Photolysis of Water: The First Step That Makes Photosynthesis Possible
Think about what a plant needs to build sugar from carbon dioxide. It needs a source of electrons — reducing power — and a source of hydrogen atoms (protons). Where do these come from? The answer is water, and the process that breaks water open is called photolysis (photo = light, lysis = splitting).
The intuition is simple: water is a stable molecule. To tear it apart, you need energy. In photosynthesis, that energy comes directly from sunlight, captured by the pigment system of Photosystem II (PS II). The result is three useful products: electrons (to replace those lost from the reaction centre), protons (to build a proton gradient for ATP synthesis), and molecular oxygen (a waste product that changed Earth's atmosphere forever).
The Precise Statement
Photolysis of water is the light-driven oxidation of water molecules, catalysed by the oxygen-evolving complex (OEC) associated with Photosystem II. The overall reaction is:
2H2Olight, PS II4H++4e−+O2
Four electrons are extracted from two water molecules. These electrons are used to replenish the electrons lost from the reaction-centre chlorophyll P680 when it is excited by light. The protons released contribute to the proton motive force across the thylakoid membrane. The oxygen gas is released into the atmosphere.
Important
Photolysis is not a single step. It is a four-step cycle driven by four successive photons. Each photon removes one electron from the OEC, and after four such events, two water molecules are fully oxidised and O2 is released.
Why This Matters for the Exam
You must remember three things:
Location: The oxygen-evolving complex on the lumenal side of the thylakoid membrane, inside PS II.
Products: 4 H+, 4 e−, and 1 O2 per two water molecules.
Purpose: The electrons replace those lost by P680+ (the oxidised form of the reaction-centre chlorophyll). Without photolysis, the electron transport chain would stop.
Watch out
A common mistake is to think photolysis happens in PS I. It does not. PS I receives electrons from plastocyanin, which ultimately come from PS II. The splitting of water is exclusive to PS II.
The Mechanism in Brief (for deeper understanding) …
When Photosystem I and Photosystem II operate together during the light reactions, electrons lost from PS II's reaction centre must be replenished from an external source. …
Photolysis of water occurs in Photosystem II, as part of non-cyclic photophosphorylation.
During the light reactions, when both Photosystem I and Photosystem II operate together (non-cyclic photophosphorylation / the Z-scheme), electrons excited and lost from the reaction centre of PS II (P680) must be replaced. This is achieved by splitting a water molecule (photolysis), which releases electrons (to replenish PS II), protons (H+, contributing to the proton gradient), and oxygen (released as a by-product — the source of atmospheric O2 from photosynthesis). This water-splitting step occurs on the inner (lumen) side of the thylakoid membrane.
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2026Set ANNUAL1 markMCQ
Q.During which process in photosynthesis, splitting of water takes place?
(a) Calvin cycle
(b) Non-cyclic photophosphorylation
(c) Oxidative phosphorylation
(d) Cyclic photophosphorylation
›Reveal solutionSolution
Photolysis of water occurs in Photosystem II, as part of non-cyclic photophosphorylation.
During the light reactions, when both Photosystem I and Photosystem II operate together (non-cyclic photophosphorylation / the Z-scheme), electrons excited and lost from the reaction centre of PS II (P680) must be replaced. This is achieved by splitting a water molecule (photolysis), which releases electrons (to replenish PS II), protons (H+, contributing to the proton gradient), and oxygen (released as a by-product — the source of atmospheric O2 from photosynthesis). This water-splitting step occurs on the inner (lumen) side of the thylakoid membrane.
Photolysis of water is carried out by the water-splitting complex associated with PS II, situated on the inner (lumen-facing) side of the thylakoid membrane.
During the light reactions of photosynthesis, Photosystem II (PS II) oxidises water molecules to replace the electrons it loses when excited by light. This reaction, splitting of water into electrons, protons (H+) and oxygen, is carried out by the water-splitting/oxygen-evolving complex, which is physically located on the inner side of the thylakoid membrane, facing the lumen. This is why the released protons accumulate in the thylakoid lumen and contribute to the proton gradient used for ATP synthesis by ATP synthase.