Final answer:
The space between the two membranes of the mitochondria, known as the intermembrane space, accumulates protons. This gradient is crucial for ATP synthesis during cellular respiration.
Step-by-step explanation:
The space between the two membranes of the mitochondria accumulates protons (hydrogen ions) during the electron transport chain of cellular respiration. The outer mitochondrial membrane serves as a barrier between the cytosol and the mitochondrion, while the inner mitochondrial membrane has many folds called cristae, which increase the surface area available for chemical reactions to occur. During the electron transport chain, protons are pumped from the mitochondrial matrix across the inner membrane into the intermembrane space, creating a proton gradient that drives the synthesis of ATP – the cell's energy currency – on the inner membrane.