What is chemiosmosis and how is ATP formed in mitochondria?

• A. Chemiosmosis uses a proton motive force across the inner mitochondrial membrane to drive ATP synthase to phosphorylate ADP to ATP.
• B. Chemiosmosis is diffusion of CO2; ATP formed by substrate-level phosphorylation.
• C. Chemiosmosis uses ATP synthase to pump protons against gradient.
• D. Chemiosmosis is glycolysis step producing NADH.

Answer: (A)

Chemiosmosis is the process by which energy stored in a proton gradient across the inner mitochondrial membrane is used to synthesize ATP. The electron transport chain pumps protons from the mitochondrial matrix into the intermembrane space, creating a proton motive force that combines a chemical gradient with an electrical potential. Protons flow back into the matrix through the ATP synthase enzyme; this flow drives the rotation of the enzyme’s rotor and the conformational changes in its catalytic sites, which phosphorylate ADP to ATP. This coupling of the proton gradient to ATP formation is oxidative phosphorylation.

This is not simply diffusion of a gas or a glycolysis step. It relies on the proton gradient and ATP synthase working together to produce most of the cell’s ATP, whereas substrate-level phosphorylation temporarily generates ATP in glycolysis and the Krebs cycle without using the gradient.