Britannica Sources used in this answer

How are ADP and phosphate converted into ATP in mitochondria?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Fluorescent confocal image of HeLa cells with green microtubules outlining cell shapes, red mitochondria, and blue nuclei. — mitochondrion Mitochondria (red) are found throughout the cytoplasm of almost all eukaryotic cells (cell nucleus is shown in blue; cytoskeleton is shown in yellow).

In mitochondria, the conversion of ADP and inorganic phosphate ( $P_{i}$ ) into ATP occurs through a process known as oxidative phosphorylation. This mechanism is explained by the chemiosmotic theory, which describes how energy from respiration is conserved.123

The process functions as follows:

Proton Gradient Formation: As electrons are transferred through the electron-transport chain embedded in the inner mitochondrial membrane (crista), energy is released. This energy is used to pump protons ( $H^{+}$ ) from the mitochondrial matrix into the intermembrane space.13
Electrochemical Potential: This pumping action creates a proton concentration gradient and an electrical potential across the inner membrane.13
ATP Synthesis: The inner membrane is largely impermeable to protons, forcing them to flow back into the matrix through a specialized enzyme complex called ATP synthase (also known as $F_{1} F_{0}$ -ATPase).13
Energy Conversion: As protons move through the enzyme's proton pore, the released energy drives the chemical reaction that binds ADP and $P_{i}$ to form ATP.13