Biology – Photosynthesis as an energy transfer process | e-Consult

The light-dependent reactions and the light-independent reactions (Calvin Cycle) are tightly coupled through the generation and utilization of energy carriers: ATP and NADPH. The light-dependent reactions capture light energy and convert it into chemical energy in the form of ATP and NADPH. Specifically, light energy drives the electron transport chain, leading to the pumping of protons (H+) into the thylakoid lumen, creating a proton gradient. This gradient is then used by ATP synthase to generate ATP (photophosphorylation). Simultaneously, electrons are passed through the electron transport chain, ultimately reducing NADP+ to NADPH.

The ATP and NADPH produced in the light-dependent reactions are then used in the Calvin Cycle to fix carbon dioxide (CO2) and produce glucose. The Calvin Cycle requires ATP for the phosphorylation of CO2 and NADPH for the reduction of the resulting 3-phosphoglycerate to glyceraldehyde-3-phosphate (G3P), a precursor to glucose. Therefore, the ATP and NADPH act as energy and reducing power, respectively, to drive the carbon fixation process. Without the ATP and NADPH generated during the light-dependent reactions, the Calvin Cycle would not be able to proceed.