Explain why a cell with a large number of mitochondria and an extensively folded inner mitochondrial membrane is likely to be metabolically very active.

OCR A-Level Biology A (H420) — 2.1 Cell structure · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Cardiac muscle cells contain far more mitochondria per cell than most other cell types in the human body. The inner membrane of each mitochondrion is folded into numerous cristae.

Model answer (4 marks)

A large number of mitochondria increases the total capacity for aerobic respiration.

Folding of the inner membrane increases the surface area available.

Greater surface area of the inner membrane allows more ATP synthase and electron‑carrier proteins to be embedded.

Therefore more ATP is synthesised per unit time, supporting a high level of metabolic activity.

Examiner tips

Use the exact terms ‘mitochondria’, ‘inner membrane’, ‘cristae’, ‘ATP synthase’, ‘electron carriers’.
Show the logical chain: more mitochondria → more total surface area → more enzymes → more ATP → high metabolism.
Keep the answer concise – 4 points, one sentence per point if possible.

Common mistakes

Using vague terms like ‘lots of energy’ instead of ‘more ATP’.
Failing to mention the role of the inner membrane surface area.

Mark scheme (4 marks)

A large number of mitochondria increases the total capacity for aerobic respiration
Folding of the inner membrane increases the surface area available
Greater surface area of the inner membrane allows more ATP synthase / electron carriers to be embedded
Therefore more ATP is synthesised per unit time, supporting a high level of metabolic activity

Key terms in this question

mitochondria · inner mitochondrial membrane