What is a proton gradient?

A proton gradient is a difference in the concentration of protons (H+ ions) across a membrane. This gradient can arise when protons accumulate on one side of a membrane, leading to a higher concentration of protons on that side compared with the other side of the membrane.

Proton gradients are of great importance in biology, as they frequently serve as energy stores. For example, during cellular respiration, mitochondria in cells generate a proton gradient across their inner membrane. These protons then flow back into the mitochondrion via an enzyme called ATP synthase. This proton flow stimulates ATP synthase to produce adenosine triphosphate (ATP), the cell’s main source of energy.

Another example is the chloroplasts in plant cells, which also generate a proton gradient to synthesise ATP during the light reaction of photosynthesis.

It is therefore the difference in proton concentration on either side of the membrane that provides this potential energy, which is utilised in biological systems, much like water dammed behind a dam provides potential energy.