Respiration: (b) the structure of the mitochondrion

The idea

The mitochondrion is the site of the later stages of aerobic respiration, and its structure is adapted to that role. It has a double membrane: a smooth outer membrane and an inner membrane folded into cristae that increase surface area. The inner membrane holds the electron transport chain and ATP synthase, and it encloses the matrix, a gel-like space containing enzymes for the Krebs cycle, mitochondrial DNA and ribosomes. The intermembrane space lies between the two membranes and is where protons accumulate during chemiosmosis. A useful method is to link each structure to its function: cristae to surface area for the electron transport chain, matrix to Krebs cycle enzymes, and the inner membrane to proton movement and ATP synthesis.

What earns the marks

• The mitochondrion is surrounded by a double membrane consisting of a smooth outer membrane and an inner membrane.
• The inner membrane is folded into cristae, which increase the surface area available for the electron transport chain and ATP synthase.
• The inner membrane is the site of the electron transport chain and ATP synthase, where oxidative phosphorylation occurs.
• The matrix is the gel-like interior containing enzymes of the Krebs cycle, mitochondrial DNA and ribosomes.
• The intermembrane space lies between the outer and inner membranes and is where protons accumulate during chemiosmosis.
• The structure is adapted to aerobic respiration by maximising surface area and compartmentalising the reactions of respiration.

Common mistakes and how to put them right

• Confusing the cristae with the matrix.

  Put it right: Cristae are folds of the inner membrane, while the matrix is the enclosed fluid space containing Krebs cycle enzymes.

• Stating that the outer membrane is folded.

  Put it right: It is the inner membrane that folds into cristae, while the outer membrane is smooth.

• Placing the Krebs cycle enzymes in the intermembrane space.

  Put it right: The Krebs cycle enzymes are in the matrix, and the intermembrane space is where protons accumulate.

Exam tips

• Always pair a named structure with its function, for example cristae with increased surface area for the electron transport chain.
• Use precise terms: inner membrane, outer membrane, cristae, matrix and intermembrane space, rather than vague words like 'inside'.
• When describing adaptation, explain how the folding increases surface area rather than just naming the cristae.