Where exactly does water splitting occur in the chloroplast?

Water splitting (photolysis) occurs on the inner side (lumen side) of the thylakoid membrane, physically associated with Photosystem II. Because the reaction takes place inside the thylakoid, the protons (H⁺) released are deposited directly into the thylakoid lumen, building the proton gradient used for ATP synthesis.

Which micronutrient is essential for the splitting of water during photosynthesis?

Manganese (Mn) is the essential micronutrient. The oxygen-evolving complex (OEC) at PSII contains a cluster of four manganese ions that accumulate oxidising equivalents across the S0–S4 states of the Kok cycle. Without manganese, the charge-accumulation steps cannot occur and water cannot be split. This was the answer to NEET 2023 Q.106.

How did Ruben and Kamen prove that oxygen in photosynthesis comes from water and not CO₂?

Samuel Ruben and Martin Kamen (1941) used heavy oxygen (¹⁸O) as a tracer. They supplied plants with H₂¹⁸O (heavy water) and found that the oxygen released was ¹⁸O₂. When ¹⁸O was supplied as C¹⁸O₂ instead, the released oxygen was normal ¹⁶O₂. This confirmed that the O₂ evolved in photosynthesis originates exclusively from the splitting of water molecules.

Why are four electrons needed to release one molecule of O₂?

One water molecule yields two electrons upon oxidation (H₂O → 2H⁺ + 2e⁻ + ½O₂). To form one complete O₂ molecule, two oxygen atoms must be released, requiring the oxidation of two water molecules and the removal of four electrons in total. The Mn₄CaO₅ cluster in the OEC accumulates these four oxidising equivalents step by step (S0→S1→S2→S3→S4→S0) before O–O bond formation and O₂ release.

What is the S-state cycle and who proposed it?

The S-state (or Kok) cycle, proposed by Bessel Kok in 1970, describes how the oxygen-evolving complex accumulates four oxidising equivalents before releasing O₂. The OEC cycles through five states (S0 to S4). Each light-driven charge separation at P680 advances the OEC by one S-state. At the S4 state, two water molecules are oxidised, O₂ is released, and the complex resets to S0.

How does water splitting contribute to the proton gradient used in chemiosmosis?

Water is split on the lumen side of the thylakoid membrane, so the four protons (H⁺) produced per O₂ are released directly into the thylakoid lumen. This increases the H⁺ concentration inside the lumen relative to the stroma, contributing to the electrochemical proton gradient (pmf) that drives ATP synthesis through ATP synthase (CF₀–CF₁).

How does water splitting in plants differ from H₂S splitting in green sulphur bacteria?

In oxygenic photosynthesis (plants, cyanobacteria), water (H₂O) is the electron donor; splitting it releases O₂ as a by-product and requires a powerful oxidant (P680⁺, ~+1.2 V). In anoxygenic photosynthesis (green sulphur bacteria), hydrogen sulphide (H₂S) is the electron donor; splitting it releases elemental sulphur (S) or sulphate, not O₂. H₂S is a much weaker reductant and can be oxidised by a less powerful reaction centre, so these bacteria use only one photosystem and do not evolve oxygen.

Splitting of Water — Photolysis — NEET Notes

NCERT grounding

NCERT Class 11 Biology, Chapter 11, Section 11.6.1 states: "The splitting of water is associated with the PS II; water is split into 2H⁺, [O] and electrons… the water splitting complex is associated with the PS II, which itself is physically located on the inner side of the membrane of the thylakoid." The same chapter's summary reinforces: "Splitting of water molecules is associated with PS II resulting in the release of O₂, protons and transfer of electrons to PS II."

"The water splitting complex is associated with PS II, which is physically located on the inner side of the membrane of the thylakoid."

NCERT Biology Class 11, Chapter 11, §11.6.1

NIOS Biology Chapter 11 adds the term explicitly: "This light-dependent splitting of water is called photolysis." Both sources agree that the oxygen released in photosynthesis comes from water, not from CO₂, and that photolysis is the event that replenishes electrons lost from P680.

Mechanism of photolysis

When Photosystem II absorbs a photon of red light (~680 nm), the reaction-centre chlorophyll a molecule designated P680 is excited. An electron from P680 is transferred to the primary electron acceptor (pheophytin), leaving P680 in a strongly oxidised state — written as P680⁺. P680⁺ has a reduction potential of approximately +1.2 V, making it the most powerfully oxidising species known in biology. This extreme oxidising power is what drives the thermodynamically unfavourable oxidation of water.

The oxidised P680⁺ extracts electrons from a nearby manganese-protein assembly embedded on the lumenal face of the thylakoid membrane. This assembly is the oxygen-evolving complex (OEC), also called the water-splitting complex. Each time P680 is excited and transfers an electron to the acceptor chain, P680⁺ draws one electron from the OEC, incrementally oxidising the manganese cluster until it has accumulated sufficient oxidising equivalents to split two water molecules simultaneously, releasing molecular oxygen.

The oxygen-evolving complex and the role of manganese

The OEC contains an inorganic core — a cluster of four manganese (Mn) ions, one calcium (Ca) ion, and five bridging oxygen atoms, often written Mn₄CaO₅. This cluster is the catalytic heart of water oxidation. The manganese ions cycle through oxidation states (Mn²⁺ through Mn⁴⁺) as they accumulate electrons removed from water.

NEET-critical fact: Manganese (Mn) is the micronutrient required for splitting of water during photosynthesis. It acts as the redox-active cofactor of the OEC — directly tested in NEET 2023.

Location of OEC

Lumen side

of thylakoid membrane

Physically associated with PSII, on the inner (lumenal) face of the thylakoid membrane.

NCERT §11.6.1

Key cofactor: Manganese

Mn₄CaO₅

inorganic core of OEC

4 Mn ions cycle through oxidation states; Ca²⁺ and Cl⁻ are also required for full activity.

NEET 2023 Q.106

Reaction stoichiometry

2H₂O

per O₂ molecule released

2H₂O → 4H⁺ + 4e⁻ + O₂. Four electrons per O₂ because each water provides 2e⁻.

Concept

The S-state cycle (Kok cycle)

Water oxidation is not a single-step event. Because P680 transfers one electron at a time — one per photon absorbed — the OEC must accumulate four oxidising equivalents before it can release one O₂ molecule. This stepwise charge accumulation is described by the S-state (Kok) cycle, proposed by Bessel Kok in 1970.

S-state cycle — charge accumulation in the OEC

One photon advances the OEC by one S-state

S0
Ground state

Fully reduced OEC after O₂ release; most reduced state of the Mn cluster.
Start / reset
S1
1 charge stored

One electron removed from OEC by P680⁺. Dark-stable state in resting leaves.
+1 oxidation
S2
2 charges stored

Second electron extracted. EPR-active state used in research to study Mn oxidation.
+2 oxidation
S3
3 charges stored

Third electron removed; OEC is highly oxidised, on the verge of water oxidation.
+3 oxidation
S4 → S0
O₂ release

4th electron extracted; OEC oxidises 2 H₂O molecules, forms O₂, resets to S0.
O₂ evolved

The S-state model explains why every fourth flash of light gives a burst of O₂ in dark-adapted leaves — a landmark observation by Pierre Joliot (1969) that Kok subsequently rationalised. The key conceptual point for NEET is that four photons must be absorbed (four charge-separation events at P680) to release one O₂ molecule.

Stoichiometry and products of water splitting

The overall reaction of photolysis, written in the form used by NCERT, is:

2H₂O

Photolysis equation

2H₂O → 4H⁺ + 4e⁻ + O₂
Protons accumulate in the lumen; electrons replenish P680⁺; O₂ diffuses out as by-product.

The three products serve distinct downstream purposes:

Product	Quantity per O₂	Immediate fate	Functional role
Electrons (e⁻)	4	Transferred to oxidised P680⁺	Replenish PSII reaction centre; continue electron transport chain
Protons (H⁺)	4	Released into thylakoid lumen	Build proton gradient (pmf) → drive ATP synthesis via CF₀–CF₁
Oxygen (O₂)	1	Diffuses out of chloroplast and leaf	By-product; sustains aerobic life on Earth

The Ruben–Kamen ¹⁸O tracer experiment

The idea that oxygen in photosynthesis comes from water rather than CO₂ was proposed conceptually by Cornelius van Niel (1930s) based on comparative biochemistry with photosynthetic bacteria. The definitive proof using isotope tracing came from Samuel Ruben and Martin Kamen in 1941.

Ruben–Kamen ¹⁸O Tracer Experiment — Two Arms

Arm A: Heavy water (H₂¹⁸O)

¹⁸O₂

oxygen released

Plants supplied with H₂¹⁸O (water labelled with heavy oxygen)
CO₂ supplied was normal C¹⁶O₂
Released gas was ¹⁸O₂ — labelled oxygen
Conclusion: O₂ originates from H₂O

Arm B: Heavy CO₂ (C¹⁸O₂)

¹⁶O₂

oxygen released

Plants supplied with C¹⁸O₂ (CO₂ labelled with heavy oxygen)
Water supplied was normal H₂¹⁶O
Released gas was normal ¹⁶O₂ — unlabelled
Conclusion: O₂ does NOT come from CO₂

Van Niel's earlier work on purple sulphur bacteria was crucial groundwork. Those bacteria use H₂S as the electron donor and release sulphur rather than O₂ — evidence that the oxidised product tracks the hydrogen donor, not CO₂. NCERT acknowledges this inference: "hence, he inferred that the O₂ evolved by the green plant comes from H₂O, not from carbon dioxide. This was later proved by using radioisotopic techniques."

Link to chemiosmosis and ATP synthesis

Because the OEC is on the inner (lumenal) face of the thylakoid membrane, every proton produced by water splitting is deposited directly into the thylakoid lumen. NCERT states explicitly: "Since splitting of the water molecule takes place on the inner side of the membrane, the protons or hydrogen ions that are produced by the splitting of water accumulate within the lumen of the thylakoids."

Three separate events collectively raise the lumenal H⁺ concentration above that of the stroma: (1) direct deposition of protons from water splitting; (2) proton pumping coupled to plastoquinone reduction and oxidation between PSII and the cytochrome b₆f complex; and (3) proton consumption at the stroma face when NADP⁺ is reduced to NADPH by PSI. Water splitting contributes the most directly and is the primary source of lumenal protons.

Anoxygenic photosynthesis — the sulphur bacteria comparison

Green sulphur bacteria (e.g., Chlorobium) and purple sulphur bacteria perform photosynthesis without splitting water. They use H₂S as the hydrogen donor instead. The generalised equation, originally formulated by van Niel, is:

Feature	Oxygenic (Plants, Cyanobacteria)	Anoxygenic (Green/Purple Sulphur Bacteria)
Electron / H donor	H₂O	H₂S
Oxidised by-product	O₂	S (sulphur) or SO₄²⁻
Photosystems used	PSI + PSII	One photosystem only
Oxygen evolution	Yes	No
OEC / Mn cluster	Present — essential	Absent
Reaction centre redox potential	~+1.2 V (P680⁺) — very oxidising	~+0.5 V — much weaker oxidant

H₂S is a much weaker reductant than H₂O (less negative redox potential), so anoxygenic bacteria do not need the extreme oxidising power of P680⁺ or the Mn-based OEC. This is precisely why they produce no O₂ and why the evolution of oxygenic photosynthesis — requiring the OEC and PSII — was such a pivotal event in Earth's history.

Figure 1

Figure 1. Schematic of water photolysis. The OEC sits on the lumenal face of the thylakoid membrane; 2H₂O are split to yield 4H⁺ (into lumen), 4e⁻ (to P680⁺), and O₂ (released as by-product). The 680 nm photon drives P680 excitation, which creates P680⁺ — the oxidising species that pulls electrons from the OEC.

Worked examples

Worked example 1

How many molecules of water must be split to produce one molecule of glucose (C₆H₁₂O₆) via oxygenic photosynthesis? Show your reasoning.

Solution: The overall equation is 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂. Twelve molecules of water appear on the substrate side. Six molecules of O₂ are released. Since 2H₂O → O₂ + 4H⁺ + 4e⁻, 12 H₂O produces 6 O₂. Therefore, 12 water molecules are split per glucose produced. The 6 H₂O on the product side come from condensation reactions in the Calvin cycle, not from photolysis.

Worked example 2

A student claims: "Water splitting produces NADPH, which is used in the Calvin cycle." Identify the error and correct the statement.

Solution: The error is the conflation of two separate events. Water splitting produces electrons (e⁻) and protons (H⁺), not NADPH directly. The electrons from water splitting replenish P680⁺, then travel through the Z-scheme (via plastoquinone, cytochrome b₆f, plastocyanin) to PSI. At PSI, after a second excitation (P700), the electrons are ultimately used to reduce NADP⁺ to NADPH via ferredoxin-NADP⁺ reductase. Corrected statement: Water splitting provides electrons that, after passing through both photosystems and the electron transport chain, reduce NADP⁺ to NADPH at PSI.

Worked example 3

Why does Mn deficiency in plants specifically impair oxygen evolution rather than the entire light reaction?

Solution: Manganese is the redox-active metal in the OEC's Mn₄CaO₅ cluster. Without Mn, the cluster cannot cycle through its S-states and cannot accumulate the four oxidising equivalents needed to split water. P680 can still absorb photons and excite electrons — the photochemical step at the reaction centre does not require Mn. However, without water as an electron donor, P680⁺ cannot be re-reduced, so PSII stalls and oxygen evolution ceases entirely. Other components of the light reaction (PSI, ATP synthase, ferredoxin) remain structurally intact but are starved of the electron supply that comes from water splitting.

Common confusion & NEET traps

Oxygenic vs Anoxygenic photosynthesis — the key contrasts

Oxygenic (plants, algae, cyanobacteria)

H₂O

electron donor

By-product: O₂
Uses PSII + PSI (Z-scheme)
Requires OEC with Mn₄CaO₅
P680⁺ is the oxidant (~+1.2 V)
Produces NADPH via PSI

Anoxygenic (green/purple sulphur bacteria)

H₂S