inner mitochondrial membrane
Summary
The inner mitochondrial membrane is the folded inner membrane (cristae) that holds the electron transport chain and ATP synthase. It is largely impermeable to H⁺, so the chain's proton pumping builds a gradient across it. The testable point: H⁺ flowing back through ATP synthase makes ATP.
Detail
Meaning: The inner mitochondrial membrane is the inner of a mitochondrion's two membranes and encloses the matrix, where the citric acid cycle runs. It is folded into cristae, which increase surface area for the electron transport chain (complexes I–IV) and ATP synthase (complex V). As electrons pass from NADH and FADH₂ to O₂, protons are pumped from the matrix into the intermembrane space. Because this membrane is largely impermeable to H⁺ and other ions, a proton gradient builds. H⁺ flowing back through ATP synthase drives ATP synthesis (chemiosmosis).
On the MCAT: Passages describe uncouplers that make the membrane leaky to H⁺. The gradient collapses, ATP synthesis falls, electron transport and O₂ use continue or increase, and energy is released as heat. Inhibiting ATP synthase backs up the gradient and slows electron transport. Questions also ask which compartment is more acidic (the intermembrane space) or where each pathway occurs.
Common confusion: Students put the gradient across the outer membrane. The outer membrane is porous to small molecules, so the intermembrane space resembles the cytosol in H⁺ concentration. The gradient exists across the inner membrane. Also, the electron transport chain pumps protons but does not make ATP; ATP synthase does.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.