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What happens during the light-dependent reactions of photosynthesis?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — photosynthesis Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

The light-dependent reactions of photosynthesis occur within the thylakoid membranes of the chloroplast and serve to convert light energy into chemical energy.12

The process involves several key steps:

Light Absorption: Chlorophyll and other pigments absorb sunlight, which excites electrons to higher energy levels. These electrons are then transferred through an electron transport chain.134
Photolysis: To replace the electrons lost by chlorophyll, water molecules are split in a process called photolysis. This releases oxygen as a by-product and contributes hydrogen ions to the interior of the thylakoid.154
Energy Synthesis: The flow of electrons establishes a proton gradient across the thylakoid membrane. This gradient provides the potential energy required to drive the synthesis of ATP from ADP. Additionally, electrons are used to reduce NADP⁺ to NADPH.154

Both ATP and NADPH are essential products that provide the energy and reducing power required for carbon fixation during the subsequent dark reactions.12