9477 / 2027

Lesson 3 of 5 / Respiration and ATP production

Oxidative phosphorylation

Why does blocking oxygen use stop most aerobic ATP production?

I can explain electron transport, oxygen and mitochondrial chemiosmosis.

  • I can recognise chloroplast and mitochondrial components in drawings and micrographs.Syllabus

    Syllabus 9477, Core 3(a). Recognise chloroplast and mitochondrial components in drawings and micrographs.

  • I can explain oxidative phosphorylation, oxygen and the electron transport chain.Syllabus

    Syllabus 9477, Core 3(h). Explain oxidative phosphorylation, oxygen and the electron transport chain.

  • I can compare chemiosmosis in photosynthesis and respiration.Syllabus

    Syllabus 9477, Core 3(l). Compare chemiosmosis in photosynthesis and respiration.

About 8 min

Make a guess. You are not marked.

Oxygen is essential for most ATP production. What does oxygen actually do?

  1. It supplies the phosphate group in each ATP.
  2. It accepts electrons at the end of the chain.
  3. It oxidises glucose directly to CO2.
Show the answer

It accepts electrons at the end of the chain.

Oxygen combines with electrons and protons to form water, keeping electrons flowing so protons keep being pumped.

The key idea

Electron transfer pumps protons out of the matrix; their return through ATP synthase drives ATP formation, with oxygen accepting electrons at the end.

Explore the idea

Connect electron transfer to ATP synthesis

Intermembrane spaceMatrixH+H+H+H+H+H+H+H+H+H+H+H+PumpingATP synthaseADP + Pi -> ATP

Energy from oxidation of reduced carriers supports proton movement towards the intermembrane space. Their controlled return to the matrix down the electrochemical gradient drives ATP synthesis.

Oxygen accepts electrons at the end of the respiratory chain and contributes to water formation. ATP is made on the matrix side of the inner mitochondrial membrane.

H+ symbols show a qualitative difference, not measured concentrations or an ATP-per-proton ratio. The gradient also has an electrical component. Pi means inorganic phosphate. Electron carriers and ATP synthase are simplified; the leak bypasses the normal coupling route.

What changes if oxygen becomes scarce?

Oxygen normally accepts electrons at the end of the respiratory chain. When this acceptance becomes limited, electron transfer and oxidation of reduced carriers slow. Less oxidised NAD and FAD are available for upstream dehydrogenations, so the link reaction and Krebs cycle can slow indirectly.

Oxidative ATP production falls; this does not imply that all ATP synthesis instantly stops. Glycolysis can still supply a small amount if NAD is regenerated, for example by fermentation.

Find mitochondrial cristae in a real micrograph

Try a real electron micrograph

Use the photograph on the right. Which internal pattern matches the diagram?

A mitochondrion diagram beside a transmission electron micrograph. The micrograph on the right shows the enclosing boundary and inner membrane folds.
Tap the image to see it full size. Image: OpenStax, Anatomy and Physiology (2016). CC BY 4.0. Used without further changes.

Different section angles change how an organelle looks. Match several visible features before naming it.

Explanation

Reduced NAD and reduced FAD supply electrons to an electron transport chain in the inner mitochondrial membrane. As electrons pass through carriers, released energy is coupled to pumping protons from the matrix into the intermembrane space. Cristae increase membrane area for this machinery.

The inner membrane limits proton leakage, allowing an electrochemical gradient to form. Protons return to the matrix through ATP synthase, driving phosphorylation of ADP using inorganic phosphate. This chemiosmotic coupling is oxidative phosphorylation, distinct from direct substrate-level phosphorylation.

Oxygen is the final electron acceptor and combines with electrons and protons to form water. Removing oxygen prevents continued electron flow through the normal aerobic chain, so reduced carriers cannot be efficiently reoxidised. This indirectly limits the link reaction and Krebs cycle as oxidised NAD and FAD become less available.

Most aerobic ATP production is linked to this pathway, but the exact total yield varies with coupling and cellular conditions. The syllabus explicitly does not require calculation of total ATP yield from oxidative phosphorylation. Explain the energy-transfer mechanism rather than memorising one universal ATP total.

Step by step
  1. 1

    Locate the membrane

    Use the inner mitochondrial membrane, not the outer membrane.

  2. 2

    Trace electrons and protons separately

    They have connected but different routes.

  3. 3

    Predict a block

    Loss of final electron acceptance prevents sustained carrier oxidation.

Worked example

Work through the evidence

An inhibitor blocks the last electron-transfer step to oxygen. Predict reduced NAD and ATP changes.

One way to explain it

Reduced NAD tends to accumulate because it cannot be efficiently reoxidised, while proton pumping and oxidative ATP production decline as electron flow stops.

Why this answer works
  • The immediate block is downstream but affects upstream carrier availability.
  • Some substrate-level ATP may still be produced if suitable pathways continue.

Watch out for this

A student says: "Oxygen supplies the phosphate group in ATP." What is wrong with this?

Show the answer

Inorganic phosphate supplies the phosphate; oxygen accepts electrons and protons to form water.

Check your understanding

Where is proton concentration elevated by the respiratory chain?

  1. The intermembrane space relative to the matrix
  2. The cytosol across the outer mitochondrial membrane
  3. The matrix because all protons are pumped inward
Show the answer

The intermembrane space relative to the matrix

The chain pumps protons out of the matrix across the inner membrane.

Quick recall

Card 1 of 3. Answer in your head, then check.

Why does blocking oxygen use stop most aerobic ATP production?

You can return to this lesson any time.