangement works as long as small reduced
organic molecules are freely available, making SLP and fermentation advantageous. As these biological
process continue, the small reduced organic molecules begin to be used up and their concentration decreases,
putting a demand on cells to be more ecient. One source of potential "ATP waste" is in the removal of
protons from the cell's cytosol, organisms that could nd other mechanisms could have a selective advantage.
Such selective pressure could have led to the rst membrane-bound proteins that could use Red/Ox reactions
as their energy source, as depicted in guire 7. In other words use the energy from a Red/Ox reaction
to move protons. Such enzymes and enzyme complexes exist today in the form of the electron transport
complexes, like Complex I, the NADH dehydrogenase.
Figure 6: Proposed evolution of an ATP dependent proton translocator
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Figure 7: As small reduced organic molecules become limiting organisms that can nd alternative
mechanisms to remove protons from the cytosol may have had and advantage. The evolution of a proton
translocator that uses the energy in a Red/Ox reaction could substitute for the ATAase.
Continuing with this line of logic, there are organisms that can now use Red/Ox reactions to translocate
protons across the membrane, instead of an ATP driven proton pump. With protons being being translocated
by Red/Ox reactions, this would now cause a build up of protons on the outside of the membrane, separating
both charge (positive on the outside and negative on the inside; an electrical potential) and pH (low pH
outside, higher pH inside). With excess protons on the outside of the cell membrane, and the F F ATPase
no longer consuming ATP to translocate protons, the pH and charge gradients can be used to drive the
F F ATPase "backwards"; that is to form or produce ATP by using the energy in the charge and pH
gradients set up by the Red/Ox pumps as depicted in gure 8. This arrangement is called an electron
transport chain (ETC).
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Figure 8: The evolution of the ETC; the combination of the Red/Ox driven proton translocators coupled
to the production of ATP by the F0 F1 ATPase.
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Solutions to Exercises in this Module
Solution to Exercise (p. 7)
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