Photosynthetic Energy is Coupled to Metabolism
The energy of the electrons in NADPH are not sufficient for anabolic reactions such as CO2 fixation. These processes also require biochemical energy in the form of ATP. The process of photosynthetic electron transfer also generates a gradient of protons (H+) across the photosynthetic membrane (Figure: Proton Gradient).
The transmembrane gradient of protons represents potential chemical energy. The chloroplast thylakoid membrane contains an ATPase complex that is able to convert the trans-membrane proton gradient into the production of ATP (Figure: ATP Formation). This entire process, including the formation of ATP is termed photophosphorylation. CLICK HERE to see this energy transfer in the context of the plant cell. The processes of photosynthetic electron transport and photophosphorylation are used to capture the energy in photons and convert it to high-energy electrons in the form of NADPH and biochemical energy in the form of ATP. With these two molecules, there is sufficient energy to carry out the fixation of CO2 into sugars via the Calvin cycle and to drive the other anabolic reactions of the plant cell. Excess energy formed by photosynthesis is conserved as starch. In the dark, the plant is able to mobilize these sugars and obtain energy by respiration in a fashion similar to the normal processes of microbes and animals.