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C1.3.11 (HL)—Photolysis of water

Photolysis splits water during the light-dependent reactions, supplying electrons and protons while releasing oxygen as a by-product, connecting the mechanism to observable photosynthesis evidence.

Syllabus
First assessment 2025
Objective
C1.3.11
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceMay 2022
Most common paperPaper1
Typical marks1–3

Common command terms

  • Describe
  • Explain

Recent exam appearances

May 2022Paper1 ["HL"] · TZ231[ 1 ]C1.3.11 (HL)—Photolysis of water
May 2021Paper1 ["HL"] · TZ231[ 1 ]C1.3.11 (HL)—Photolysis of water
November 2019Paper2 ["HL"] · TZ04(b)(ii)[ 3 ]C1.3.11 (HL)—Photolysis of water
November 2017Paper2 ["HL"] · TZ04(b)[ 3 ]C1.3.11 (HL)—Photolysis of water
May 2013Paper2 ["HL"] · TZ27(c)[ 8 ]C1.3.11 (HL)—Photolysis of water
Practice this objective

Coverage 2013–2022 · Updated 15 Jul 2026

Photolysis and NADP reduction

HL only
Thylakoid membrane showing PSII, electron carriers, PSI, photolysis, proton movement and NADP reduction.

In photosystem II, light-driven photolysis splits water; later, photosystem I supplies excited electrons that reduce NADP on the stromal side of the thylakoid.

2H2O→4H++4e−+O22H₂O → 4H⁺ + 4e⁻ + O₂

Photolysis replaces electrons lost by photosystem II. Its protons contribute to the thylakoid proton gradient and its oxygen is released as waste; oxygen generation profoundly changed Earth's living and geological systems.

At photosystem I, NADP accepts two excited electrons originating from photosystem I and one H⁺ from the stroma to form reduced NADP (NADPH), which carries reducing power to the Calvin cycle.

Use one consistent pair of names: NADP/reduced NADP or NADP⁺/NADPH. The released O₂ comes from water, not carbon dioxide, and photolysis does not directly make glucose.

Photolysis of water

HL only

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through structured response, commonly using Describe / Explain.

Command terms

Describe / Explain

What earns marks

Build the answer around this relationship: Photolysis uses light energy to split water.

Representative question

Question 1

[Maximum number: 8]

Some of the water carried to the leaves of a plant is used in photosynthesis. Explain the role of water in the light-dependent reactions of photosynthesis.

Trace Photosynthesis Mechanism

HL only

HL photosynthesis links light-dependent reactions to the Calvin cycle. Photosystems are pigment-protein arrays in thylakoid membranes; antenna pigments broaden absorption and pass energy to reaction-centre chlorophyll. PSII uses light to split water, supplying electrons, protons, and oxygen. Electron transport pumps protons into the thylakoid space, ATP synthase makes ATP by photophosphorylation, PSI re-excites electrons, and NADP is reduced. The Calvin cycle uses ATP and reduced NADP: Rubisco fixes CO2 to RuBP to form GP, GP is reduced to triose phosphate, most triose phosphate regenerates RuBP, and some becomes wider biomolecules.

  • Trace energy: pigments -> reaction centres -> ATP and reduced NADP.
  • Trace electrons/protons: water -> PSII -> carriers -> PSI -> NADP; proton gradient -> ATP synthase.
  • Trace carbon: CO2 -> RuBP/GP -> triose phosphate -> RuBP regeneration and biomolecule synthesis.
  • Trace exchange: light-dependent reactions supply ATP/reduced NADP; Calvin cycle returns ADP/NADP.

Concept essentials

  • Photolysis uses light energy to split water.
  • Electrons from water replace electrons lost by Photosystem II.
  • Photolysis produces protons that support the thylakoid proton gradient.
  • Oxygen is released as a by-product of water splitting.