12.2.1—Four stages of aerobic respiration location

Mitochondria are adapted for aerobic respiration and ATP production by separating reactions across a double membrane. The matrix, inner membrane and intermembrane space provide distinct conditions for respiratory enzymes, electron transfer and proton-gradient-driven ATP synthesis.

• Outer membrane: a smooth outer boundary that is permeable to several small molecules, helping define the organelle while allowing exchange with the cytoplasm.
• Inner membrane: a less-permeable membrane folded into cristae. It contains electron-transfer proteins and ATP synthase, so the folds provide a large working surface for oxidative phosphorylation.
• Intermembrane space: the compartment in which protons accumulate during electron transfer. Keeping it separated from the matrix allows a proton concentration gradient to form across the inner membrane.
• Matrix: the aqueous internal compartment containing enzymes and other components needed for mitochondrial reactions. Its separation from the inner membrane keeps matrix chemistry distinct from the membrane-based proton gradient.

Structure → function: the double-membrane arrangement maintains compartmentalisation; cristae increase inner-membrane area for electron-transfer proteins and ATP synthase; and the sealed inner membrane helps retain the proton gradient. Together these features allow electron-transfer energy to be coupled to ATP synthesis. Cells with greater ATP demand may contain more mitochondria or mitochondria with more extensive cristae, within the source-supported boundary.

This card explains mitochondrial structure and its functional consequences. It does not replace the later cards on the ordered stages of respiration, individual pathway chemistry or numerical ATP yield. Cristae are folds of the inner membrane, not separate organelles, and ATP is made by coupling a gradient to ATP synthase rather than by the folds alone.