What is photolysis of water?

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Q. What is photolysis of water?

Tamil Nadu DgeTamil Nadu HSC First Year (DGE) Board 2020Subjective· 2mImportance★★★★★

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Concept understanding — Light Reaction

Light Reaction: The Engine That Runs on Sunlight

Imagine a solar panel. Sunlight hits it, and out comes electricity. The light reaction in photosynthesis is exactly that — a solar panel built into every green leaf. But instead of electricity, it produces two chemical fuels: ATP (the energy currency of the cell) and NADPH (a reducing agent that carries high-energy electrons). These two molecules are then used in the next phase (the Calvin cycle) to build glucose from carbon dioxide.

The key insight: the light reaction is the only part of photosynthesis that directly uses light. Everything after it runs on the ATP and NADPH it produces.

Where does it happen?

Inside the chloroplast, there are flattened, disc-like sacs called thylakoids. Their membranes are packed with pigment molecules (chlorophyll a, chlorophyll b, carotenoids) and protein complexes. This is the stage. The space inside the thylakoid is the lumen; the fluid outside is the stroma.

What actually happens? (The precise statement)

When a photon of light strikes a chlorophyll molecule, an electron gets excited — it jumps to a higher energy level. That excited electron is then passed through a chain of carriers embedded in the thylakoid membrane, like a bucket brigade. As it moves, its energy is used to pump protons ( $H^+$ ) from the stroma into the thylakoid lumen, building up a concentration gradient. That gradient drives ATP synthesis (via ATP synthase). Meanwhile, the electron eventually ends up reducing NADP $^+$ to NADPH.

But here's the catch: the chlorophyll that lost an electron needs to replace it. That electron comes from water ( $H_2O$ ). Splitting water releases oxygen gas ( $O_2$ ) as a byproduct — that's the oxygen we breathe.

2H_2O + 2NADP^+ + 3ADP + 3P_i \xrightarrow{\text{light}} O_2 + 2NADPH + 3ATP

The two photosystems: a closer look

There are two distinct pigment systems, Photosystem II (PSII) and Photosystem I (PSI), named in the order they were discovered, not the order they work. Light hits both.

PSII absorbs light, excites an electron, and passes it to an electron acceptor. The electron hole in PSII is filled by splitting water. This releases $O_2$ and protons into the lumen.
The electron travels down an electron transport chain (cytochrome $b_6f$ complex), pumping protons along the way.
The electron reaches PSI, which also absorbs light and re-excites the electron to an even higher energy level.
That high-energy electron is finally used to reduce NADP $^+$ to NADPH.

The proton gradient built up during step 2 drives ATP synthase to make ATP. This entire flow is called non-cyclic photophosphorylation — the most common pathway.

Note

There is also a cyclic pathway where electrons from PSI cycle back to the electron transport chain instead of reducing NADP $^+$ . This produces only ATP, no NADPH or oxygen. It happens when the cell needs extra ATP.

Why does water split? …

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Photolysis (light-splitting) of water is the reaction, occurring at the oxygen-evolving complex of Photosystem II, that breaks water into protons, electrons, and molecular oxygen; the electrons replace those PS II loses to light excitation, and the O2 released is the oxygen we breathe.

During the light reaction of photosynthesis, when Photosystem II's reaction centre (P680) absorbs light energy, it becomes excited and loses (donates) an electron to the electron transport chain. To keep functioning, PS II must continuously replace this lost electron. This replacement comes from splitting water molecules, a reaction called photolysis:

2H2O -> 4H+ + 4e- + O2

This reaction is catalysed by the oxygen-evolving complex (OEC, a manganese-containing protein cluster) associated with PS II, and it was demonstrated experimentally by Robert Hill (the "Hill reaction"), who showed that isolated chloroplasts, in the presence of a suitable electron acceptor and light, could split water and evolve oxygen even without CO2 being fixed.

Significance of photolysis:

It supplies the electrons needed to keep the light reaction's electron flow going (replacing electrons continuously lost from PS II). …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set ANNUAL1 markMCQ
$Q. Hill reaction occurs in which part of the chloroplast ?$
(a) Stroma
(b) Inner membrane
(c) Grana
(d) Outer membrane
›Reveal solutionSolution
The Hill reaction occurs in the grana (thylakoids) of the chloroplast, so the answer is (C).

The chloroplast has two main regions:

Grana (stacks of thylakoids) — site of the light reaction, including the Hill reaction (light-driven splitting of water and release of O2), electron transport and ATP/NADPH formation. …
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CBSE 2022Set ANNUAL1 markMCQ
$Q. The light reaction Occurs in$
(a) Grana
(b) Stroma
(c) Cytoplasma
(d) Mitochondria
›Reveal solutionSolution

Light reactions occur on thylakoids (grana); dark reactions in stroma → (a).

From the NCERT Class 11 Biology account of photosynthesis in higher plants:

…

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CBSE 2020Set ANNUAL1 markMCQ
$Q. The first event in photosynthesis is :$
(a) Photoexcitation of chlorophyll are electron emission
(b) Photolysis of water
(c) Release of oxygen
(d) Synthesis of ATP
›Reveal solutionSolution

The photosynthetic light reaction begins with light being absorbed by chlorophyll, exciting an electron out of the reaction-centre chlorophyll -- this photoexcitation and electron emission is the very first step, and everything else (electron transport, water-splitting, ATP/NADPH synthesis) follows from it.

When light strikes chlorophyll in the reaction centre of a photosystem, it excites an electron to a higher energy state, and that excited electron is emitted/passed to an electron acceptor, starting the electron transport chain. The chlorophyll molecule, now electron-deficient, is replenished by an electron pulled from the splitting of water (photo …

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CBSE 2020Set ANNUAL1 markMCQ
$Q. Identify true statement regarding light reaction of photosynthesis.$
(a) Splitting of water molecule is associate with PS I.
(b) PS I and PS II are involved in the formation of NADPH+H+.
(c) The reaction center of PS I is chlorophyll "a" with absorption peak at 680 nm.
(d) The reaction center of PS II is chlorophyll "a" with absorption peak at 700 nm.
›Reveal solutionSolution
NADPH formation requires the combined, linear action of both photosystems (the Z-scheme): PS II supplies the electrons (from water splitting) that flow via the electron transport chain to PS I, where they are finally used to reduce NADP+ to NADPH.

Check each option against the actual light reaction mechanism:

"Splitting of water molecule is associated with PS I" — FALSE. Photolysis of water (2H2O -> 4H+ + 4e- + O2) occurs at the oxygen-evolving complex associated with Photosystem II (PS II), not PS I. The electrons released here replace those lost from PS II's reaction centre.

"PS I and PS II are involved in the formation of NADPH+H+" — TRUE. In the Z-scheme (non-cyclic/linear electron flow), light-excited electrons leave PS II, pass down an electron transport chain (plastoquinone -> cytochrome b6f -> plastocyanin) to PS I, are re-energised by light at PS I, and are finally passed via ferredoxin to the enzyme ferredoxin-NADP+ reductase, which reduces NADP+ to NADPH + H+. Since the electrons that end up reducing NADP+ originated at PS II and were re-energised at PS I, both photosystems are genuinely involved in NADPH formation. …
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CBSE 2020Set ANNUAL1 mark
$Q. Fill in the blank: In photosynthesis, the light reaction is completed in __________. (choose: Grana / stroma)$
›Reveal solutionSolution

The light-dependent reactions occur in the grana (thylakoid membranes); the light-independent (dark) reactions occur in the stroma.

The chloroplast is differentiated into grana (stacks of thylakoid membranes bearing chlorophyll and other pigments organized into Photosystem I and II) and the stroma (the fluid matrix around the grana). Light energy is absorbed by pigments in the thylakoid membranes, driving photolysis of water, electron transport, and ATP + NADPH synthesis — all within the grana. The stro …

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CBSE 2018Set BOTANY1 mark
$Q. Where does the light reaction of photosynthesis take place?$
›Reveal solutionSolution

The light reaction of photosynthesis takes place in the thylakoid membranes (grana) of the chloroplast.

The light reaction (also called the light-dependent reaction, or the Hill reaction) of photosynthesis occurs in the thylakoid membrane system of the chloroplast — the flattened, membrane-bound sacs (thylakoids) stacked into structures called grana, which are embedded in the stroma. This is where the photosynthetic pigments (chlorophylls, carotenoids), the two photosystems (PS I and PS II), and the electron transport chain components are located. Here, light …

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Q.What is photolysis of water?

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