Locate the four stages of aerobic respiration
Place glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation in the correct eukaryotic cell compartments.
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.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where does glycolysis take place?
What you will learn
- State the location of each aerobic respiration stage
- Relate mitochondrial compartments to respiration
Locate the four stages of aerobic respiration
| Feature | Meaning |
|---|---|
| Glycolysis | Cytoplasm · glucose to pyruvate |
| Link reaction | Mitochondrial matrix · acetyl-CoA formation |
| Krebs cycle | Mitochondrial matrix · oxaloacetate regenerated |
| Oxidative phosphorylation | Inner mitochondrial membrane · electron transport and chemiosmosis |
Build the explanation
In eukaryotic cells, glycolysis occurs in the cytoplasm. The link reaction and Krebs cycle occur in the mitochondrial matrix. Oxidative phosphorylation takes place on the inner mitochondrial membrane. These locations help explain why pyruvate and reduced coenzymes must reach the mitochondrion after glycolysis. The stages are connected by products: glycolysis produces pyruvate, the link reaction produces acetyl groups carried by coenzyme A, and reduced NAD and FAD transfer hydrogen to carriers in the inner membrane.
Apply it carefully
Do not place glycolysis in the mitochondrial matrix or oxidative phosphorylation in the cytoplasm. The mitochondrial matrix is enclosed by the inner membrane; the cristae are folds of that membrane. Stage locations are not the same as a claim that the entire mitochondrion is one reaction space.
Pause and explain
Which stage occurs on the inner mitochondrial membrane?
Worked example
Where do the Krebs cycle and oxidative phosphorylation occur?
Show the worked solution
- Separate the matrix stages from the membrane stage.
- The Krebs cycle is in the matrix; oxidative phosphorylation is on the inner membrane.
Answer Krebs cycle: mitochondrial matrix. Oxidative phosphorylation: inner mitochondrial membrane.
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Complete the location sequence: glycolysis → link reaction → Krebs cycle → oxidative phosphorylation.
Give me a hint
Use cytoplasm, matrix and inner membrane.
Compare my reasoning
- Glycolysis occurs in the cytoplasm.
- The link reaction and Krebs cycle occur in the matrix; oxidative phosphorylation occurs on the inner mitochondrial membrane.
Glycolysis: cytoplasm; link reaction: mitochondrial matrix; Krebs cycle: mitochondrial matrix; oxidative phosphorylation: inner mitochondrial membrane.
Look for these in your work
- Glycolysis location correct
- Both matrix stages correct
- Membrane stage correct
A micrograph shows folds in the inner mitochondrial membrane. Relate these cristae to oxidative phosphorylation without claiming that folds make ATP by themselves.
Give me a hint
Connect membrane location to reaction site and surface area.
Compare my reasoning
- Oxidative phosphorylation occurs on the inner mitochondrial membrane.
- Cristae increase the membrane surface available for its components; the folds alone do not catalyse the process.
Oxidative phosphorylation occurs on the inner mitochondrial membrane. Cristae increase the available membrane surface for associated carriers and ATP-synthesis machinery; the folds themselves are not a reaction or an energy source.
Look for these in your work
- Correct membrane named
- Surface-area role explained
- Structure not mistaken for mechanism
A cell extract contains glycolysis enzymes but lacks intact mitochondria. Which stage can it still carry out, and why?
Give me a hint
Use the compartment map, not a prediction of a full cell's energy yield.
Compare my reasoning
- Glycolysis occurs in the cytoplasm.
- The extract may support glycolysis if required substrates and conditions are present; mitochondrial stages need their mitochondrial compartments.
It can carry out glycolysis because glycolysis occurs in the cytoplasm, provided enzymes and substrates remain functional. The matrix and inner-membrane stages cannot be inferred without the required mitochondrial structures and conditions.
Look for these in your work
- Glycolysis located correctly
- Conditions qualified
- Mitochondrial limitation stated
Common mistakes
- Locating glycolysis in mitochondria
- Locating the Krebs cycle on cristae
- Treating cristae as the cause of ATP synthesis
What can you explain now?
Where does glycolysis take place?
Compare with the explanation
Cytoplasm
Glycolysis precedes the mitochondrial stages in eukaryotic respiration.
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Make it stick
Which aerobic stages occur in the cytoplasm, matrix and inner mitochondrial membrane?
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