Edexcel A-Level Biology: Photosynthesis and Respiration — Complete Revision Guide (9BI0)

Photosynthesis and Respiration is the most mechanistically dense topic on Edexcel A-Level Biology B (9BI0) — and the topic where genuine A* candidates separate from competent A-grade ones. Once you can trace electron flow through the photosystems and the electron-transport chain, balance the carbon and energy accounting per glucose across glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation, and explain chemiosmosis as the central energy-coupling principle, you have the framework for nearly every Paper 2 metabolism question.

This guide is a topic-by-topic walkthrough of the energy and biological processes content. It covers light-dependent reactions of photosynthesis, the Calvin cycle, factors affecting photosynthesis rate, glycolysis, the link reaction and Krebs cycle, oxidative phosphorylation and chemiosmosis, anaerobic respiration and respiratory substrates, factors affecting respiration, the practical methods used to measure photosynthesis and respiration, and the integration of catabolism and anabolism via shared intermediates and ATP. For each topic you will find the core ideas, common pitfalls, a worked example, and a link into the LearningBro Energy and Biological Processes course.

What the Edexcel 9BI0 Specification Covers

Edexcel A-Level Biology B (9BI0) is examined in three written papers. Topic 5 — On the Wild Side — sits in Paper 2 (Energy, Exercise and Coordination), with synoptic Paper 3 questions routinely returning to chemiosmosis, ATP yield calculations, and limiting-factor reasoning.

Photosynthesis-and-respiration questions tend to fall into three styles: short recall on stage products and locations; calculations of ATP yield, respiratory quotient, or limiting factors; and extended-response questions integrating multiple stages or comparing photosynthesis and respiration mechanistically. The table below maps the main sub-topics to a typical paper weighting.

Sub-topic	Spec area	Typical paper weight
Light-dependent reactions	Topic 5	6–8 marks
Calvin cycle	Topic 5	4–6 marks
Factors affecting photosynthesis	Topic 5 / Paper 3	4–6 marks
Glycolysis	Topic 5	4–6 marks
Link reaction and Krebs	Topic 5	6–8 marks
Oxidative phosphorylation	Topic 5	8–12 marks
Anaerobic respiration	Topic 5	4–6 marks
Factors affecting respiration	Topic 5	4–6 marks
Practicals	Paper 3	6–10 marks
Metabolic integration	Topic 5	4–6 marks

These weights are estimates. What is reliable is that an oxidative-phosphorylation or chemiosmosis question and an ATP-yield calculation appear on most papers.

Light-Dependent Reactions of Photosynthesis

The light-dependent reactions occur on the thylakoid membrane inside chloroplasts. They convert solar energy into chemical energy in the form of ATP and NADPH, with O₂ as a by-product from photolysis of water.

The Z-scheme describes electron flow: photons absorbed by photosystem II (P680) excite electrons → captured by primary electron acceptor → flow down plastoquinone (PQ) → cytochrome b6f complex (pumps protons into the thylakoid lumen) → plastocyanin (PC) → photosystem I (P700) → ferredoxin → NADP⁺ + H⁺ → NADPH. PSII's lost electrons are replaced by photolysis of water: 2 H₂O → 4 H⁺ + 4 e⁻ + O₂ (the O₂ comes from water, NOT from CO₂ — Hill's classic isotope experiment proved this).

The proton gradient across the thylakoid membrane drives ATP synthase by chemiosmosis, regenerating ATP from ADP + Pᵢ — exactly the same mechanism as oxidative phosphorylation in mitochondria, but at a different membrane.

Worked example. Predict the consequence of a herbicide that blocks plastoquinone reduction at PSII. Without PQ reduction, the electron-transport chain stalls; the proton gradient collapses; ATP synthesis halts; NADP⁺ cannot be reduced to NADPH; the Calvin cycle stops; the plant dies. Real example: DCMU (diuron) is exactly this herbicide — it binds the QB pocket of PSII, blocking PQ reduction. It is also the standard biochemistry tool for isolating the light reactions experimentally.

A common pitfall is to think O₂ from photosynthesis comes from CO₂ — it comes from H₂O. Another is to confuse PSI and PSII (PSII does the photolysis; PSI hands electrons to NADP⁺). A third is to miss NADPH as the reducing power — ATP alone is not enough to drive the Calvin cycle.

See the light-dependent reactions lesson for Z-scheme diagrams.

The Calvin Cycle (Light-Independent Reactions)

The Calvin cycle runs in the chloroplast stroma and fixes CO₂ into organic molecules using ATP and NADPH from the light reactions. Three stages: carbon fixation (CO₂ + RuBP → 2 × glycerate-3-phosphate, GP — catalysed by RuBisCO); reduction (GP → triose phosphate, TP — using ATP and NADPH); regeneration (5/6 of TP recycled back to RuBP via a complex web of carbohydrate isomerisations).

Stoichiometry per glucose: 6 CO₂ must be fixed → 6 turns of the cycle → 12 GP → 12 TP made (using 12 ATP + 12 NADPH for reduction); 10 TP recycle to 6 RuBP (using 6 more ATP); 2 TP exit to make 1 glucose. Total per glucose: 18 ATP + 12 NADPH.

RuBisCO is the most abundant protein on Earth — and yet it is slow (turnover ~3 s⁻¹) and partially substrate-confused: it has an oxygenase activity that catalyses photorespiration when O₂ is high relative to CO₂ (typical at high temperature, drought, or low CO₂). Photorespiration wastes fixed carbon; C4 and CAM plants have evolved CO₂-concentrating mechanisms to suppress it.

Worked example. A plant has access to abundant ATP and NADPH but limited CO₂. Predict the rate-limiting step in the Calvin cycle and the consequence. RuBisCO is the limiting enzyme — without CO₂ substrate, the carbon-fixation step stalls. RuBP accumulates; GP and TP fall; the regeneration phase has nothing to recycle. The cycle slows to a halt despite abundant ATP and NADPH supply. This is exactly the situation when stomata close under drought stress — the limiting factor switches from energy to substrate.

A common pitfall is to call the Calvin cycle the "dark reactions" — it can run briefly in the dark using stored ATP/NADPH but stops within minutes without light reactions. Another is to miss the 3-turn requirement for one TP exit (the recycling consumes 5/6 of the TP made).

See the Calvin cycle lesson for stoichiometry diagrams.

Factors Affecting Photosynthesis

Three principal factors limit photosynthesis rate: light intensity (photon supply for the light reactions), CO₂ concentration (substrate for RuBisCO), and temperature (enzyme kinetics for both stages).

Limiting-factor logic (Blackman): at any given conditions, one factor is rate-limiting. Increasing the limiting factor raises the rate; increasing a non-limiting factor has no effect. Read this off rate-vs-factor curves: where the curve plateaus, another factor has become limiting.

Temperature has two effects on photosynthesis: enzyme kinetics rise with temperature up to the optimum (~25 °C for most temperate plants), then drop sharply as enzymes denature and stomatal closure under heat stress cuts CO₂ supply. The downward arm of the rate-vs-temperature curve has two causes, not one.

Worked example. A pondweed bubble experiment shows the rate of photosynthesis rises linearly with light intensity up to ~10,000 lux, then plateaus. The rate at the plateau is doubled when bicarbonate (a CO₂ source) is added to the water. Identify the limiting factor at low light and at the plateau. At low light: light intensity is rate-limiting (Z-scheme is photon-starved). At the plateau: CO₂ is limiting (the Calvin cycle is substrate-starved). Adding bicarbonate raises the CO₂-limited plateau, confirming the diagnosis.

A common pitfall is to think only ONE factor limits at a time — factors interact, and the steepest slope on the curve dominates locally. Another is to miss temperature's two distinct effects.

See the factors affecting photosynthesis lesson for limiting-factor curves.

Glycolysis

Glycolysis is the cytoplasmic 10-step pathway converting glucose (6C) → 2 pyruvate (3C each), with net production of 2 ATP and 2 NADH. It is anaerobic-compatible — does not require mitochondria — and is the only ATP-producing pathway in cells without mitochondria (mature red blood cells; bacteria of all kinds).

Two phases: investment (uses 2 ATP to phosphorylate glucose → fructose-1,6-bisphosphate, splitting then to 2 × glyceraldehyde-3-phosphate) and payoff (4 ATP and 2 NADH produced, net +2 ATP and +2 NADH).

The regulatory choke point is phosphofructokinase (PFK) at the F6P → F1,6BP step — allosterically inhibited by ATP and citrate (feedback signals that energy is plentiful), activated by AMP and fructose-2,6-bisphosphate. Once F6P is committed to F1,6BP, glucose is committed to glycolysis.

Worked example. Calculate the net ATP and NADH yield from 3 glucose molecules entering glycolysis. Per glucose: net 2 ATP, 2 NADH, 2 pyruvate. For 3 glucose: 6 ATP, 6 NADH, 6 pyruvate. The 6 NADH must shuttle into mitochondria (glycerol-3-phosphate or malate-aspartate shuttle) to feed the ETC.

A common pitfall is to confuse total ATP made (4) with net ATP (2 — must subtract the 2 invested). Another is to think glycolysis requires mitochondria — it doesn't.

See the glycolysis lesson for the 10-step pathway and PFK regulation.

The Link Reaction and Krebs Cycle

Pyruvate enters mitochondria via the pyruvate carrier and is processed by the link reaction (catalysed by pyruvate dehydrogenase, PDH): pyruvate (3C) → acetyl-CoA (2C) + CO₂ + NADH. Per glucose: 2 pyruvate → 2 acetyl-CoA + 2 NADH + 2 CO₂.

Each acetyl-CoA enters the Krebs cycle: acetyl-CoA + oxaloacetate → citrate → α-ketoglutarate → succinyl-CoA → succinate → fumarate → malate → oxaloacetate (regenerating the starting molecule). Per turn: 3 NADH + 1 FADH₂ + 1 ATP (substrate-level) + 2 CO₂. The cycle runs twice per glucose (one turn per acetyl-CoA), so per glucose: 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂.

The Krebs cycle is amphibolic: it both oxidises substrates AND donates intermediates to biosynthesis (citrate → cytoplasmic acetyl-CoA for fatty acid synthesis; α-ketoglutarate → glutamate → other amino acids; oxaloacetate → aspartate). Replenishment via anaplerotic reactions (pyruvate carboxylase: pyruvate + CO₂ → oxaloacetate) keeps the cycle topped up when intermediates are siphoned off.

Worked example. Calculate the total NADH, FADH₂, ATP, and CO₂ produced per glucose from glycolysis through the end of Krebs (excluding the ETC). Glycolysis: 2 ATP + 2 NADH (cytoplasmic). Link reaction (×2): 2 NADH + 2 CO₂. Krebs (×2): 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂. Total: 10 NADH + 2 FADH₂ + 4 ATP + 6 CO₂. The 6 CO₂ matches the 6 carbons in the original glucose — all carbon has been released as CO₂ by the end of Krebs. ATP production at this stage is via substrate-level phosphorylation; the bulk yield comes next via oxidative phosphorylation.

A common pitfall is to think the Krebs cycle runs once per glucose — it runs twice (once per acetyl-CoA). Another is to miss the amphibolic nature.

See the link reaction / Krebs lesson for cycle diagrams and balance sheets.

Oxidative Phosphorylation and Chemiosmosis

Oxidative phosphorylation is where most ATP is made — on the inner mitochondrial membrane, by chemiosmosis driven by the electron-transport chain.

NADH donates electrons at Complex I (NADH dehydrogenase) → ubiquinone (CoQ) → Complex III (cytochrome bc₁) → cytochrome c → Complex IV (cytochrome c oxidase) → O₂ → H₂O. FADH₂ enters at Complex II (succinate dehydrogenase) — bypassing Complex I.

Complexes I, III, and IV pump protons (H⁺) from matrix to intermembrane space, building a proton-motive force (combined chemical and electrical gradient). H⁺ flows back through ATP synthase (a rotary motor — F₀ rotor turns 120° per H⁺ pair, F₁ catalytic head synthesises ATP from ADP + Pᵢ).

Yields: ~2.5 ATP per NADH (passes Complex I), ~1.5 ATP per FADH₂ (skips Complex I, fewer protons pumped). Total per glucose: ~30–32 ATP combined across all stages. The exact number depends on the shuttle used to bring glycolytic NADH into mitochondria (glycerol-3-phosphate → ~30; malate-aspartate → ~32).

Mitchell's chemiosmotic hypothesis (1961) replaced the long search for a "high-energy chemical intermediate" linking electron transport to ATP synthesis. The proton-motive force is the link.

Worked example. Cyanide blocks Complex IV. Predict the consequences and explain why cyanide poisoning is rapidly lethal. Without Complex IV, electrons cannot reach O₂; the entire ETC stalls; NADH and FADH₂ cannot be reoxidised; the proton gradient collapses; ATP synthesis halts within seconds. Cells with no fermentation backup (heart, brain) die first. Cyanide is rapidly lethal because the brain has only ~30 seconds of stored ATP and cannot survive without fresh production.

A common pitfall is to think O₂ is consumed throughout respiration — it is consumed only at Complex IV. Another is to confuse electron flow (down the energy gradient) with proton flow (against the gradient by pumps, then back through ATP synthase).

See the oxidative phosphorylation lesson for ETC and chemiosmosis diagrams.

Anaerobic Respiration

Without O₂, the ETC stalls and oxidative phosphorylation halts. Cells then rely on glycolysis alone (2 ATP per glucose) — but only if NAD⁺ can be regenerated for the next glycolytic round. Two fermentation strategies achieve this:

Animal cells: pyruvate → lactate via lactate dehydrogenase (LDH), reducing NADH → NAD⁺. Lactate is exported, taken up by liver, converted back to pyruvate, then to glucose via gluconeogenesis (the Cori cycle). Reversible.

Yeast and some bacteria: pyruvate → acetaldehyde → ethanol via alcohol dehydrogenase (ADH), regenerating NAD⁺. CO₂ is released. Irreversible — yeast cells eventually die in their own ethanol at >12% concentration.

Worked example. During an intense sprint, muscle cells rapidly run out of O₂. Predict the metabolic shift and its consequence. Glycolysis continues (uses no O₂), but the ETC stalls. Pyruvate accumulates; LDH converts it to lactate, regenerating NAD⁺ for further glycolysis. The result: 2 ATP per glucose (vs ~30 aerobic) — much less efficient, but enough to maintain power for ~30 seconds. Lactate is exported to liver via the Cori cycle for reuse, but also acidifies muscle, contributing to fatigue. After the sprint, oxygen debt is repaid as the lactate is reconverted to pyruvate and oxidised aerobically.

Respiratory substrates beyond glucose: lipids yield ~106 ATP per palmitate (vs ~30 per glucose) because they are more reduced. Proteins yield ~5 ATP per amino acid on average; rarely used unless other fuels are exhausted (long fasting, starvation). Respiratory quotient RQ = CO₂ produced / O₂ consumed: glucose RQ = 1.0; lipid RQ ≈ 0.7; protein RQ ≈ 0.8.

A common pitfall is to think lactate is "waste" — it is recycled in the Cori cycle. Another is to forget the purpose of fermentation (NAD⁺ regeneration) — the ATP yield is incidental.

See the anaerobic respiration lesson for fermentation and Cori-cycle diagrams.

Factors Affecting Respiration

Respiration rate depends on temperature (Q₁₀ ~2–3 in the biological range, falling and reversing as enzymes denature), substrate availability (glucose, fat, amino acid concentrations follow Michaelis-Menten kinetics at each enzymatic step), O₂ concentration (rate-limiting only in severe hypoxia — Complex IV's Km for O₂ is ~1 μM), and inhibitors (cyanide, rotenone, oligomycin block specific complexes for diagnostic purposes).

Q₁₀ calculation: Q₁₀ = (rate at T+10) / (rate at T). At biological temperatures it is typically 2–3, falling below 2 near optimum and reversing sharply above the denaturation cliff (~50 °C for most enzymes).

Worked example. A respirometer measures O₂ consumption of 40 μL/min at 20 °C and 100 μL/min at 30 °C. Calculate Q₁₀ and predict the rate at 25 °C. Q₁₀ = 100 / 40 = 2.5. At 25 °C, the rate would be approximately √2.5 × 40 = ~63 μL/min (geometric interpolation, since rate scales exponentially with temperature in the biological range).

See the factors affecting respiration lesson for Q₁₀ and rate-vs-temperature diagrams.

Photosynthesis and Respiration Practicals

The Edexcel 9BI0 spec specifies two core practicals here.

CP7 (Factors affecting photosynthesis): typically uses leaf-disk floating assays (vacuum-infiltrate disks with bicarbonate, time how long they take to float as O₂ accumulates) or pondweed bubble counting (Cabomba/Elodea in NaHCO₃ solution at varying light intensities). The Hill reaction with DCPIP isolates the light reactions experimentally — DCPIP is reduced (decolourised) at the same site as NADP⁺.

CP12 (Rate of yeast respiration / O₂ consumption): methylene blue redox indicator (decolourised by NADH, gives direct visual readout of metabolic activity), or respirometer measuring O₂ consumption via manometer (KOH absorbs CO₂ so the manometer reflects net O₂ uptake).

Worked example. A leaf disk floating experiment uses bicarbonate solution at saturating concentration. The light intensity is varied; rate plateaus at high light. A second experiment without bicarbonate shows a much lower plateau. Identify the experimental design choice that creates this difference. Bicarbonate provides CO₂ at saturating concentration, removing CO₂ as a limiting factor. Without bicarbonate, CO₂ becomes limiting at low light intensities, and the plateau is set by CO₂ supply rather than light. This isolates the light-dependent rate from the CO₂-dependent rate.

A common pitfall is to confuse net O₂ evolution (photosynthesis − respiration — what a leaf-disk measures in normal conditions) with gross O₂ evolution (photosynthesis alone — requires subtracting dark-respiration).

See the practicals lesson for protocol diagrams.

Energy Transfer and Metabolic Pathways

Catabolism (glycolysis, β-oxidation, amino acid oxidation, Krebs, ETC) and anabolism (gluconeogenesis, fatty acid synthesis, biosynthesis from amino acids) are integrated through shared intermediates (acetyl-CoA, pyruvate, oxaloacetate) and ATP as the central energy currency.

β-oxidation of fatty acids: each cycle removes 2 C as acetyl-CoA, producing 1 NADH + 1 FADH₂. Palmitate (C16) yields 8 acetyl-CoA + 7 NADH + 7 FADH₂ via 7 cycles. Acetyl-CoA enters Krebs (8 turns); applying OXPHOS yields → ~106 ATP per palmitate vs ~30 per glucose. Lipids yield ~3.5× more ATP per molecule because they are more reduced.

Fed-vs-fasted state switching (Topic 8 cross-link): postprandial insulin → glycogen synthesis + lipogenesis; fasting glucagon → glycogenolysis + gluconeogenesis + lipolysis + β-oxidation; prolonged fasting → ketogenesis from acetyl-CoA in the liver.

The pentose phosphate pathway (cytoplasmic, parallel to glycolysis) produces NADPH (for biosynthesis and antioxidant defence) and ribose-5-phosphate (for nucleic acid synthesis). It does not produce ATP — its purpose is reductive biosynthesis.

Worked example. A long-distance runner depletes muscle glycogen in the first hour. Predict the metabolic shift over the next 90 minutes. With glycogen depleted, blood glucose falls; glucagon rises, suppressing glycogen synthesis and activating lipolysis. Adipose tissue releases fatty acids; muscle takes up and β-oxidises them. The Cori cycle continues to recycle some lactate. After ~90 minutes, the liver also shifts to ketogenesis, producing ketone bodies (β-hydroxybutyrate, acetoacetate) that the brain can use as alternative fuel. The metabolic state has switched from carbohydrate-dominated to lipid-dominated combustion.

A common pitfall is to think fats enter at the start of glycolysis — they enter as acetyl-CoA at the level of Krebs via β-oxidation. Another is to think the body burns one fuel at a time — it always burns a mixture, with proportions shifting by intensity, duration, and fed/fasted state.

See the metabolic pathways lesson for catabolic-pathway integration diagrams.

Common Mark-Loss Patterns

• Saying O₂ from photosynthesis comes from CO₂ (it comes from photolysis of H₂O).
• Confusing PSI and PSII.
• Calling the Calvin cycle the "dark reactions" (it requires the products of light reactions).
• Missing the 3-turn requirement for one TP exit from Calvin cycle.
• Confusing total ATP made (4 in glycolysis) with net ATP (2).
• Thinking the Krebs cycle runs once per glucose (it runs twice).
• Thinking O₂ is consumed throughout respiration (only at Complex IV).
• Confusing electron flow with proton flow in oxidative phosphorylation.
• Thinking ATP synthase is just an enzyme (it is a rotary motor).
• Calling lactate "waste" (it's recycled in the Cori cycle).
• Forgetting the purpose of fermentation (NAD⁺ regeneration).
• Thinking fats and proteins enter at the start of glycolysis (fats enter via β-oxidation as acetyl-CoA; amino acids at various points).

How to Revise This Topic

• Master the ATP balance sheet for one glucose end-to-end: glycolysis (2) + link (2 × NADH × 2.5) + Krebs (2 × (3 NADH × 2.5 + 1 FADH₂ × 1.5 + 1 ATP)) ≈ 30–32 ATP. Be able to derive this from memory.
• Drill the Z-scheme for light-dependent reactions until you can label every component (PSII, PQ, cytochrome b6f, PC, PSI, Fd, NADP⁺ → NADPH) and explain proton pumping.
• Memorise the regulatory checkpoints: PFK in glycolysis (allosterically inhibited by ATP/citrate), PDH (covalent + allosteric regulation), key Krebs enzymes (citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase).
• Practice limiting-factor analyses on rate-vs-light curves at multiple CO₂ levels.
• For Paper 3 extended-response questions, structure your answer in three stages: name the stage, explain the mechanism, link it to the next stage. Same template works for Z-scheme, glycolysis, Krebs, ETC.
• Use the LearningBro Examiner Mode to drill 6-mark and 9-mark questions with full AO breakdown.

Linking to Other Topics

Energy and Biological Processes is heavily synoptic. Cells, viruses and reproduction provides the chloroplast and mitochondrial ultrastructure (thylakoids, cristae) that underpins both photosynthesis and respiration. Biological molecules supplies enzymes, ATP, NADH/NADPH, the genetic code that encodes them, and the lipids/proteins that serve as alternative respiratory substrates. Exchange and transport delivers O₂ and removes CO₂ — VO₂max integrates whole-body metabolism. Control systems coordinates fed-vs-fasted state via insulin and glucagon. And the endosymbiotic origin of chloroplasts and mitochondria (Topic 2) explains why both organelles use chemiosmosis and 70S ribosomes.

Final Word

Photosynthesis and Respiration is one of the most rewarding topics on 9BI0 — the mechanisms are precise, the calculations are clean, and a fluent grasp pays off across most of Paper 2. Drill the Z-scheme, master one feedback loop in detail (PFK regulation works well), learn the ATP balance sheet, and practice extended-response questions until the language flows automatically. The full LearningBro Energy and Biological Processes course walks through every sub-topic with diagrams, worked examples, AI tutor feedback, and Examiner Mode marking. Get this section right and the metabolic vocabulary you build here will support most of Paper 2 and many synoptic Paper 3 questions.