Inside a mitochondrion

Schematic, simplified representation of a mitochondrion. Inset: in the matrix (light gray), an enzyme (yellow) delivers protons (orange) and electrons stripped from digested food to a proton pump (blue shape on the right; electrons and additional stations are not shown). The flow of electrons within the pump provides the energy to make it transfer protons from the matrix to the intermembrane space (dark gray). The protons can then return from the intermembrane space to the matrix via the ATP synthase (mushroom-like blue shape on the left). In the process, the ATP synthase is pressed into service and produces ATP (green).