Compare the two routes for ATP synthesis
Distinguish substrate-linked phosphorylation from chemiosmosis in mitochondrial and chloroplast membranes.
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- ATP transfers energy for cellular work, and enzymes control metabolic reactions.
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Quick readiness check
What drives ATP synthesis in chemiosmosis?
What you will learn
- State where substrate-linked phosphorylation occurs
- Describe chemiosmosis as ATP synthesis across organelle membranes
Build the explanation
ATP is synthesised by phosphorylation of ADP. In substrate-linked phosphorylation, a phosphate group is transferred directly from a phosphorylated substrate to ADP during a metabolic reaction. In chemiosmosis, energy stored in a proton gradient across a membrane drives protons back through ATP synthase, coupling proton movement to ATP synthesis. Chemiosmosis occurs across membranes in mitochondria and chloroplasts, whereas substrate-linked phosphorylation is tied directly to particular reaction steps.
Apply it carefully
Keep the routes distinct: substrate-linked phosphorylation transfers phosphate directly from an intermediate, while chemiosmosis uses a proton gradient across a membrane. At this syllabus level, describe ATP synthase's role in coupling proton flow to ATP synthesis without adding molecular detail that is not required.
Pause and explain
Which organelle membranes use chemiosmosis to synthesise ATP?
Worked example
A phosphate is transferred directly from a metabolic intermediate to ADP. Which ATP synthesis route is shown?
Show the worked solution
- Check whether a membrane proton gradient is required.
- Direct phosphate transfer from a substrate is substrate-linked phosphorylation.
Answer Substrate-linked phosphorylation.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare substrate-linked phosphorylation and chemiosmosis in one sentence each.
Give me a hint
Focus on what supplies the phosphate or the energy for ATP synthesis.
Compare my reasoning
- Substrate-linked: phosphate is transferred directly from a phosphorylated substrate to ADP.
- Chemiosmosis: a proton gradient drives proton flow through ATP synthase, which synthesises ATP.
Substrate-linked phosphorylation transfers phosphate directly from a metabolic substrate to ADP. Chemiosmosis uses energy in a proton gradient across a membrane to drive proton flow through ATP synthase and form ATP.
Look for these in your work
- Direct transfer stated
- Gradient stated
- ATP synthase role stated
A diagram shows protons crossing a mitochondrial membrane through ATP synthase. Identify the route and the immediate energy source.
Give me a hint
Look for a proton gradient and a membrane protein.
Compare my reasoning
- The process is chemiosmosis.
- The proton electrochemical gradient powers ATP synthesis as protons flow through ATP synthase.
This is chemiosmosis. The proton electrochemical gradient stores energy, and proton flow through ATP synthase is coupled to ADP phosphorylation.
Look for these in your work
- Chemiosmosis identified
- Gradient as source
- Coupling to ATP synthesis
A chloroplast uses light-driven reactions to establish a proton gradient. Explain how that gradient can lead to ATP formation without saying that light directly phosphorylates ADP.
Give me a hint
Follow the intermediate: membrane gradient, proton flow, ATP synthase.
Compare my reasoning
- The gradient forms across a chloroplast membrane.
- Protons return through ATP synthase, driving ATP synthesis by chemiosmosis.
Light-driven reactions establish a proton gradient across a chloroplast membrane. As protons flow back through ATP synthase, the enzyme couples that flow to ATP synthesis; light does not directly transfer a phosphate to ADP.
Look for these in your work
- Gradient across membrane
- ATP synthase included
- Direct light transfer rejected
Common mistakes
- Confusing substrate-linked phosphorylation with oxidative phosphorylation
- Saying light directly adds phosphate to ADP
- Omitting the membrane proton gradient from chemiosmosis
What can you explain now?
What drives ATP synthesis in chemiosmosis?
Compare with the explanation
Protons moving down their electrochemical gradient through ATP synthase
Proton flow is coupled to phosphorylation of ADP.
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Make it stick
What distinguishes direct substrate-linked phosphorylation from chemiosmosis?
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