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Bis2A 06.2 Oxidative Phosphorylation
Mitch Singer
Based on Oxidative Phosphorylation† by
OpenStax College
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0‡
Abstract
By the end of this section, you will be able to:
• Describe how electrons move through the electron transport chain and what happens to their energy
levels
• Explain how a proton (H+ ) gradient is established and maintained by the electron transport chain
In the last module we discussed the various ways cells synthesize ATP and had a detailed discussion
on substrate level phosphorylation. The second primary mechanism for ATP and energy formation is by
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 membrane potential that can then be used to do work. One of the "machines"
that can be driven by the membrane potential, also referred to as the proton motive force or PMF, is
the F1 F0 ATPase. Unlike SLP, which directly synthesizes ATP, Oxidative Phosphorylation is an indirect
mechanism. It is derived from a process that begins with moving electrons through a series of electron
transporters or carriers that undergo red/ox reactions. The energy released from these reactions leads to
the movement of protons across a membrane. This accumulation of protons on ones 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. this is called an electrical potential due to the charge
separation. In addition, the accumulation of protons also causes a pH gradient to form across the membrane
and is referred to as the chemical potential. Together this is called an electro-chemical gradient across
the membrane. Think of this as a cellular capacitor, as the charge and pH gradient grows more and more
energy is stored and can be used to do work, such as driving the F F ATPase and generating ATP indirectly.
Below the basic concepts of oxidative phosphorylation are described. Remember for ATP synthesis to
occur two criteria must be met, the rst is the formation of the membrane potential via a series of red/ox
reactions, referred to as an electron transport chain and second, a membrane bound, proton driven F F
ATPase, that uses the potential energy from the PMF to drive the formation of ATP by allowing protons to
move from the higher concentration on one side of the membrane to the other side of lower concentration.
Nowhere is molecular oxygen required for this to happen. Oxygen is a terric terminal electron acceptor
and allows for a very ecient way to generate a large PMF, however, other compounds such as hydrogen
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sulde can also act as terminal electron acceptors. The eukaryotic mitochondrion has evolved an incredibly
ecient electron transport chain to maximize ATP production for every 2 high energy electrons that enter
the chain. While this mitochondrial electron transport chain is what we (eukaryotes) use, the diversity of
the electron transport chain in nature is one of the most amazing features of life on this planet. Think about
all of the unique and inhospitable places there are on this planet, yet some form of life can survive there.
It makes one think about the possibility of life on other worlds. Remember all that is required is a donor
of high energy electrons, carriers to move the electrons by red/ox reactions in a membrane, and a terminal
electron acceptor. As we will discuss below, as long as the terminal electron acceptor has a higher anity
for the electrons than the electron donor, the electrons will move, and the energy can be captured by the
cell.
1 Electron Transport Chain
Where do the electrons come from?
The electron transport chain, or ETC, is made up of a group of protein complexes that undergo a series
of red/ox reactions to translocate protons across the membrane to generate a PMF. Electrons enter the ETS
from a high energy electron donor, often times this is in the form of NADH or FADH , which are generated
during catabolism (oxidation of carbon compounds, such as sugars or proteins or fats). Use the electron
tower below (gure 1) as a reference guide to orient you as to where each component sits. Depending on
the complexity of the ETC being used, electrons can enter at a variety of places depending upon the energy
level of those entering electrons. To enter the ETC (electrons being donated to a red/ox complex within
the chain), the electron donor must have a lower electronegativity than the electron acceptor (the complex
that is taking the electrons). The donor will become oxidized and the acceptor will become reduced. The
dierence in the reduction potential between the donor and acceptor is the measure of energy released. If
sucient energy is released the cell can use it to do work, and in the case of an ETC that work would include
translocating a proton from one side of the membrane to the other, setting up a PMF.
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Figure 1: Electron Tower
Note: electrons entering the ETC do not have to come from NADH or FADH . Many other compounds
can serve as electron donors, the only requirement is that there exists an enzyme that can oxidize the electron
donor and then reduce another compound. Even a small amount of energy 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
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2H + 2 e /H has a reduction potential (E ) of -0.42 eV. If these electrons are eventually donated to
oxygen then the ∆E of the reaction is 1.24 eV and that is equivalent to a lot of energy, 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 eV. These bacteria use oxygen as their terminal electron acceptor and in this case,
the ∆E of the reaction is approximately 0.62. Not so great, but 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
harvested 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 (some are integral) red/ox complexes
that move electrons from a donor source, such as NADH, to a nal acceptor, such as oxygen (that's what
we use). Each 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 the substrate (source of electrons) for the next reaction. In other words,
think of the ETC as a series of complexes that passes electrons to the next complex, which eventually uses
some oxidized compound as the nal substrate (referred to as the terminal electron acceptor).
The total dierence in the reduction potential of the electron donor and the nal electron acceptor can
be thought of as the total energy available for the system. How that energy is released, in one big chunk
or in small aliquots is dependent upon the number of red/ox complexes the electrons will travel through.
Each complex (in general) can be thought of as a release valve, where the energy being generated by the
various red/ox reactions can be captured by the translocation of a proton. NOTE, not all complexes can
generate enough energy to translocate a proton. The number of protons being translocated is important
because in general it takes 3 protons to enter the F F ATPase to generate 1 ATP molecule. The more
protons translocated per 2 electrons that enter the chain, the more ATP that can be made.
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How do ETC complexes transfer electrons?
As previously mentioned the ETC is composed of a series of complexes that undergo a series of red/ox
reactions. These complexes are in fact multiprotein enzyme complexes referred to as oxidoreductases or
simply reductases. The one exception to this 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.
During the red/ox reaction the electrons are not carried directly on the proteins within the complex, but on
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
• Cytochromes are proteins that contain a heme prosthetic group. The Heme is capable of carrying a
single electron.
•
Iron-Sulfur proteins contain a non-heme iron-sulfur clusters that can carry an electron. The prosthetic group is often abbreviated as Fe-S
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, we breath in oxygen,
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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.
However, many other organisms, all of them microbes (and include bacterial, archaeal and eykaryotic
members) can use other compounds as terminal electron acceptors. These other compounds include common
ions as nitrate (NO ), reduction potential of +0.42, and nitrite (NO ), reduction potential of +0.72, or
tetrathionate (S O ) reduction potential of +0.024. When the terminal electron acceptor is not molecular
oxygen (O ) then the process is considered anaerobic and 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 1 shows a generic electron transport chain, composed of two integral membrane complexes; Complex
I and complex II . A reduced high energy electron donor, designated HD (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) the energy released is used to translocate
a proton from one side of the membrane to the other. The net result is that one surface 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 red/ox
reactions, regenerating Complex I which can repeat the 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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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. 12.)
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) provides 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, 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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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.
1.1 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
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.
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1.2 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
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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. 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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1.3 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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1.4 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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2 Links
Here are some useful links to videos on electron transport chains
• YouTubeElectron Transport Chain1
• YouTubeElectron Transport Chain #22
3 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.
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. Recall that 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 the force
of the hydrogen ions diusing through it, down their electrochemical gradient. The turning of parts of this
1 https://www.youtube.com/watch?v=j6_e39ueJBo
2 http://cnx.org/content/m56011/latest/hhttps://search.yahoo.com/yhs/search;_ylt=AwrT6V2ZoXBV_NQAav4nnIlQ;_ylc=X1MDMTM1MT
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molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the potential energy
of the hydrogen ion gradient.
:
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 an uncoupler that makes the inner mitochondrial membrane leaky to protons. It was used until 1938 as a weight-loss drug. What eect would you expect DNP to have on
the change in pH across the inner mitochondrial membrane? Why do you think this might be an
eective weight-loss drug?
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
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.
Links
How ATP is made from ATP synthase
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3 https://www.youtube.com/watch?v=PjdPTY1wHdQ
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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.
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?
4 ATP Yield
The number of ATP molecules generated from the catabolism of glucose varies. For example, the number
of hydrogen ions that the electron transport chain complexes can pump through the membrane varies between species. Another source of variance stems from the shuttle of electrons across the membranes of the
mitochondria. (The NADH generated from glycolysis cannot easily enter mitochondria.) Thus, electrons are
picked up on the inside of mitochondria by either NAD or FAD . As you have learned earlier, these FAD
molecules can transport fewer ions; consequently, fewer ATP molecules are generated when FAD acts as a
carrier. NAD is used as the electron transporter in the liver and FAD acts in the brain.
Another factor that aects the yield of ATP molecules generated from glucose is the fact that intermediate compounds in these pathways are used for other purposes. Glucose catabolism connects with the
pathways that build or break down all other biochemical compounds in cells, and the result is somewhat
messier than the ideal situations described thus far. For example, sugars other than glucose are fed into
the glycolytic pathway for energy extraction. Moreover, the ve-carbon sugars that form nucleic acids are
made from intermediates in glycolysis. Certain nonessential amino acids can be made from intermediates of
both glycolysis and the citric acid cycle. Lipids, such as cholesterol and triglycerides, are also made from
intermediates in these pathways, and both amino acids and triglycerides are broken down for energy through
these pathways. Overall, in living systems, these pathways of glucose catabolism extract about 34 percent
of the energy contained in glucose.
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5 Section Summary
The electron transport chain is the portion of aerobic respiration that uses free oxygen as the nal electron
acceptor of the electrons removed from the intermediate compounds in glucose catabolism. The electron
transport chain 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 through a series of redox reactions, with a small amount of free energy used at three points to
transport hydrogen ions across a membrane. This process contributes to the gradient used in chemiosmosis.
The electrons passing through the electron transport chain gradually lose energy, High-energy electrons
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donated to the chain by either NADH or FADH complete the chain, as low-energy electrons reduce oxygen
molecules and form water. The level of free energy of the electrons drops from about 60 kcal/mol in NADH
or 45 kcal/mol in FADH to about 0 kcal/mol in water. The end products of the electron transport chain
are water and ATP. A number of intermediate compounds of the citric acid cycle can be diverted into the
anabolism of other biochemical molecules, such as nonessential amino acids, sugars, and lipids. These same
molecules can serve as energy sources for the glucose pathways.
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6 Art Connections
Exercise 2
(Solution on p. 12.)
Exercise 3
(Solution on p. 12.)
Figure 4 Dinitrophenol (DNP) is an uncoupler that makes the inner mitochondrial membrane leaky
to protons. It was used until 1938 as a weight-loss drug. What eect would you expect DNP to
have on the change in pH across the inner mitochondrial membrane? Why do you think this might
be an eective weight-loss drug?
Figure 5 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?
7 Review Questions
Exercise 4
What compound receives electrons from NADH?
a.
b.
c.
d.
FMN
ubiquinone
cytochrome c
oxygen
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Exercise 5
Chemiosmosis involves ________.
a.
b.
c.
d.
(Solution on p. 12.)
(Solution on p. 12.)
the movement of electrons across the cell membrane
the movement of hydrogen atoms across a mitochondrial membrane
the movement of hydrogen ions across a mitochondrial membrane
the movement of glucose through the cell membrane
8 Free Response
Exercise 6
(Solution on p. 12.)
Exercise 7
(Solution on p. 12.)
How do the roles of ubiquinone and cytochrome c dier from the other components of the electron
transport chain?
What accounts for the dierent number of ATP molecules that are formed through cellular respiration?
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Solutions to Exercises in this Module
Solution to Exercise (p. 6)
To be discussed in class
to Exercise (p. 11)
Figure 4 After DNP poisoning, the electron transport chain can no longer form a proton gradient, and ATP
synthase can no longer make ATP. DNP is an eective diet drug because it uncouples ATP synthesis; in
other words, after taking it, a person obtains less energy out of the food he or she eats. Interestingly, one of
the worst side eects of this drug is hyperthermia, or overheating of the body. Since ATP cannot be formed,
the energy from electron transport is lost as heat.
to Exercise (p. 11)
Figure 5 After cyanide poisoning, the electron transport chain can no longer pump electrons into the
intermembrane space. The pH of the intermembrane space would increase, the pH gradient would decrease,
and ATP synthesis would stop.
to Exercise (p. 11)
A
to Exercise (p. 11)
C
to Exercise (p. 11)
Q and cytochrome c are transport molecules. Their function does not result directly in ATP synthesis in
that they are not pumps. Moreover, Q is the only component of the electron transport chain that is not a
protein. Ubiquinone and cytochrome c are small, mobile, electron carriers, whereas the other components of
the electron transport chain are large complexes anchored in the inner mitochondrial membrane.
to Exercise (p. 11)
Few tissues except muscle produce the maximum possible amount of ATP from nutrients. The intermediates
are used to produce needed amino acids, fatty acids, cholesterol, and sugars for nucleic acids. When NADH is
transported from the cytoplasm to the mitochondria, an active transport mechanism is used, which decreases
the amount of ATP that can be made. The electron transport chain diers in composition between species,
so dierent organisms will make dierent amounts of ATP using their electron transport chains.
Glossary
Denition 1: ATP synthase
(also, F1F0 ATP synthase) membrane-embedded protein complex that adds a phosphate to ADP
with energy from protons diusing through it
Denition 2: prosthetic group
(also, prosthetic cofactor) molecule bound to a protein that facilitates the function of the protein
Denition 3: ubiquinone
soluble electron transporter in the electron transport chain that connects the rst or second complex
to the third
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