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photosystem I
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Diagram of the light reactions of photosynthesis
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photosystem I

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 I (PSI) is a crucial component of photosynthesis, found in the thylakoid membranes of chloroplasts. It's one of two photochemical reaction centers that absorb sunlight. PSI primarily absorbs light with a wavelength of about 700 nanometers.12

During the light-dependent reactions of photosynthesis, PSI captures light energy, exciting electrons. These energized electrons are then passed along an electron transport chain, contributing to the production of ATP and NADPH, which are energy-carrying molecules essential for sugar synthesis. To replace the lost electrons, PSI receives them from photosystem II, which in turn obtains electrons from the splitting of water molecules. This process is fundamental to converting light energy into chemical energy that plants use to grow.134

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4 Britannica Sources
  1. photosynthesis - The pathway of electrons
  2. photosystem
  3. cell - Trapping of light
  4. plant - Basic mechanisms

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