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Energy and Respiration: Question 2

Syllabus 12.2

Multiple choice A2 1 mark

Aerobic respiration in a eukaryotic cell is often described as occurring in four stages: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation.

Which option correctly identifies where each of these four stages takes place?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall where each stage of aerobic respiration occurs

  • Glycolysis takes place in the cytoplasm (cytosol) of the cell. It does not require oxygen directly and does not need the mitochondrion.
  • Once pyruvate has been transported into the mitochondrion, the link reaction takes place in the mitochondrial matrix.
  • The Krebs cycle also takes place in the mitochondrial matrix, immediately following on from the link reaction.
  • Oxidative phosphorylation takes place on the inner mitochondrial membrane (the folded cristae), where the electron transport chain proteins and ATP synthase are located.

So, moving through the four stages, respiration starts in the cytoplasm and then becomes compartmentalised inside the mitochondrion: first in the matrix (link reaction and Krebs cycle), and finally on the inner membrane (oxidative phosphorylation).

Step 2: Evaluate each option

  • A: correct, cytoplasm for glycolysis, mitochondrial matrix for both the link reaction and the Krebs cycle, and the inner mitochondrial membrane for oxidative phosphorylation.
  • B: incorrect, this swaps glycolysis and the link reaction; glycolysis takes place in the cytoplasm, not the matrix, and pyruvate (the product of glycolysis) must be transported into the mitochondrion before the link reaction, which happens in the matrix, not the cytoplasm.
  • C: incorrect. This places oxidative phosphorylation in the mitochondrial matrix, but the electron transport chain and ATP synthase are located on the inner mitochondrial membrane, not free in the matrix.
  • D: incorrect. This places the link reaction in the cytoplasm, but the link reaction (like the Krebs cycle) takes place in the mitochondrial matrix, after pyruvate has entered the mitochondrion.

Final answer

  • Glycolysis occurs in the cytoplasm; the link reaction and Krebs cycle occur in the mitochondrial matrix; oxidative phosphorylation occurs on the inner mitochondrial membrane, option A.