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photosystem II
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Diagram of the light reactions of photosynthesis
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photosystem II

biology
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Diagram of the light reactions of photosynthesis
Diagram of the light reactions of photosynthesis The light reactions occur in two photosystems (units of chlorophyll molecules). Light energy (indicated by wavy arrows) absorbed by photosystem II causes the formation of high-energy electrons, which are transferred along a series of acceptor molecules in an electron transport chain to photosystem I. Photosystem II obtains replacement electrons from water molecules, resulting in their split into hydrogen ions (H+) and oxygen atoms. The oxygen atoms combine to form molecular oxygen (O2), which is released into the atmosphere. The hydrogen ions are released into the lumen. Additional hydrogen ions are pumped into the lumen by electron acceptor molecules. This creates a high concentration of ions inside the lumen. The flow of hydrogen ions back across the photosynthetic membrane provides the energy needed to drive the synthesis of the energy-rich molecule adenosine triphosphate (ATP). High-energy electrons, which are released as photosystem I absorbs light energy, are used to drive the synthesis of nicotine adenine dinucleotide phosphate (NADPH). Photosystem I obtains replacement electrons from the electron transport chain. ATP provides the energy and NADPH provides the hydrogen atoms needed to drive the subsequent photosynthetic dark reaction, or Calvin cycle.

Photosystem II (PSII) is a protein complex crucial for photosynthesis, located in the thylakoid membranes of chloroplasts. It captures light energy, exciting electrons within chlorophyll molecules. These energized electrons are then passed along an electron transport chain.12

To replace the lost electrons, PSII splits water molecules in a process called photolysis. This splitting releases oxygen as a byproduct, along with hydrogen ions that contribute to generating ATP, the cell's energy currency. The excited electrons, along with hydrogen ions, are used to create NADPH, another energy-carrying molecule. Both ATP and NADPH are then used in the subsequent stages of photosynthesis to produce sugars.132

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4 Britannica Sources
  1. photosynthesis - The pathway of electrons
  2. photosynthesis - Evidence of two light reactions
  3. photosynthesis - Step two: The conversion of light energy to ATP
  4. cell - Trapping of light

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