The photosynthetic process relies on electron flow that involves several complexes in the thylakoid membranes of photosynthetic organisms. This flux of electrons can follow two competing pathways. Linear electron transfer from water is coupled to the generation of a transmembrane potential difference that allows for the synthesis of ATP and NADPH, while cyclic electron transport only allows for ATP production. These two pathways are both critical for the reduction of CO2 and likely coexist to establish an ATP/NADPH ratio that is required for sustained reduction of CO2 into carbohydrates.