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A-Level Biology light-dependent reactions of photosynthesis
What is photosynthesis (light-dependent reactions)?
The light-dependent reactions of photosynthesis happen in the thylakoid membranes of the chloroplast. Light excites electrons in chlorophyll, which drive an electron transport chain that pumps H⁺ into the thylakoid lumen. The H⁺ gradient powers ATP synthase (photophosphorylation). Water is split (photolysis) to replace lost electrons, producing O₂. NADP is reduced to NADPH. ATP and NADPH then power the Calvin cycle in the stroma.
Worked example
Trace the path of an electron from water through to NADPH in non-cyclic photophosphorylation.
- Light hits Photosystem II (PSII), exciting an electron to a higher energy level.
- The excited electron leaves PSII; it's replaced by an electron from photolysis of water (2H₂O → 4H⁺ + 4e⁻ + O₂).
- The electron passes down the electron transport chain in the thylakoid membrane. As it falls in energy, H⁺ is pumped into the thylakoid lumen.
- The electron reaches Photosystem I (PSI), where light re-excites it to a higher energy level.
- The re-excited electron from PSI is passed to NADP, reducing it to NADPH (with an H⁺).
- Meanwhile, the H⁺ accumulated in the lumen flows back through ATP synthase, generating ATP from ADP + Pi.
The three products of the light-dependent stage
Each ATP and NADPH molecule made in the thylakoids is used in the Calvin cycle to fix CO₂ into G3P (glyceraldehyde-3-phosphate). Oxygen is released as a by-product of photolysis. Without the light-dependent reactions, the Calvin cycle stops within seconds — ATP and NADPH are not stored in significant amounts.
- ATP — chemical energy currency for the Calvin cycle.
- NADPH — reducing power (electrons + H⁺) to convert glycerate-3-phosphate to G3P.
- O₂ — released through stomata into the atmosphere.
Cyclic vs non-cyclic photophosphorylation
Non-cyclic (described above) involves both photosystems and produces ATP + NADPH + O₂. Cyclic uses only Photosystem I; the electron cycles back to PSI via the electron transport chain, generating only ATP (no NADPH, no O₂). Cells use cyclic photophosphorylation to top up ATP when the Calvin cycle demand for ATP exceeds NADPH demand.
Photolysis and oxygen evolution
PSII contains the oxygen-evolving complex, which splits water using light energy: 2H₂O → 4H⁺ + 4e⁻ + O₂. The electrons replace those lost from PSII. The H⁺ contributes to the lumen gradient. The O₂ is the source of atmospheric oxygen that supports aerobic respiration in nearly every organism.
Where it appears in the exam
AQA 7402, Edexcel 9BI0 and OCR H420 all examine the light-dependent stage on the synoptic A-Level Biology papers. Common formats: trace-the-electron diagrams, six-mark essay questions on the stages, and the practical-skills paper on measuring photosynthesis rate.
Common mistakes
- Confusing PSI (Photosystem I, the SECOND in the electron chain) with PSII (Photosystem II, the FIRST). PSII fires first.
- Saying water 'donates electrons to NADP'. Water donates electrons to PSII; NADPH is reduced at the end of PSI, not directly by water.
- Treating cyclic and non-cyclic photophosphorylation as alternatives. They run simultaneously in the same chloroplast.
- Forgetting to mention oxygen as a product of photolysis (a common 6-mark loss).
- Confusing the light-dependent stage (thylakoids) with the Calvin cycle (stroma).
Frequently asked
- Why is the light-dependent stage called 'non-cyclic' even though some electrons cycle?
- The MAIN pathway in the light-dependent stage is non-cyclic — electrons flow from water through both photosystems to NADP, ending at NADPH. The auxiliary cyclic pathway (electrons looping back to PSI) is a separate process, not the default.
- Where exactly do the H⁺ ions go?
- H⁺ accumulates in the thylakoid lumen (the inside of the thylakoid disc). The lumen acts as a reservoir of H⁺ at much higher concentration than the stroma. The gradient is what drives ATP synthase as H⁺ flows back through it into the stroma.
- Does the light-dependent stage require CO₂?
- No — that's the Calvin cycle. The light-dependent stage only needs light, water (for photolysis), ADP + Pi (to make ATP) and NADP (to be reduced). CO₂ fixation happens in the stroma during the Calvin cycle.
A-Level Biology glossary terms
- Photosynthesis (light-dependent reactions)The light-dependent reactions of photosynthesis happen in the thylakoid membranes of chloroplasts. Light excites electrons in chlorophyll, driving an electron transport chain that pumps H⁺ into the thylakoid lumen. The H⁺ gradient drives ATP synthase (photophosphorylation). Water is split (photolysis) to replace lost electrons, producing O₂. NADP is reduced to NADPH. ATP and NADPH then power the Calvin cycle in the stroma.
- Calvin cycleThe Calvin cycle is the second stage of photosynthesis, happening in the chloroplast stroma. It uses ATP and NADPH (from the light-dependent reactions) to fix carbon dioxide into glyceraldehyde-3-phosphate (G3P), eventually producing glucose. Three phases: CO₂ fixation (catalysed by RuBisCO), reduction (G3P from GP using ATP + NADPH), and regeneration of ribulose bisphosphate (RuBP). Named after Melvin Calvin (1961 Nobel Prize). Independent of light directly, but stops when ATP/NADPH run out.
- ChemiosmosisChemiosmosis is the synthesis of ATP driven by a proton (H⁺) gradient across a membrane. In photosynthesis, light energises electrons that pump H⁺ into the thylakoid lumen; the gradient drives ATP synthase. In respiration, the electron transport chain pumps H⁺ into the mitochondrial inter-membrane space; ATP synthase converts the flow-back into ATP. The principle (proposed by Peter Mitchell, 1978 Nobel Prize) is central to both energy-converting processes in cells.
- DNA replicationDNA replication is the process by which a cell copies its DNA before division. The double helix unwinds at a replication fork. Each strand serves as a template; DNA polymerase adds complementary nucleotides (A-T, G-C). The leading strand is synthesised continuously; the lagging strand is built in Okazaki fragments. The result is two identical double helices, each with one original (parent) strand and one new (daughter) strand — hence 'semi-conservative'. Demonstrated by Meselson and Stahl in 1958.
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