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A2 · Practice questions · Mitochondria, glycolysis and the Krebs cycle

Follow the carbon, count the hydrogen.

Six original Cambridge-style questions on mitochondria, glycolysis, the link reaction and the Krebs cycle, including tallies of products per glucose.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Place, carbon, hydrogen.

01
Recall
[4 marks]

State where each of the four stages of aerobic respiration occurs in a eukaryotic cell.

  • Glycolysis: cytoplasm. ✓
  • Link reaction: mitochondrial matrix. ✓
  • Krebs cycle: mitochondrial matrix. ✓
  • Oxidative phosphorylation: inner mitochondrial membrane (cristae). ✓
02
Explain
[4 marks]

Outline the stages of glycolysis.

  • Glucose is phosphorylated using two ATP, forming fructose 1,6-bisphosphate (6C). ✓
  • Fructose 1,6-bisphosphate splits (lysis) into two triose phosphate molecules (3C). ✓
  • Each triose phosphate is oxidised (dehydrogenated) to pyruvate (3C); the hydrogen reduces NAD. ✓
  • Four ATP are made by substrate-linked phosphorylation, a net gain of two ATP and two reduced NAD per glucose. ✓
03
Explain
[3 marks]

Describe the link reaction, including the role of coenzyme A.

  • Pyruvate enters the mitochondrial matrix, where it is decarboxylated (CO2 removed). ✓
  • It is also dehydrogenated; the hydrogen reduces NAD. ✓
  • The remaining acetyl group (2C) combines with coenzyme A, forming acetyl coenzyme A, which carries it to the Krebs cycle. ✓
04
Explain
[4 marks]

Outline the Krebs cycle, using the terms decarboxylation and dehydrogenation.

  • The acetyl group (2C) from acetyl coenzyme A combines with oxaloacetate (4C), the acceptor, to form citrate (6C). ✓
  • Citrate is converted back to oxaloacetate in a series of small steps. ✓
  • Decarboxylation removes carbon as CO2 (6C → 5C → 4C); dehydrogenation removes hydrogen, which reduces NAD and FAD. ✓
  • One ATP is formed per turn by substrate-linked phosphorylation; oxaloacetate is regenerated to accept another acetyl group. ✓
05
Calculation
[3 marks]

Calculate the total number of molecules of reduced NAD, reduced FAD and carbon dioxide produced from one molecule of glucose in glycolysis, the link reaction and the Krebs cycle.

  • Reduced NAD: 2 (glycolysis) + 2 (link reaction) + 6 (two turns of the Krebs cycle) = 10. ✓
  • Reduced FAD: 2 (one per turn of the Krebs cycle). ✓
  • CO2: 2 (link reaction) + 4 (Krebs cycle) = 6, equal to the six carbon atoms in glucose. ✓
06
Analysis
[3 marks]

Electron micrographs show that mitochondria in heart muscle cells have many more cristae than those in skin cells. Explain the significance of this observation.

  • Cristae are folds of the inner membrane that carry the electron transport chain and ATP synthase. ✓
  • More cristae give a larger surface area for oxidative phosphorylation, so more ATP can be made. ✓
  • Heart muscle contracts continuously and needs a large, constant supply of ATP; skin cells are less active. ✓

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