What you'll learn
The electron transport chain and ATP synthesis is the final stage of aerobic respiration and the one that produces the overwhelming majority of the cell's ATP. Glycolysis, the link reaction and the Krebs cycle together yield only four ATP directly per glucose molecule; this stage yields around twenty-six to twenty-eight more. The mechanism is chemiosmosis, and understanding it explains not only respiration but also the light-dependent reactions of photosynthesis, which use the same principle. By the end of this topic you should be able to describe the structure of the mitochondrion in relation to its function, explain electron transfer and proton pumping, describe chemiosmosis and the action of ATP synthase, account for the total ATP yield, explain why the theoretical and actual yields differ, and explain the effect of respiratory inhibitors and uncouplers.
Key terms and definitions
Electron transport chain — a series of carriers in the inner mitochondrial membrane along which electrons pass, releasing energy
Cristae — the folds of the inner mitochondrial membrane
Intermembrane space — the compartment between the inner and outer mitochondrial membranes
Matrix — the fluid interior of the mitochondrion, enclosed by the inner membrane
Oxidative phosphorylation — the synthesis of ATP using energy from the oxidation of reduced coenzymes
Chemiosmosis — the synthesis of ATP driven by the movement of protons down an electrochemical gradient
Proton gradient — the difference in proton concentration across the inner membrane
Electrochemical gradient — a gradient of both concentration and electrical charge
ATP synthase — the enzyme complex that catalyses ATP synthesis as protons flow through it
Final electron acceptor — oxygen, which accepts electrons and protons to form water
Respiratory inhibitor — a substance that blocks a step in the chain, such as cyanide
Uncoupler — a substance that allows protons to cross the membrane without passing through ATP synthase
Core concepts
Mitochondrial structure and its relation to function
The mitochondrion is bounded by two membranes, and the compartments they create are what make chemiosmosis possible.
The outer membrane is smooth and permeable to small molecules, separating the mitochondrion from the cytoplasm.
The inner membrane is folded into cristae, which greatly increase its surface area. This matters because the electron carriers and ATP synthase complexes are embedded in this membrane, so a larger area allows more of them and therefore a higher rate of ATP synthesis. The inner membrane is also relatively impermeable to protons, which is essential: without that impermeability the gradient could not be maintained.
The intermembrane space between the two membranes is small, which allows a proton gradient to be established rapidly since relatively few protons are needed to raise the concentration substantially.
The matrix contains the enzymes of the link reaction and Krebs cycle, along with circular DNA and 70S ribosomes.
Cells with high energy demands, such as muscle, liver and the epithelium of the proximal convoluted tubule, contain numerous mitochondria with densely packed cristae.
The electron transport chain
Reduced NAD and reduced FAD produced in the earlier stages deliver their hydrogen to the inner membrane.
Each reduced coenzyme is oxidised, releasing hydrogen atoms which split into protons and electrons. The coenzyme is thereby regenerated and returns to the matrix to accept more hydrogen, which is essential because the supply of NAD and FAD is limited.
The electrons pass along a series of electron carriers embedded in the inner membrane. Each carrier is at a slightly lower energy level than the last, so at each transfer the electrons release a small quantity of energy.
This released energy is used to pump protons from the matrix, across the inner membrane, into the intermembrane space. Because the membrane is impermeable to protons, they accumulate there.
Reduced NAD delivers its electrons at the start of the chain and so passes three proton-pumping sites, whereas reduced FAD delivers its electrons further along and passes only two. This is why reduced NAD yields more ATP than reduced FAD, and it is a point examiners like.
Oxygen as the final electron acceptor
At the end of the chain the electrons, together with protons from the matrix, are accepted by oxygen, forming water.
Oxygen's role is limited to this single step, but it is indispensable. If oxygen is absent, the electrons have nowhere to go, so the carriers remain reduced and electron transfer stops. Proton pumping ceases, the gradient collapses, and no ATP is made by chemiosmosis.
The consequences work backwards through the whole pathway. With the chain halted, reduced NAD and reduced FAD cannot be reoxidised, so NAD and FAD are not regenerated. The Krebs cycle and link reaction then stop because they require these coenzymes, and only glycolysis can continue, and then only if NAD is regenerated anaerobically.
This is why the absence of oxygen reduces ATP yield so dramatically, and it is the explanation questions are usually seeking.
Chemiosmosis and ATP synthase
The accumulation of protons in the intermembrane space creates an electrochemical gradient across the inner membrane: a higher concentration of protons on one side, and a difference in electrical charge.
Protons can only cross the membrane back into the matrix through channels associated with ATP synthase, since the membrane is otherwise impermeable to them.
As protons flow down the electrochemical gradient through ATP synthase, the energy released drives the enzyme to catalyse the synthesis of ATP from ADP and inorganic phosphate.
This mechanism is chemiosmosis, and the ATP produced this way is described as coming from oxidative phosphorylation, since it depends on the oxidation of the reduced coenzymes.
The same mechanism, with the gradient across the thylakoid membrane instead, produces ATP in the light-dependent reactions of photosynthesis, which is a comparison worth being able to draw.
Accounting for the ATP yield
Each reduced NAD entering the chain yields approximately 2.5 to 3 ATP, and each reduced FAD approximately 1.5 to 2 ATP, the exact figures depending on the source consulted.
Using the conventional values of 3 and 2, the ten reduced NAD produced per glucose yield 30 ATP and the two reduced FAD yield 4, giving 34 from oxidative phosphorylation. Adding the four ATP from substrate-level phosphorylation in glycolysis and the Krebs cycle gives a theoretical total of 38 ATP per glucose molecule.
The actual yield is lower, usually given as around 30 to 32, and being able to explain the discrepancy is a common higher-level question.
Some ATP is used to transport pyruvate from the cytoplasm into the matrix by active transport. Some is used to transport the reduced NAD produced in glycolysis across the inner membrane, since the membrane is impermeable to it and a shuttle mechanism is required, which in some tissues delivers the hydrogen to FAD rather than NAD and so yields less. The proton gradient also leaks slightly, since the membrane is not perfectly impermeable, and some of the gradient is used to drive the transport of other substances rather than ATP synthesis.
Inhibitors and uncouplers
Respiratory inhibitors block a specific carrier in the chain. Cyanide is the standard example, binding to the final carrier and preventing the transfer of electrons to oxygen.
The effect is the same as the absence of oxygen: electron transfer stops, proton pumping ceases, the gradient collapses and ATP synthesis by chemiosmosis stops. Reduced coenzymes accumulate, the Krebs cycle halts, and the cell must rely on anaerobic respiration. Since tissues such as the brain cannot meet their ATP demand this way, cyanide is rapidly fatal.
Uncouplers act differently and the distinction is examinable. An uncoupler makes the inner membrane permeable to protons, so they leak back into the matrix without passing through ATP synthase. Electron transfer and proton pumping continue, and may even accelerate, but the energy of the gradient is released as heat rather than being captured as ATP.
Brown adipose tissue in newborn mammals and in hibernating animals contains a natural uncoupling protein for precisely this purpose, generating heat rather than ATP. This is a good illustration that the two processes, electron transport and ATP synthesis, are separate and merely coupled by the proton gradient.
Worked examples
Example 1: Explaining the effect of cyanide (5 marks)
Explain why cyanide, which binds to the final electron carrier, causes ATP production to fall sharply.
Cyanide prevents electrons being passed from the final carrier to oxygen. Electrons therefore cannot leave the chain, so the carriers remain reduced and electron transfer along the chain stops.
Because no energy is released by electron transfer, protons are no longer pumped from the matrix into the intermembrane space. The existing proton gradient dissipates, so protons no longer flow through ATP synthase and chemiosmosis ceases. Oxidative phosphorylation therefore stops, eliminating the great majority of ATP production.
In addition, reduced NAD and reduced FAD cannot be reoxidised, so NAD and FAD are not regenerated. Without them the link reaction and Krebs cycle stop, removing the remaining substrate-level ATP from those stages.
Only glycolysis can continue, and then only while NAD is regenerated anaerobically, yielding just two ATP per glucose.
Example 2: Comparing an inhibitor with an uncoupler (5 marks)
Compare the effects of a respiratory inhibitor with those of an uncoupler on oxygen consumption and ATP production.
A respiratory inhibitor blocks a carrier in the chain, so electron transfer stops. Oxygen is therefore not reduced and oxygen consumption falls to near zero. Proton pumping stops, the gradient collapses and ATP production by chemiosmosis ceases.
An uncoupler makes the inner membrane permeable to protons so that they return to the matrix without passing through ATP synthase. Electron transfer continues, and because the gradient never builds up to oppose further pumping, it may proceed faster than normal, so oxygen consumption remains high or increases. However, the energy of the gradient is dissipated as heat rather than captured, so ATP production falls.
The distinguishing observation is therefore oxygen consumption: an inhibitor reduces it while an uncoupler maintains or raises it, yet both reduce ATP synthesis.
Example 3: Explaining the yield difference (4 marks)
Explain why the theoretical yield of 38 ATP per glucose is not achieved in practice.
Some ATP is expended transporting pyruvate from the cytoplasm, where glycolysis occurs, into the mitochondrial matrix by active transport.
Further ATP is expended transporting the reduced NAD produced during glycolysis into the mitochondrion, since the inner membrane is impermeable to it and a shuttle system is required. In some tissues this shuttle delivers the hydrogen to FAD rather than NAD, which yields less ATP because FAD delivers electrons further along the chain and so drives fewer proton pumps.
The inner membrane is not perfectly impermeable to protons, so some leak back without passing through ATP synthase and their energy is lost as heat.
Finally, part of the proton gradient is used to drive the transport of other substances across the membrane rather than to synthesise ATP. The actual yield is therefore about 30 to 32 ATP per glucose.
Common mistakes and how to avoid them
The most frequent error is stating that oxygen is needed throughout respiration. It is required only as the final electron acceptor at the end of the chain, though its absence stops the earlier aerobic stages indirectly.
Students often say that ATP is made as the electrons pass along the chain. The energy from electron transfer pumps protons; ATP is made separately as protons flow back through ATP synthase.
Another common slip is placing the proton gradient the wrong way round. Protons are pumped from the matrix into the intermembrane space, so their concentration is higher there.
Many candidates treat reduced NAD and reduced FAD as equivalent. FAD delivers electrons further along the chain, drives fewer proton pumps and therefore yields less ATP.
In inhibitor questions, answers often stop at the chain stopping without following the consequences back to the Krebs cycle. The accumulation of reduced coenzymes and the halting of earlier stages carry marks.
Finally, candidates frequently confuse uncouplers with inhibitors. An uncoupler does not block the chain; it disconnects it from ATP synthesis.
Exam technique for "The electron transport chain and ATP"
Describe the process as a sequence of four linked events: coenzyme oxidised, electrons pass along carriers releasing energy, protons pumped creating a gradient, protons return through ATP synthase making ATP. Each is typically a separate mark.
Always name the compartments. Saying that protons are pumped into the intermembrane space is worth more than saying they are pumped across the membrane.
When explaining the effect of any disruption, follow the consequences backwards through the pathway as well as forwards. Examiners reward the recognition that stopping the chain stops the Krebs cycle.
Relate a structural feature to a function wherever possible: cristae increase surface area for carriers and ATP synthase, impermeability maintains the gradient, and a narrow intermembrane space allows the gradient to build quickly.
Quote ATP yields with the qualification that values vary, and show the arithmetic from the number of reduced coenzymes rather than simply stating a total.
Quick revision summary
The mitochondrion has an outer membrane, an inner membrane folded into cristae that carry the electron carriers and ATP synthase and is impermeable to protons, a narrow intermembrane space, and a matrix holding the link reaction and Krebs cycle enzymes. Reduced NAD and reduced FAD are oxidised at the inner membrane, releasing protons and electrons and regenerating the coenzymes. Electrons pass along carriers of successively lower energy, and the energy released pumps protons from the matrix into the intermembrane space, creating an electrochemical gradient. Reduced NAD drives three pumping sites and reduced FAD only two, which is why NAD yields more ATP. Oxygen is the final electron acceptor, combining with electrons and protons to form water, and without it the chain stops and the Krebs cycle halts because coenzymes cannot be regenerated. Protons return through ATP synthase, and the energy of their flow drives ATP synthesis — chemiosmosis, the same mechanism used across the thylakoid membrane in photosynthesis. The theoretical yield of 38 ATP per glucose falls to about 30 to 32 because of active transport of pyruvate and reduced NAD, proton leakage and other uses of the gradient. Inhibitors such as cyanide block the chain and reduce oxygen consumption, while uncouplers let protons bypass ATP synthase, maintaining oxygen consumption but releasing the energy as heat.