Adaptations of mitochondria and chloroplastsSpec B2.2.4, B2.2.5
In short
The mitochondrion is adapted for aerobic respiration by a double membrane with a small intermembrane space, an inner membrane folded into cristae for a large surface area, and a matrix holding Krebs cycle enzymes and substrates. The chloroplast has a large area of thylakoid membranes with photosystems, small volumes inside thylakoids, and Calvin cycle enzymes in the stroma.
| Feature | Adaptation |
|---|---|
| Double membrane | The outer membrane separates the mitochondrion from the cytoplasm; the inner membrane holds the electron transport chain and ATP synthase. |
| Small volume of intermembrane space | Protons pumped into it by the electron transport chain quickly build up to a high concentration, giving a steep gradient for making ATP by chemiosmosis. |
| Cristae | Folds of the inner membrane give a large surface area for many electron transport chains and ATP synthase molecules. |
| Matrix | Compartmentalizes the enzymes and substrates of the Krebs cycle, keeping them together at high concentration. |
| Feature | Adaptation |
|---|---|
| Thylakoid membranes | Stacked into grana, giving a large surface area for photosystems, so more light can be absorbed. |
| Small volume of fluid inside thylakoids | Protons pumped into the thylakoid space quickly reach a high concentration, giving a steep gradient for making ATP. |
| Stroma | Compartmentalizes the enzymes and substrates of the Calvin cycle, close to the ATP and NADPH made by the thylakoids. |
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(opens full size in a new tab)Exam tip:
For an 'explain the adaptation' question, give the feature and then its effect: for example, a small intermembrane space, so a high proton concentration builds up quickly.
Written and checked against the IB Biology HL specification · Updated October 2026