Step 1: Photosystems

Diagram showing the two photosystems in the light-depend reaction.

The light-dependent portion of photosynthesis is carried out by two consecutive photosystems (photosystem I and photosystem II) in the thylakoid membrane of the chloroplasts. The photosystems are driven by the excited chlorophyll molecules.

To begin photosynthesis, the chlorophyll molecule in photosystem II is excited by sunlight and the energy produced helps to break down a water molecule (H₂O) into ½O₂ (with electrons removed) and 2H⁺. The removed electrons are excited by the light energy. When the electrons prepare to come to their rest state, they go through an oxidative phosphorylation process and produces an ATP molecule.

As the electrons are coming to a resting state, they are excited again in photosystem I and raised to a even higher energy state. The excited electrons are then used to produce NADP⁺ + H⁺. The highly energetic NADPH molecule is then fed into the Calvin Cycle to conduct carbon fixation.

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