Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis Flashcards
The electron transport chain is a collection of molecules embedded in the inner membrane of the mitochondrion in
eukaryotic cells
In prokaryotes, these molecules reside in the
plasma membrane
The folding of the inner membrane to form cristae increases its surface area, providing space for thousands of copies of each component of the electron transport chain in a
mitochondrion.
Most components of the chain are proteins, which exist in
multiprotein complexes numbered I through IV
Tightly bound to these proteins are ____________________, nonprotein components such as cofactors and coenzymes essential for the catalytic functions of certain enzymes.
prosthetic groups
During this electron transport, electron carriers alternate between ___________________ as they accept and then donate electrons.
reduced and oxidized states
Each component of the chain becomes reduced when it accepts electrons from its “uphill” neighbor, which has a lower affinity for
electrons (in other words, is less electronegative)
It then returns to its oxidized form as it passes electrons to its
“downhill,” more electronegative neighbor.
free energy change during electron transport. During glycolysis and the citric acid cycle, electrons from food molecules are transferred to NAD+ and FAD forming
NADH and FADH2
these electrons carriers bring electrons to the electron transport chain in the inner
mitochondrial membrane
there the energy of electrons is converted to a form that powers the synthesis of
ATP
the overall function of the electron transport chain is to
receive electrons from NADH and FADH2 and move them through a series of redox reactions
when the electrons are in the NADH and FADH2 molecules they have a relatively high energy level and they lose a little energy with each
redox exchange
the electron transport chain is made up of
four protein complexes, a smaller cytochrome protein, and an organic molecular called ubiquinone or Q
Electrons acquired from glucose by NAD+ during glycolysis and the citric acid cycle are transferred from NADH to the first molecule of the
electron transport chain in complex I
This molecule is a flavoprotein, so named because it has a prosthetic group called
flavin mononucleotide (FMN).
In the next redox reaction, the flavoprotein returns to its oxidized form as it passes electrons to an iron-sulfur protein ( Feּּ* S in complex I), one of a family of proteins with both
iron and sulfur tightly bound.
The iron-sulfur protein then passes the electrons to a compound called
ubiquinone
Most of the remaining electron carriers between ubiquinone and oxygen are proteins called
cytochromes
Their prosthetic group, called a _______________, has an iron atom that accepts and donates electrons.
heme group
The electron transport chain has several types of cytochromes, each named “cyt” with a letter and number to distinguish it as a different protein with a slightly different
electron-carrying heme group.
The last cytochrome of the chain, Cyt aּּᴈ, passes its electrons to oxygen, which is very
electronegative.
Each oxygen atom also picks up a pair of hydrogen ions (protons) from the aqueous solution, neutralizing the charge of the added electrons and forming
water.
Another source of electrons for the electron transport chain is FADH2 , the other reduced product of the
citric acid cycle.
FADH2 adds its electrons from within complex II, at a lower energy level than
NADH does.
The electron transport chain makes no
ATP directl;ly
How does the mitochondrion (or the plasma membrane in prokaryotes) couple this electron transport and energy release to ATP synthesis?
a mechanism called chemiosmosis.
Populating the inner membrane of the mitochondrion or the prokaryotic plasma membrane are many copies of a protein complex called ,
ATP synthase, the enzyme that makes ATP from ADP and inorganic phosphate
ATP synthase uses the energy of an existing ion gradient to power
ATP synthesis.
The power source for ATP synthase is a difference in the concentration of H+ on opposite sides of the
inner mitochondrial membrane.
This process, in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP, is called
chemiosmosis
ATP synthase is a multisubunit complex with
four main parts, each made up of multiple polypeptide
Protons move one by one into binding sites on one of the parts (the rotor), causing it to spin in a way that catalyzes ATP production from
ADP and inorganic phosphate
Establishing the gradient is a major function of the electron transport chain, which is shown in its
mitochondrial location
The chain is an energy converter that uses the exergonic flow of electrons from NADH and FADH2 to pump H+ across the membrane, from the .
mitochondrial matrix into the intermembrane space
The H+ has a tendency to move back across the membrane, diffusing down its gradient. And the ATP synthases are the only sites that provide a route through the membrane for
H+
Researchers have found that certain members of the electron transport chain accept and release protons (H+) along with
electrons
In eukaryotic cells, the electron carriers are spatially arranged in the inner mitochondrial membrane in such a way that H+ is accepted from the mitochondrial matrix and deposited in the
intermembrane space
emphasizing the capacity of the gradient to perform work.
proton-motive force
The force drives H+ back across the membrane through the H+ channels provided by
ATP synthases.
is an energy-coupling mechanism that uses energy stored in the form of an gradient across a membrane to drive cellular work.
chemiosmosis
In mitochondria, the energy for gradient formation comes from exergonic redox reactions along the
electron transport chain, and ATP synthesis is the work performed
Chloroplasts use chemiosmosis to generate ATP during
photosynthesis
In these organelles, light (rather than chemical energy) drives both electron flow down an electron transport chain and the resulting
H+ gradient formation
Prokaryotes, as already mentioned, generate H+ gradients across their
plasma membranes.
They then tap the proton-motive force not only to make ATP inside the cell but also to rotate their flagella and to
pump nutrients and waste products across the membrane
respiration overall function:
harvesting the energy of glucose for ATP synthesis.
During respiration, most energy flows in this sequence:
glucoseּּ → NADHּּ → electron transport chain ּּ→ proton motive ּּ→ ATP
We can do some bookkeeping to calculate the ATP profit when cellular respiration oxidizes a molecule of glucose to
six molecules of carbon dioxide
The three main departments of this metabolic enterprise are
glycolysis, pyruvate oxidation and the citric acid cycle, and the electron transport chain, which drives oxidative phosphorylation.
The tally adds the 4 ATP produced directly by substrate-level phosphorylation during glycolysis and the citric acid cycle to the many more molecules of
ATP generated by oxidative phosphorylation.
Each NADH that transfers a pair of electrons from glucose to the electron transport chain contributes enough to the proton-motive force to generate a maximum of about
3 ATP.
There are three reasons we cannot state an exact number of ATP molecules generated by the breakdown of
one molecule of glucose.
First, phosphorylation and the redox reactions are not directly coupled to each other, so the ratio of the number of NADH molecules to the number of ATP molecules is not a
whole number.
Second, the ATP yield varies slightly depending on the type of shuttle used to transport electrons from the
cytosol into the mitochondrion
A third variable that reduces the yield of ATP is the use of the proton-motive force generated by the redox reactions of
respiration to drive other kinds of work
Recall that the complete oxidation of a mole of glucose releases
686 kcal of energy under standard conditions ( ּּ∆G= -686 kcal/mol)
Phosphorylation of ADP to form ATP stores at least
7.3 kcal per mole of ATP.
Therefore, the efficiency of respiration is
7.3 kcal per mole of ATP times 32 moles of ATP per mole of glucose divided by 686 kcal per mole of glucose, which equals 0.34.
Thus, about 34% of the potential chemical energy in glucose has been transferred to
ATP;
The rest of the energy stored in glucose is lost as
heat
