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How does photosynthesis convert sunlight into stored chemical energy?

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Underwater view of a glowing chain of tunicates with a scuba diver and boat visible near the water's surface in the background. — photochemical reaction Chain of fluorescent tunicates.

Photosynthesis converts sunlight into stored chemical energy through a two-stage process involving light-dependent and light-independent reactions.

The Light Reactions

The process begins in the chloroplast membranes, where pigments like chlorophyll absorb sunlight. This energy excites electrons, which are then transferred through an electron transport chain. As electrons move, they facilitate the transport of hydrogen ions across the membrane, creating a proton gradient that stores energy, much like a charged battery. During this phase, water is split—a process called photolysis—to replace the lost electrons, releasing oxygen as a byproduct. This stage generates ATP and NADPH, which serve as energy carriers.123

The Dark Reactions (Carbon Fixation)

In the subsequent light-independent reactions, such as the Calvin-Benson cycle, the plant uses the ATP and NADPH produced earlier to convert carbon dioxide into energy-rich sugars like glucose. This chemical energy can then be stored as starch or used to fuel the organism's metabolic needs.1435