A Simple Version of Photosynthesis

The 1988 Nobel Prize went to Johann Deisenhofer, Robert Huber, and Hartmut Michel for solving the structure of the first photosystem [see Nobel Laureates]. The photosystem was isolated from a purple bacterium and those bacteria have a relatively simple form of photosynthesis compared to cyanobacteria and chloroplasts.

It's worth looking at this simple version because it illustrates the main principles of photosynthesis without getting bogged down in excessive detail.

The type of photosystem is called photosystem II or PSII. Photons of light are absorbed by the chlorophyll molecules (P870) in this complex. Excited electrons are ejected from the chlorophyll molecules and they pass down a short path where they are picked up by quinone (Q). When Q acquires a pair of electrons, it brings in two protons from the cytoplasm (below) to form QH₂.


QH₂ diffuses in the membrane to another protein complex called the cytochrome bc₁ complex. This is the same complex that works in membrane associated electron transport, or respiration (as in mitochondria and non-photosynthetic bacteria: see Ubiquinone and the Proton Pump). The cytochrome bc₁ complex catalyzes the oxidation of QH₂ causing the release of protons on the outside of the membrane. The reaction—one of the most important reactions in biochemistry—is called the Q-cycle.

The net effect of these reactions is a light-driven proton pump that creates a gradient across the membrane. This is exactly what happens in respiration as well. The proton gradient, or protonmotive force, drives the synthesis of ATP by ATP synthase, another membrane protein.

The electrons that were ejected from the chlorophylls need to be replaced. The original electrons are passed on to cytochrome c by the cytochrome bc₁ complex during the Q-cycle reactions. Cytochrome c then diffuses back to the photosystem were it resupplies electrons to the chlorophylls in a cyclic pathway.

This is how light drives the synthesis of ATP.