The process of splitting water molecules in the presence of light energy is called .

Photolysis is the process by which water molecules (H₂O) are split into hydrogen (H⁺) ions, electrons, and oxygen (O₂) using light energy. This process takes place during the light-dependent reactions of photosynthesis, which occur in the thylakoid membranes of chloroplasts in green plants, algae, and some bacteria. The term "photolysis" is derived from the Greek words “photo” meaning light and “lysis” meaning splitting.

When sunlight strikes the chlorophyll pigment, it excites the electrons within the photosystem II complex. This energy is then used to break down water molecules into their components:

2H2O→4H++4e−+O22H₂O → 4H⁺ + 4e⁻ + O₂2H2O→4H++4e−+O2

The oxygen released from this reaction diffuses out of the leaf into the atmosphere, while the hydrogen ions and electrons are used to generate ATP (adenosine triphosphate) and NADPH, two important energy carriers. These molecules then drive the Calvin cycle, which converts carbon dioxide (CO₂) into glucose (C₆H₁₂O₆).

Photolysis is essential because it is the primary source of atmospheric oxygen and provides the reducing power needed for photosynthesis. Without photolysis, plants could not produce oxygen or store solar energy as chemical energy.

In contrast:

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  Electrolysis uses electrical energy to split water, not light.

  
  


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  Radiolysis involves radiation to decompose water molecules.

  
  


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  Thermolysis is the breakdown caused by heat.

  
  

Thus, photolysis is a unique light-driven reaction that sustains life on Earth by supplying oxygen and energy for the biosphere.