01Structure explains function
AP questions love asking WHY an organelle is built the way it is. The answer is almost always surface area or compartmentalization.
The inner mitochondrial membrane is folded into cristae. More folds = more membrane = more electron transport chains and ATP synthase = more ATP.
Chloroplasts stack membranes into thylakoids (grana). The light reactions happen on thylakoid membranes, so stacking packs in more photosystems. The fluid around them, the stroma, is where the Calvin cycle runs.
02Membranes make separate environments
Lysosomes keep their interior at about pH 5, which their enzymes need. If one leaks, the enzymes barely work in the neutral cytosol — a built-in safety feature.
The ER and Golgi form an endomembrane system: membranes are passed between them by vesicles, so proteins can be modified step by step in order.
03Notebook box
Mitochondrion: outer membrane → intermembrane space → inner membrane (cristae, ETC + ATP synthase) → matrix (Krebs cycle).
Chloroplast: outer + inner membrane → stroma (Calvin cycle) → thylakoid membrane (light reactions) → thylakoid lumen (H+ builds up).
Worked example: Why do muscle cells with folded cristae make more ATP than cells with smooth inner membranes? → Folding increases surface area for ETC proteins and ATP synthase.