In bioenergetics, the universal underlying mechanism for creating biochemical energy involves a chemical gradient across a biological membrane system. The ultimate goal is to make ATP, the fundamental energy currency of living systems. The coupling of a chemical gradient to generate biochemical energy is called chemiosmosis.
Photosynthesis and Cellular Respiration are often presented as opposing processes, but their function is the same- to make ATP. These processes just use different energy sources to accomplish this feat (photosynthesis-light; cellular respiration-glucose). The chemical gradient used by biological systems is a proton gradient. So, energy sources (light or glucose) are used to pump protons across a biological membrane. The proton imbalance across this barrier results in potential energy that can be used for biochemical purposes because energetics favors the flow of these protons back down the gradient to the side of the membrane with fewer protons. The enzyme ATP synthase takes advantage of this proton potential, allowing protons to flow back over the membrane, but coupling that energy to form ATP.
The proton motive force has two components: the chemical pH gradient (∆pH) and the electrochemical gradient (∆Ψ) from the ionic imbalance. In the context of photosynthesis, the ∆pH component plays a major role in facilitating mechanisms of light energy dissipation to shunt energy away from photosynthesis when the system is functioning at maximal capacity. The physical relationship as to how the two components are regulated remains unclear, but something must be controlling them since the photosynthetic machinery is particularly sensitive at optimizing the competing processes of photochemical efficiency and photoprotection.
While the chemiosmotic theory is readily accepted today, when this new idea was postulated by Peter Mitchell in 1961, it was a radical idea. He had to argue effectively with his colleagues for more than a decade before there was enough support for his hypothesis (and problems with alternative hypotheses) to gain acceptance. One key experiment was done by Andre Jagendorf to show that chloroplasts use a proton gradient to make ATP. He soaked chloroplasts in low pH buffer to ensure lots of H+ ions were present in the thylakoid lumen. These low pH-adapted chloroplasts were then incubated in a high pH buffer in the presence of ADP and Pi. Upon incubation in the higher pH buffer, a gradient was formed across the thylakoid membrane because the thylakoid lumen was still acidic (low pH). This condition resulted in a burst of ATP formation at the expense of the H+ gradient. Note: All of this was done in the dark to eliminate the possibility that the photosynthetic machinery itself was making the ATP. This experiment showed that an artificial pH gradient induced by soaking in buffers with different pH lent great support to the chemiosmotic hypothesis.