IB Biology HLCellular RespirationPaper 1 & 2~14 min read
Oxidative Phosphorylation
Everything so far has been preparation. Ten reduced NAD and two reduced FAD are queued up holding electrons, and this is the stage that finally converts them into ATP — roughly thirty-two of them.
📚 What you need to know
The electron transport chain is a series of redox reactions in membrane proteins (electron carriers) embedded in the inner mitochondrial membrane.
Carriers sit close together, so electrons pass from carrier to carrier, releasing energy in a controlled way at each step.
Reduced NAD and reduced FAD deliver their electrons and are reoxidised to NAD and FAD.
The energy released is used to pump protons from the matrix into the intermembrane space.
The cristae are impermeable to protons, so a proton (electrochemical) gradient builds up.
Protons return to the matrix by facilitated diffusion through ATP synthase, which turns like a water wheel and phosphorylates ADP. This is chemiosmosis.
About 3 ATP are produced per reduced NAD, contributing to a total of about 32 ATP per glucose from this stage.
Oxygen is the final (terminal) electron acceptor. It is reduced and combines with protons to form water.
Without oxygen the electrons have nowhere to go, the chain stops, and NAD and FAD cannot be regenerated.
Passing electrons down the chain
Reduced NAD from glycolysis, the link reaction and the Krebs cycle arrives at the inner mitochondrial membrane and hands over a pair of electrons to the first carrier. In doing so it is reoxidised back to NAD and released to collect more hydrogen.
Removing electrons from hydrogen atoms leaves H+ ions (protons) behind — and those protons turn out to be the important part.
The electrons are then passed from one carrier to the next. Each transfer is a redox reaction that releases a small amount of energy in a controlled way, and that energy is used to pump protons across the membrane.
Building the gradient: chemiosmosis
The carrier proteins pump protons from the matrix into the intermembrane space. Because the cristae are impermeable to protons, they cannot leak back — so positively charged protons accumulate on one side. That difference in concentration and charge is a proton gradient, also called an electrochemical gradient.
The only way back into the matrix is through a membrane protein called ATP synthase. Protons flow down their gradient through it by facilitated diffusion, and the flow turns the enzyme rather like water turning a water wheel. As it turns, ATP synthase catalyses the phosphorylation of ADP to make ATP.
DefinitionChemiosmosis is the process in which energy stored in a proton gradient is used to make ATP as protons diffuse back across a membrane through ATP synthase.
The two arrows to follow are the green ones going up and the blue one coming down. Everything else on this diagram exists to make those two things happen.
🧠
The dam
Electrons do the pumping, like a pump filling a reservoir uphill. ATP synthase is the turbine in the dam wall. The energy is not in the electrons at the end — it is in the height of the water.
Oxygen: the final electron acceptor
At the end of the chain the electrons have to go somewhere. Oxygen takes them, which is why it is called the final or terminal electron acceptor. Oxygen is reduced by the electrons and combines with protons from the matrix to form water.
The end of the chain
½O2 + 2H+ + 2e− → H2O
This is not a minor housekeeping detail. Accepting those electrons is what keeps the whole chain flowing. Remove oxygen and:
the electrons have nowhere to go, so the chain stops;
reduced NAD and reduced FAD cannot be reoxidised, so no further hydrogen transport is possible;
no proton gradient is maintained, so chemiosmosis stops and ATP is no longer made;
without enough ATP, cells cannot carry out the reactions they need to survive.
Examiners ask “why is oxygen so important?” constantly, and the weak answer is “for respiration”. The strong answer is two lines: oxygen is the final electron acceptor, and without it the electron transport chain cannot continue because the electrons have nowhere to go. Learn those two lines word for word.
Where the ATP finally comes from
Roughly 3 ATP are made for each reduced NAD delivered to the chain, and about 2 for each reduced FAD. Added up across a whole glucose molecule, this stage contributes about 32 ATP — the vast majority of the total.
Drawn to scale. This is why an organism that loses its oxygen supply loses almost nine tenths of its ATP income immediately.
Why the exact number varies. Different sources quote 30, 32, 36 or 38 ATP per glucose. The differences come from how many protons ATP synthase needs per ATP and what it costs to move cytoplasmic reduced NAD into the mitochondrion. The biology is identical; only the accounting assumptions differ.
Worked examples
WE 1
Describe chemiosmosis
Describe how a proton gradient is used to produce ATP in the mitochondrion. (5 marks)
Point 1: the energy source
Electrons pass along carriers in the inner membrane, releasing energy at each redox step.
Point 2: pumping
That energy is used to pump protons from the matrix into the intermembrane space.
Point 3: the gradient
The membrane is impermeable to protons, so they accumulate and an electrochemical gradient forms.
Point 4: the return
Protons diffuse back into the matrix through ATP synthase by facilitated diffusion.
Point 5: the product
The flow turns ATP synthase, which phosphorylates ADP to ATP. This process is chemiosmosis.
Pump out, build a gradient, let them back in through the enzymename ATP synthase and say facilitated diffusion — both are marking points
WE 2
Explain the role of oxygen
Cyanide blocks the last carrier in the electron transport chain. Explain why this stops ATP production even though glucose is still present. (4 marks)
Point 1: the immediate effect
Electrons can no longer be passed to oxygen, the final electron acceptor, so the chain stops.
Point 2: the knock-on effect on carriers
Reduced NAD and reduced FAD cannot be reoxidised, so no further hydrogen can be transported.
Point 3: the gradient
No protons are pumped, so the proton gradient collapses and chemiosmosis stops.
Point 4: the yield
Oxidative phosphorylation produces no ATP; only the small amount from glycolysis remains, which is not enough for the cell to function.
No acceptor → no flow → no gradient → no ATPbuild the chain of consequences — each link is a separate mark
WE 3
Calculate an ATP yield
Assume each reduced NAD yields 3 ATP and each reduced FAD yields 2 ATP. A cell delivers 6 reduced NAD and 2 reduced FAD to the electron transport chain. Calculate the ATP produced. (2 marks)
Step 1: from reduced NAD
6 × 3 = 18 ATPStep 2: from reduced FAD
2 × 2 = 4 ATPStep 3: total
18 + 4 = 22
22 ATPuse the conversion figures the question gives you, not ones you memorised
💡 Exam tips
Locate this stage precisely: the inner mitochondrial membrane and its cristae.
Say protons are pumped from the matrix into the intermembrane space — direction matters.
Use the words electrochemical gradient, facilitated diffusion, ATP synthase and chemiosmosis.
State that the membrane is impermeable to protons; without that, no gradient could form.
Learn the two-line answer on why oxygen matters.
Remember reduced NAD and FAD are reoxidised here, ready to be reused.
⚠ Common mistakes
Saying oxygen accepts protons only. It accepts the electrons, then combines with protons to form water.
Saying ATP synthase pumps protons. The carrier proteins pump; ATP synthase lets them back through.
Getting the gradient backwards. Protons build up in the intermembrane space, not the matrix.
Calling this substrate-level phosphorylation. That happens in glycolysis and the Krebs cycle.
Saying oxygen is used in the Krebs cycle. It is used only here.
Forgetting water is produced. It is the product of the final reaction.
Up next: Respiratory Substrates. Everything so far assumed glucose. The last page of this topic looks at what happens when a cell burns fat or protein instead, and why fat carries so much more energy.
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