8876 / 2027

Lesson 5 of 6 / Photosynthesis and respiration

Use oxygen at the end of electron transfer

Why does oxygen availability affect ATP supply?

I can identify oxidative-phosphorylation inputs, products and mitochondrial location.

  • I can identify inputs and outputs of glycolysis, link reaction, Krebs cycle and oxidative phosphorylation.Syllabus

    Syllabus 8876, Core 3(d). Identify inputs and outputs of glycolysis, link reaction, Krebs cycle and oxidative phosphorylation.

  • I can locate glycolysis in cytoplasm and other aerobic stages in mitochondria.Syllabus

    Syllabus 8876, Core 3(e). Locate glycolysis in cytoplasm and other aerobic stages in mitochondria.

About 5 min

Make a guess. You are not marked.

Is the oxygen you breathe in turned into the carbon dioxide you breathe out?

  1. It accepts electrons and becomes water.
  2. Oxygen joins carbon to make CO2.
  3. It is, inside the lungs.
Show the answer

It accepts electrons and becomes water.

Oxygen is the final electron acceptor. CO2 comes from the carbon in glucose.

The key idea

Reduced carriers are oxidised, oxygen accepts electrons to form water, and energy transfer supports ATP formation from ADP and phosphate.

Explore the idea

Oxygen keeps electron transfer going

Inner mitochondrial membraneOxygen + electrons + H+Water formedNAD/FAD regeneratedADP + phosphate to ATP

The membrane system couples energy transfer to ATP formation. Oxygen is the final acceptor and contributes to water; it is not simply converted into carbon dioxide.

Original functional schematic. The three blocks are not named or counted electron-transfer complexes; detailed machinery and ATP totals are outside this H1 requirement.

Original models of required stage inputs and outputs. Shapes are not full chemical formulae; exact pathway ATP yields are not required here.

Explanation

Oxidative phosphorylation occurs at the inner mitochondrial membrane. Reduced NAD and reduced FAD supply electrons to electron-transfer components and are oxidised back to NAD and FAD. These carriers can then participate again in earlier stages.

Energy released through electron transfer is coupled to ATP formation from ADP and inorganic phosphate. H1 requires the connection and overall inputs and products, not names of the electron-transfer complexes or a detailed ATP-synthase mechanism.

Oxygen is the final electron acceptor and combines with electrons and hydrogen ions to form water. Removing oxygen prevents sustained transfer to this final acceptor, reducing the ability to regenerate oxidised carriers and maintain this ATP supply.

The main stage-level inputs are reduced carriers, oxygen, ADP and phosphate; products include oxidised carriers, water and ATP. Oxygen is not required because it is transformed into carbon dioxide: the oxygen-consumption and carbon-dioxide-release steps are different.

Name the enzyme that makes the ATP: ATP synthase. The energy released as electrons pass along the carriers is used to pump hydrogen ions (protons) from the matrix into the space between the two mitochondrial membranes. The hydrogen ions then flow back into the matrix through ATP synthase, a protein in the inner membrane, and this flow drives the formation of ATP from ADP and phosphate. H1 needs the name and this outline, not the detailed mechanism.

The inner membrane is folded into cristae. The folds give a large surface area, so the membrane holds many electron carriers and many ATP synthase molecules, and more ATP can be made by oxidative phosphorylation.

Oxidative phosphorylation makes most of the ATP in aerobic respiration. A small amount is made directly in glycolysis and in the Krebs cycle, by transferring a phosphate group from a substrate molecule to ADP. This is called substrate-level phosphorylation. It does not need oxygen, an electron transport chain or ATP synthase.

Step by step
  1. 1

    Oxidise the carriers

    Reduced NAD and FAD donate electrons and are regenerated in oxidised form.

  2. 2

    Couple energy transfer to ATP

    ADP and phosphate are used to form ATP.

  3. 3

    Accept electrons at the end

    Oxygen contributes to water formation, allowing sustained flow.

Worked example

Why earlier stages can slow

Explain why lack of oxygen can eventually affect the link reaction and Krebs cycle even though oxygen is not their direct reactant.

One way to explain it

Without a final electron acceptor, sustained oxidation of reduced carriers is restricted. Less oxidised NAD and FAD is regenerated for earlier reactions, so their continued operation is limited.

Why this answer works
  • Trace the recycling of carriers between stages.
  • Distinguish direct use of oxygen from indirect dependence on it.

Watch out for this

A student says: "Respiratory oxygen is changed directly into the carbon dioxide breathed out." What is wrong with this?

Show the answer

Oxygen acts as the final electron acceptor and forms water. Carbon dioxide is released from carbon-containing respiratory substrates in earlier stages.

Check your understanding

Which set lists important oxidative-phosphorylation inputs?

  1. Glucose, oxidised NAD and carbon dioxide.
  2. Glucose, ADP and phosphate, with ATP made only by substrate-level phosphorylation.
  3. Reduced carriers, oxygen, ADP and phosphate.
  4. Reduced carriers and oxygen, with ATP used to form ADP and phosphate.
Show the answer

Reduced carriers, oxygen, ADP and phosphate.

Their roles account for carrier oxidation, water formation and ATP production.

Quick recall

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

Why does oxygen availability affect ATP supply?

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