What you need to know
- 7.4ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. is synthesised by oxidative phosphorylationMaking ATP using energy from electrons passed along the electron transport chain, with oxygen as the final electron acceptor. linked to the electron transport chainA series of carriers in the inner mitochondrial membrane. Energy from electrons passing along it pumps H⁺ ions into the intermembrane space. in mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place., including chemiosmosisMaking ATP as H⁺ ions diffuse back down their gradient through ATP synthase. and ATP synthaseThe enzyme in the inner mitochondrial membrane and thylakoid membrane that makes ATP as H⁺ ions flow through it..
No carrier names such as cytochrome are required. No single ATP-per-glucose total. The proton gradient is across the inner membrane, not the outer one.
By the end, you should be able to…
- Describe electron flow from reduced coenzymeA non-protein molecule that helps an enzyme work, often by carrying hydrogen or chemical groups, such as NAD. to oxygen.
- Explain chemiosmosis and the role of ATP synthase.
- Predict what happens if oxygen is absent or the membrane is leaky.
Build your understanding
Electrons, then a gradient
reduced NADNAD that has accepted hydrogen in glycolysis, the link reaction or the Krebs cycle. It passes electrons to the electron transport chain. and reduced FADFAD that has accepted hydrogen in the Krebs cycle. It passes electrons to the electron transport chain. are oxidised on the inner mitochondrial membrane. Their hydrogen is split into electrons and protons. Electrons pass along a chain of carriers at successively lower energy. The energy released as electrons move is used to pump protons out of the matrix into the intermembrane spaceThe space between the inner and outer mitochondrial membranes, where H⁺ ions build up..
The inner membrane is the site. cristaeThe folds of the inner mitochondrial membrane. They give a large surface area for the electron transport chain and ATP synthase. give it a large surface areaThe total area of a surface. A larger surface area gives a faster rate of diffusion. for carriers and for ATP synthaseThe enzyme in the inner mitochondrial membrane and thylakoid membrane that makes ATP as H⁺ ions flow through it.. The outer membrane is not where this gradient is built. You do not need the names of the individual carriers.
Chemiosmosis makes the ATP
Pumping produces an electrochemical gradient: the intermembrane space has a higher concentration of protons than the matrix. Protons flow back into the matrix through ATP synthase. That flow drives the phosphorylation of ADPAdenosine diphosphate. It is formed when ATP is hydrolysed, and is phosphorylated to make ATP again.. ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. made this way is oxidative phosphorylationMaking ATP using energy from electrons passed along the electron transport chain, with oxygen as the final electron acceptor., because it is coupled to the oxidation of the reduced coenzymeA non-protein molecule that helps an enzyme work, often by carrying hydrogen or chemical groups, such as NAD..
chemiosmosisMaking ATP as H⁺ ions diffuse back down their gradient through ATP synthase. is this use of a proton gradient to make ATP. It is not “hydrogen diffusing into ATP”. ATP synthase is the channel and the enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction.. substrate-level phosphorylationMaking ATP by transferring a phosphate group directly from a substrate molecule to ADP, as in glycolysis and the Krebs cycle., in glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD. and the Krebs cycleA cycle of reactions in the mitochondrial matrix in which acetyl groups are broken down, releasing CO₂ and making reduced NAD, reduced FAD and ATP., does not use this gradient.
Oxygen is the final acceptor
At the end of the chain, electrons and protons combine with oxygen to form water. That is the reunion in the overall equation. If oxygen is missing, electrons cannot leave the chain, pumping stops, and ATP synthase no longer has a useful gradient. Reduced coenzymes are not reoxidised, so the link reactionThe stage in the mitochondrial matrix in which pyruvate is decarboxylated and dehydrogenated and joined to coenzyme A to form acetyl coenzyme A. and Krebs cycle stop too. Glycolysis can continue only if NAD is regenerated another way, which in animal cells means lactateThe product of anaerobic respiration in animals, formed when pyruvate is reduced by reduced NAD..
A hole in the inner membrane would let protons leak back without passing through ATP synthase. Electron transport might continue, and oxygen might still be used, but ATP synthesis would fall. Heat would increase. That is an uncoupler argument. You do not need a named poison unless a question gives it. Do not attach a single number of ATP per glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. to this page.
Follow the process
Reduced NAD and reduced FAD are oxidised on the inner membrane → their hydrogen splits into electrons and protons → electrons pass along carriers, and the energy released pumps protons into the intermembrane space → protons flow back into the matrix through ATP synthase, and ADP is phosphorylated → electrons, protons and oxygen form water at the end of the chain. If oxygen is missing, the chain backs up, pumping stops, and the coenzymes stay reduced. A leaky inner membrane can let protons return without ATP synthase, so ATP falls and more energy leaves as heat even while oxygen is still used. Do not memorise an ATP total per glucose, or a fixed “three ATP per reduced NAD”.
NOT: there is no single ATP total to memorise, and glycolysis is not where most ATP is made.
- Electrons come from reduced coenzymes
- Protons are higher in the intermembrane space
- ATP synthase uses the return flow
- Oxygen accepts the electrons and protons
Two traps this diagram blocks
- Do not put the proton gradient the wrong way round.
- Do not write a fixed number of ATP per glucose as if it were required.
From the diagram alone
Question
Explain the role of oxygen in oxidative phosphorylation. [3]
Show a model answer
Oxygen is the final electron acceptor. It combines with electrons from the chain and with protons to form water. Without it the chain stops and reduced coenzymes are not reoxidised.
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: most of the ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. appears at this stage. Y: oxidation of the reduced coenzymeA non-protein molecule that helps an enzyme work, often by carrying hydrogen or chemical groups, such as NAD. drives a proton gradient across the inner membrane. Z: protons returning through ATP synthaseThe enzyme in the inner mitochondrial membrane and thylakoid membrane that makes ATP as H⁺ ions flow through it. phosphorylate ADPAdenosine diphosphate. It is formed when ATP is hydrolysed, and is phosphorylated to make ATP again.. Incomplete: “Hydrogen turns into ATP.” Improved: “Electrons from reduced NADNAD that has accepted hydrogen in glycolysis, the link reaction or the Krebs cycle. It passes electrons to the electron transport chain. pass along the chain and pump protons out. The protons re-enter through ATP synthase, and that flow makes the ATP. Oxygen only accepts the electrons at the end, forming water.”
Three nouns, in order
electron transport chainA series of carriers in the inner mitochondrial membrane. Energy from electrons passing along it pumps H⁺ ions into the intermembrane space., proton gradient, ATP synthase. oxidative phosphorylationMaking ATP using energy from electrons passed along the electron transport chain, with oxygen as the final electron acceptor. is the result.
Say which side is acid
More H⁺ in the intermembrane spaceThe space between the inner and outer mitochondrial membranes, where H⁺ ions build up.. Flow is back into the matrix.
Avoid this misconception
The claim: ATP synthase pumps the protons out and oxygen is used in the Krebs cycleA cycle of reactions in the mitochondrial matrix in which acetyl groups are broken down, releasing CO₂ and making reduced NAD, reduced FAD and ATP..
The correction: The chain pumps protons out as electrons flow. ATP synthase lets them back in. Oxygen accepts the electrons at the end of the chain, forming water.
How a mitochondrion uses a proton gradient to make ATP
Most of the ATP from aerobic respiration is made on the inner membrane of the mitochondria. Electrons and protons from reduced NAD and reduced FAD are the starting point. The model below lets you follow them and see what happens when something goes wrong.
What the model shows
A close-up of a small piece of the inner membrane. Electrons (blue) pass along a chain of carriers in the membrane, and the energy they release pumps protons (H⁺, red) up into the intermembrane space. ATP synthase lets the protons flow back down into the matrix and spins as they pass.
What to look for
Watch the H⁺ pile up above the membrane, and watch ATP synthase speed up as the gradient grows. Then switch oxygen off, or make the membrane leaky, and compare ATP made, oxygen used and heat with normal.
What it tells us
ATP is made by protons flowing back through ATP synthase (chemiosmosis), not directly by oxygen. Oxygen matters because it takes the electrons at the end of the chain: without it, the chain stops, the coenzymes stay reduced and the Krebs cycle stops too.
Be careful what you claim
All the numbers are relative, chosen to show cause and effect, not real measurements, and the model does not give a total of ATP per glucose. The carriers are not named because you do not need their names.
Try it in the model: wait for the gradient to build, then switch oxygen off and watch the reduced coenzyme bar and ATP synthase.
Follow the protons
A schematic slice of the inner mitochondrial membrane. Build the proton gradient, then take oxygen away, make the membrane leaky, or make the cell use more ATP. All values are relative.
What you see
- A slice of the inner mitochondrial membrane: intermembrane space above, matrix below.
- Reduced NAD and FAD giving electrons (blue dots) to a chain of carriers; H⁺ (red) pumped up into the intermembrane space; oxygen accepting electrons to form water; ATP synthase turning as H⁺ flows back.
- Gauges for the gradient, reduced coenzymes and ATP vs ADP, and a trace of ATP made, oxygen used and heat.
How to use it
- Switch oxygen on or off. Drag the membrane leak (an uncoupler). Set how fast the cell uses ATP.
- Compare the readouts with the normal values shown beside them.
- Answer the questions under the model; the guide walks you through the lesson’s what-ifs.
What it shows
- Chemiosmosis: the electron transport chain builds a proton gradient across the inner membrane, and protons flowing back through ATP synthase make ATP.
- No oxygen: the chain backs up, coenzymes stay reduced and the Krebs cycle stops.
- A leaky membrane: oxygen is still used, but ATP falls and heat rises.
Limits of the model
- All numbers are relative, chosen to show cause and effect. The model does not give an ATP total per glucose (the lesson explains why).
- Only oxidative phosphorylation is shown; glycolysis and anaerobic routes are left out, and carriers are not named (not required).
- One gradient value stands for both the concentration and the charge difference across the membrane.
Guided tour · step 1 of 7
- Watch reduced NAD hand electrons to the chain in the simulator.
- Follow the electrons along the carriers and the H⁺ being pumped up.
- Wait until the H⁺ gradient gauge passes 60 %.
Energy released as electrons pass from carrier to carrier pumps protons from the matrix into the intermembrane space, building a gradient across the inner membrane.
Controls
Compared with a normal, resting cell
1× is the normal, resting cell (oxygen on, no leak). Relative values, chosen to show cause and effect; there is no ATP total per glucose.
Which side?
No oxygen
Stretch
The sequence
Reduced NAD and FAD are oxidised on the inner membrane → electrons pass along the carriers, and the energy released pumps H⁺ into the intermembrane space → H⁺ flow back into the matrix through ATP synthase, which phosphorylates ADP → electrons, H⁺ and oxygen form water. Using ATP (ATP → ADP + Pi, the ATP cycle) supplies the ADP that keeps ATP synthase turning.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Where in the mitochondrion does the electron transport chain sit?
Show suggested answer
On the inner membrane, including the cristae.
Practice question
Retrieval and explanation · suggested answer
What is chemiosmosis in this context?
Show suggested answer
Protons flow down their gradient, through ATP synthase, and that flow drives ATP synthesis.
Practice question
Retrieval and explanation · suggested answer
Why does a lack of oxygen stop oxidative phosphorylation?
Show suggested answer
Oxygen is the final electron acceptor. Without it the chain stops, protons are not pumped, and ATP synthase is not driven.
Practice question
Retrieval and explanation · suggested answer
How is oxidative phosphorylation different from substrate-level phosphorylation?
Show suggested answer
Oxidative phosphorylation uses the proton gradient from the electron transport chain. Substrate-level phosphorylation transfers phosphate directly during glycolysis or the Krebs cycle.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A chemical makes the inner membrane freely permeable to protons but does not block the electron carriers. Oxygen use continues. What happens to ATP synthesis, and why might the cell heat up? Assume the carriers still pass electrons to oxygen.
Show a hint
The gradient, not the presence of oxygen, drives ATP synthase.
Explore the reasoning
Protons leak back without going through ATP synthase, so the gradient collapses and oxidative phosphorylation falls even though oxygen is still reduced. Energy from electron transport is transferred as heat instead of being captured as ATP. This is an assumption about a leak, not a named inhibitor to memorise.
Recall this lesson
Lesson recall questions
Question 1 of 5: Name the inner-mitochondrial process that uses the electron transport chain to make ATP.
Oxidative phosphorylation
Practice question
Retrieval and explanation · suggested answer
Name the inner-mitochondrial process that uses the electron transport chain to make ATP.
Lesson: 7A 4: The Electron Transport Chain
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The gradient was the disputed idea
7A · The Electron Transport ChainPeter Mitchell received the 1978 Nobel Prize in Chemistry for the chemiosmotic theory. The award summary describes it as a contribution to understanding biological energy transfer. It covers…Specification 7.4Read & use the scienceClose article
Why this matters
Peter Mitchell received the 1978 Nobel Prize in Chemistry for the chemiosmotic theory. The award summary describes it as a contribution to understanding biological energy transfer. It covers the idea you use here for mitochondria, and the same kind of gradient is used in chloroplasts. The page is not a list of carrier proteins. 7.4: oxidative phosphorylation depends on a proton gradient and ATP synthase, fed by the electron transport chain.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.4 Oxidative phosphorylation, electron transport, chemiosmosis and ATP synthase.
Use the science
Why is the prize relevant to mitochondria and still not a reason to memorise cytochrome names?
Explore a possible response
The theory is about the proton gradient driving ATP synthesis. Carrier names are not in specification 7.4.
Explore the source
- Nobel Prize in Chemistry 1978: chemiosmotic theoryNobel Prize Outreach
Brown fat spends a gradient on heat
7A · The Electron Transport ChainBrown adipose tissue contains mitochondria with UCP1, a protein that can let protons cross the inner membrane without driving ATP synthase. Energy from the gradient is then released…Specification 7.4Read & use the scienceClose article
Why this matters
Brown adipose tissue contains mitochondria with UCP1, a protein that can let protons cross the inner membrane without driving ATP synthase. Energy from the gradient is then released mainly as heat. This is a deliberate physiological use of uncoupling, not a statement that ATP synthase is unnecessary in ordinary respiration. Your 7.4 model helps predict the trade-off: electron transport may continue to build a gradient, but a proton leak leaves less gradient available for oxidative phosphorylation. Researchers study how cold exposure regulates this tissue. A school diagram of one mitochondrion cannot tell you the whole-body rate of heat production.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.4 Oxidative phosphorylation, electron transport, chemiosmosis and ATP synthase.
Use the science
If proton leakage through UCP1 increased while electron supply stayed similar, why might oxygen use continue while ATP yield falls?
Explore a possible response
Electron transport can still transfer electrons to oxygen, but some proton flow returns through UCP1 rather than ATP synthase. Less of the gradient drives ATP synthesis and more energy becomes heat.
Explore the source
- Brown fat as a regulator of metabolismScientific review via NIH PubMed Central