amazon.co.ukA-Level Biology revision on photosynthesis: chloroplast structure and pigments, the light-dependent reaction, photolysis and photophosphorylation, the Calvin cycle, limiting factors and required practicals (AQA RP7/RP8, OCR PAG11), with multi-board exam practice.
Photosynthesis occurs in the chloroplast: a double-membraned organelle with thylakoid membranes stacked into grana connected by lamellae, suspended in the stroma. Structure-function links to quote: thylakoid membranes hold the photosystems (PSI and PSII) and electron transport chain for the light-dependent reaction; their stacking (grana) gives a large surface area for light absorption; the stroma contains the Calvin-cycle enzymes (RuBisCO), sugars and starch grains for the light-independent reaction; chloroplasts have their own circular DNA and 70S ribosomes (endosymbiotic origin) so they can make some photosynthetic proteins quickly.
Pigments absorb light: chlorophyll a is the primary pigment at the reaction centre of both photosystems (absorbs blue-violet and red, reflects green); chlorophyll b and carotenoids (xanthophylls, carotene) are accessory pigments broadening the wavelengths absorbed, transferring energy to the reaction centre. The absorption spectrum (light absorbed per wavelength) matches the action spectrum (photosynthesis rate per wavelength) - their overlap is evidence that the pigments absorb the light that drives photosynthesis. Chromatography separates pigments (AQA RP7; OCR PAG11): crush leaves in solvent, spot on chromatography paper/silica TLC plate, run in solvent; pigments separate by solubility and affinity for the stationary phase; calculate Rf = distance moved by pigment ÷ distance moved by solvent front and identify against known Rf values. Exam questions ask for the method, the Rf calculation and why pigments separate (different solubilities/adsorption). Chlorophyll appears red under UV fluorescence when excited - occasionally examinable as evidence of energy absorption.
The light-dependent reaction (LDR) occurs in the thylakoid membranes and produces ATP and NADPH (reduced NADP) plus oxygen - all for the Calvin cycle. Sequence: (1) photolysis: light energy (absorbed by PSII) splits water - 2H₂O → 4H⁺ + 4e⁻ + O₂ - replacing electrons lost from chlorophyll, releasing protons into the lumen and oxygen as a waste product; (2) light excites electrons in the chlorophyll a reaction centre of PSII to a higher energy level; (3) excited electrons pass along an electron transport chain of carriers from PSII to PSI, losing energy; (4) this energy pumps H⁺ from the stroma into the thylakoid lumen, building a steep proton gradient (also fed by photolysis protons); (5) H⁺ flow back into the stroma through ATP synthase, driving photophosphorylation of ADP + Pi to ATP (chemiosmosis - exactly parallel to mitochondria); (6) at PSI, electrons are re-excited and, along with H⁺ from the stroma, reduce NADP to NADPH (NADP + 2H⁺ + 2e⁻ → NADPH). NADP is the terminal electron acceptor - the direct analogue of oxygen in respiration, a favourite comparison question (compare chemiosmosis in mitochondria vs chloroplasts: membrane, proton source, acceptor, products).
Herbicide questions test the same sequence: DCMU/atrazine block electron transport at PSII, so photolysis stops, no proton gradient forms, no ATP/NADPH - the Calvin cycle stops; DCPIP (blue → colourless when reduced) substitutes for NADP as the electron acceptor in the dehydrogenase practical (AQA RP8), letting you measure electron transport rate by the rate of decolourisation. Cyclic photophosphorylation (electrons return to PSI producing ATP only, no NADPH) is examinable on some boards (OCR/WJEC) as balancing the ATP:NADPH ratio.
The light-independent reaction (Calvin cycle) occurs in the stroma and uses ATP and NADPH from the LDR to fix CO₂ into carbohydrate. Sequence: (1) carbon fixation: RuBisCO catalyses CO₂ combining with ribulose bisphosphate (RuBP, 5C) to form two molecules of glycerate 3-phosphate (GP, 3C); (2) reduction: each GP is reduced to triose phosphate (TP) using ATP and NADPH from the LDR (ATP provides energy, NADPH provides H/electrons); (3) regeneration: five of every six TP molecules regenerate RuBP (using ATP), so the cycle continues; (4) the remaining one in six TP is the product used to synthesise glucose (hexose: two TP combine), starch (storage), cellulose (walls), lipids and amino acids (with nitrogen). Balance the numbers: 3 CO₂ fixed → 6 GP → 6 TP → 5 TP regenerate 3 RuBP + 1 TP surplus. Interpretation questions are the highest-value skill: in darkness, GP accumulates (cannot be reduced without NADPH/ATP) while RuBP and TP fall; when light returns, TP recovers first. If CO₂ is removed, GP and TP fall (no fixation) while RuBP accumulates (cannot be used) - learn both directions and which molecule moves which way under each change; this distinguishes candidates who memorise from those who understand the cycle. RuBisCO is the most abundant enzyme on Earth and is inefficient - it also catalyses oxygen fixation (photorespiration), reducing yield; the 6-marker often asks why RuBisCO's oxygenase activity wastes fixed carbon.
Regulation and yield: the cycle's output depends on the balance between carboxylation (using RuBP) and reduction (using ATP/NADPH) - if either runs short, TP is diverted away from RuBP regeneration and sugar synthesis slows; stromal phosphate concentration and stromal pH also regulate the cycle, since ATP synthase needs a proton gradient maintained by the LDR. RuBisCO's inefficiency is a high-value point: it works slowly (~3 reactions per second) and also catalyses the reaction of O₂ with RuBP (photorespiration), wasting fixed carbon and ATP with no sugar gain - explaining why crop yields are limited and why C4 plants (maize) with CO₂-concentrating mechanisms outperform C3 plants in hot, dry conditions.
The limiting factor is the factor in shortest supply determining the rate. Three are examinable: light intensity - rate increases linearly at low light (light is limiting), then plateaus at high light because another factor (CO₂ or temperature) becomes limiting; the plateau level depends on CO₂ concentration and temperature. CO₂ concentration - atmospheric CO₂ (~0.04%) is usually the main limiting factor for C3 crops; increasing CO₂ (greenhouses at ~0.1%) raises the rate until light or temperature limits. Temperature - rate follows enzyme kinetics: increases to an optimum (~25-30 °C for most plants), then falls as enzymes (RuBisCO) denature; also increases photorespiration and transpiration. Graph interpretation is the standard exam skill: on a light-response curve, the initial gradient shows the photosystems' efficiency, the plateau shows the next limiting factor, and the x-intercept is the compensation point - the light intensity at which photosynthesis rate = respiration rate, so net gas exchange is zero; below it, plants respire more than they photosynthesise and lose biomass. Two curves at different CO₂ concentrations: the higher CO₂ curve plateaus higher - identify which factor limits on each section (low light = light; plateau at low CO₂ = CO₂; plateau at high CO₂ = temperature or another factor).
Agricultural optimisation (glasshouse questions, AQA/Edexcel): commercial glasshouses control light (supplementary lighting, reflective surfaces), CO₂ (burners/enrichment to ~0.1%), temperature (heating, ventilation) and water (hydroponics) to raise yields; the economics matter - added CO₂/light must raise yield enough to pay for it (marginal cost vs marginal yield), an A* evaluation point. Application to exam scenarios: given a glasshouse scenario, identify the limiting factor from the data (e.g. yield rises with CO₂ enrichment until it plateaus - then light or temperature limits), and suggest the next intervention. Photosynthesis and global carbon: net primary productivity feeds ecosystems (see ecology topic), and increased atmospheric CO₂ can raise photosynthesis (CO₂ fertilisation) but is limited by other factors - linking this topic to climate-change questions.
CO₂ fertilisation and crop data: raising CO₂ raises photosynthesis until another factor limits - interpret two-curve graphs by identifying the limiting factor on each segment (initial slope = light; plateau at low CO₂ = CO₂; plateau at high CO₂ = temperature/enzymes). Economics is the evaluation angle: enrichment only pays if the marginal yield increase exceeds the marginal cost of fuel/lamps - so commercial glasshouses enrich CO₂ during daylight when light is not limiting, and supplement light only when natural light falls below the compensation point; ventilation prevents overheating. Global context: elevated atmospheric CO₂ raises NPP slightly (CO₂ fertilisation) but the effect is limited by nutrients and water - linking photosynthesis to ecosystem productivity questions.
AQA RP7 (chromatography of leaf pigments): crush leaves in propanone (solvent), spot onto chromatography paper or a TLC plate, run in a suitable solvent, then calculate Rf values for each pigment (Rf = pigment distance ÷ solvent distance) and identify chlorophyll a, chlorophyll b, carotene and xanthophyll. Marks for: safe solvent handling, applying small concentrated spots, keeping the origin above the solvent line, and correct Rf calculations with comparison to reference values.
AQA RP8 (dehydrogenase activity in chloroplasts): isolate chloroplasts in cold isotonic buffer (cold - slows enzyme damage; isotonic - prevents osmotic lysis), add DCPIP, and illuminate; DCPIP (blue) accepts electrons from the electron transport chain and decolourises, substituting for NADP; the rate of decolourisation (measured by colorimetry against a calibration curve, or by time to decolourise) estimates the rate of electron transport. Variables to test: light intensity (distance from lamp), wavelength (coloured filters), herbicide presence (DCMU stops decolourisation). Controls and errors: keep a dark control tube (should not decolourise - confirms light-dependence), keep temperature constant, use the same chloroplast concentration; quote sources of error (variation in chloroplast concentration, lamp heating the samples - use a water bath/heat filter).
OCR PAG11 covers measuring photosynthesis rate: pondweed (Elodea) oxygen production (counting bubbles or collecting O₂ in a gas syringe - volume per minute), or data-logging CO₂/pH. Bubbles are imprecise (size varies, some dissolve); gas-syringe collection is more accurate. Interpretation: rate ∝ oxygen produced; compare with respiration at night; the compensation point explains why plants grown indoors need supplementary light. Isotope evidence (occasionally examinable): ¹⁸O-labelled water shows the oxygen evolved in photosynthesis comes from water (photolysis), not CO₂ - a classic experiment supporting the LDR mechanism.