OCR A-Level Biology: Photosynthesis and Respiration — Complete Revision Guide (H420)
OCR A-Level Biology: Photosynthesis and Respiration
Photosynthesis and respiration is the bioenergetic core of Module 5 on OCR A-Level Biology A (H420), and the most chemically intensive course on the specification. It develops the two reciprocal metabolic pathways that move carbon and energy through the biosphere — photosynthesis fixing carbon dioxide into hexose at the cost of light energy, respiration oxidising hexose back to carbon dioxide while capturing energy as adenosine triphosphate — and shows that both pathways share a common architectural principle: an electron transport chain coupled by chemiosmosis to a rotary ATP synthase. Why does the Z-scheme of the light-dependent reactions span two photosystems rather than one? Why is glycolysis in the cytoplasm but the Krebs cycle in the mitochondrial matrix? Why does the respiratory quotient (RQ) of a germinating seed shift from one to less than one over a week? Each answer routes back through the structural and enzymatic logic developed across the twelve lessons of this course.
Course 9 of 12 on the LearningBro OCR A-Level Biology learning path, this course depends on substantial prior content — the biochemistry of glucose, ATP, NAD, FAD and chlorophyll developed in biological molecules, the ultrastructure of mitochondria and chloroplasts (and the endosymbiotic theory) developed in cell structure, and the enzyme and nucleic-acid content of nucleic acids and enzymes, which supplies the regulatory framework for dehydrogenases and the structural framework for RuBisCO. It then feeds forward into biodiversity and evolution and cloning, biotechnology and ecosystems, where the energy-transfer accounting and the ecological efficiency of trophic levels recur. Get the chemiosmotic principle fluent here and the bioenergetic content of the rest of Modules 5 and 6 falls into place.
Guide Overview
The Photosynthesis and Respiration course is structured as twelve lessons that move from the overview of photosynthesis through the light-dependent and light-independent stages, then through the four phases of respiration, anaerobic alternatives and quantitative respirometry.
- Photosynthesis Overview and Chloroplast Structure
- Photosynthetic Pigments
- Light-Dependent Stage: Photolysis and Photophosphorylation
- Cyclic vs Non-Cyclic Photophosphorylation
- Light-Independent Stage: Calvin Cycle
- Limiting Factors of Photosynthesis
- Respiration Overview and Mitochondrial Structure
- Glycolysis
- Link Reaction and Krebs Cycle
- Oxidative Phosphorylation and Chemiosmosis
- Anaerobic Respiration
- Respiratory Substrates and the Respirometer
OCR H420 Specification Coverage
This course covers Module 5 sub-modules 5.2.1 (photosynthesis) and 5.2.2 (respiration) in full. Each spec strand is mapped to one or more lessons below (refer to the official OCR specification document for exact wording).
| Sub-topic | Spec area | Primary lesson(s) |
|---|---|---|
| Photosynthesis overview and chloroplast ultrastructure | 5.2.1 | Photosynthesis Overview and Chloroplast Structure |
| Photosynthetic pigments and absorption spectra | 5.2.1 | Photosynthetic Pigments |
| Light-dependent reactions; Z-scheme; photolysis | 5.2.1 | Light-Dependent Stage: Photolysis and Photophosphorylation |
| Cyclic and non-cyclic photophosphorylation | 5.2.1 | Cyclic vs Non-Cyclic Photophosphorylation |
| Light-independent reactions; the Calvin cycle | 5.2.1 | Light-Independent Stage: Calvin Cycle |
| Limiting factors and the Blackman principle | 5.2.1 | Limiting Factors of Photosynthesis |
| Respiration overview and mitochondrial ultrastructure | 5.2.2 | Respiration Overview and Mitochondrial Structure |
| Glycolysis | 5.2.2 | Glycolysis |
| Link reaction and Krebs cycle | 5.2.2 | Link Reaction and Krebs Cycle |
| Electron transport chain, chemiosmosis and ATP synthase | 5.2.2 | Oxidative Phosphorylation and Chemiosmosis |
| Anaerobic respiration in yeast and mammals | 5.2.2 | Anaerobic Respiration |
| Respiratory substrates and the respiratory quotient | 5.2.2 | Respiratory Substrates and the Respirometer |
Bioenergetic content is examined across all three H420 papers, but Module 5.2 is particularly heavy on Paper 1 (Biological processes) for mechanistic short-answer items and on Paper 3 (Unified biology) for synoptic items combining bioenergetics with cell structure, biochemistry or ecology. Quantitative respirometry, Hill-reaction kinetics and Calvin-cycle response to light-on/light-off perturbations are reliable Paper 3 fixtures.
Photosynthesis Overview and Chloroplast Structure
The photosynthesis overview lesson establishes the net reaction (six carbon dioxide plus six water yielding one glucose plus six oxygen, requiring light energy) and partitions it into the light-dependent stage on the thylakoid membrane and the light-independent stage in the stroma. The chloroplast ultrastructure is then mapped to function: the double envelope (outer and inner membranes with an intermembrane space, derived from the endosymbiosis of a cyanobacterial ancestor and revisited in cell structure), the stroma containing the Calvin-cycle enzymes, the thylakoid membrane system organised into stacked grana connected by intergranal lamellae, and the thylakoid lumen where photolysis-derived protons accumulate to drive ATP synthesis.
The lesson reinforces the structural reasoning: the thylakoid stacking maximises membrane surface area for the light-dependent reactions, the impermeability of the thylakoid membrane to protons is the precondition for chemiosmosis, and the location of the Calvin-cycle enzymes in the stroma places them in direct contact with the ATP and reduced NADP produced by the light reactions on the adjacent thylakoid membrane. The historical anchor is Cornelis van Niel's 1931 generalisation that photosynthesis is, at root, a light-driven hydrogen transfer — water in plants, hydrogen sulphide in purple sulphur bacteria. A common mark-loss pattern is to describe the thylakoid lumen as "inside the chloroplast" without specifying that it is the topologically distinct compartment enclosed by the thylakoid membrane.
Photosynthetic Pigments
The photosynthetic pigments lesson develops the chlorophyll family (chlorophyll a with peak absorbance in the blue and red, chlorophyll b with peak absorbance shifted slightly within those ranges) and the accessory pigments (carotenoids, xanthophylls) that extend the photosynthetically active wavelength range. The pigments are bound to integral thylakoid proteins in photosystem complexes, with each photosystem reaction centre containing a special pair of chlorophyll a molecules (P680 in photosystem II, P700 in photosystem I).
This lesson is anchored to PAG 6 (Chromatography), where chloroplast pigments extracted in ethanol are separated on thin-layer plates run with a non-polar solvent; the Rf value of each separated pigment is then matched to its absorbance spectrum and to the action spectrum of intact leaf photosynthesis. A common pitfall is to describe chlorophyll a as "absorbing all visible wavelengths"; it absorbs preferentially in the blue and red bands and reflects in the green, which is why leaves are green.
Light-Dependent Stage: Photolysis and Photophosphorylation
The light-dependent stage lesson develops the Z-scheme in full mechanistic detail. A photon absorbed at the P680 reaction centre of photosystem II raises an electron to a higher energy level; the excited electron is passed to a primary acceptor and then along the electron transport chain — plastoquinone (PQ) carries it across the thylakoid membrane to the cytochrome b₆f complex, which uses the redox energy to pump protons from stroma into thylakoid lumen; the electron is then transferred to plastocyanin (PC), the mobile copper-containing carrier that delivers it to photosystem I. A second photon absorbed at the P700 reaction centre re-excites the electron, which passes through another primary acceptor and ferredoxin (Fd) to NADP⁺ reductase, where it reduces NADP⁺ to NADPH on the stromal face.
The electron lost from P680 is replaced by photolysis of water at the lumenal face of photosystem II: two water molecules yield four protons, four electrons (which restore the photosystem II reaction centre to its reduced state) and one molecule of oxygen released as the by-product. The accumulating proton gradient across the thylakoid membrane drives ATP synthesis as protons flow back into the stroma through ATP synthase — this is photophosphorylation, the photosynthetic analogue of oxidative phosphorylation.
The historical anchor for photolysis is Robert Hill's 1939 demonstration that isolated chloroplasts can reduce an artificial electron acceptor (now classically DCPIP) on illumination, even in the absence of carbon dioxide — paraphrased here to indicate the experimental result rather than to quote any source. A common mark-loss pattern is to claim that photolysis "produces glucose"; photolysis produces oxygen, protons and electrons, and glucose is assembled later in the Calvin cycle. Another is to place photolysis on the stromal face rather than the lumenal face of photosystem II.
Cyclic vs Non-Cyclic Photophosphorylation
The cyclic vs non-cyclic photophosphorylation lesson contrasts the two routes for electron flow through the photosystems. Non-cyclic photophosphorylation is the Z-scheme as described above: electrons flow from water through PSII, the cytochrome b₆f complex, PSI and ferredoxin to NADP⁺, producing both ATP and NADPH and releasing oxygen by photolysis.
In cyclic photophosphorylation, electrons leaving the primary acceptor of photosystem I are returned via ferredoxin to the cytochrome b₆f complex rather than passing on to NADP⁺ reductase. The cycle produces ATP via proton pumping at cytochrome b₆f but no NADPH and no oxygen. The functional role is to supplement ATP supply when the Calvin cycle's ATP-to-NADPH demand ratio exceeds the non-cyclic stoichiometric output. A common pitfall is to claim that cyclic photophosphorylation "produces oxygen as well" or to confuse the directionality of electron flow at the cytochrome b₆f complex.
Light-Independent Stage: Calvin Cycle
The Calvin cycle lesson develops the stromal carbon-fixation pathway in three phases. Carbon-fixation: ribulose-1,5-bisphosphate (RuBP, a five-carbon sugar phosphate) is condensed with carbon dioxide by the enzyme RuBisCO, producing an unstable six-carbon intermediate that immediately splits into two molecules of glycerate-3-phosphate (GP, a three-carbon acid). Reduction: GP is phosphorylated by ATP and reduced by NADPH (both delivered from the adjacent light-dependent stage) to produce triose phosphate (G3P). Regeneration: most G3P (five out of every six molecules) is fed back through a series of phosphorylated intermediates to regenerate RuBP, with ATP investment; the remaining G3P is used for hexose, starch, sucrose and amino-acid biosynthesis.
The historical anchor is the radiolabelling work of Melvin Calvin and Andrew Benson in the early 1950s on Chlorella cultures with carbon-14 carbon dioxide, traced by paper chromatography — paraphrased here to indicate the experimental approach rather than to quote any source. A common mark-loss pattern is to confuse GP (the three-carbon acid) with G3P (the three-carbon sugar phosphate), or to claim that "one turn of the cycle makes one glucose"; six turns of the cycle fix six carbon dioxides and produce twelve G3P, of which two are used to assemble one hexose and ten are recycled to regenerate RuBP.
Limiting Factors of Photosynthesis
The limiting factors lesson develops the Blackman principle that the rate of photosynthesis at any moment is set by whichever variable is currently in shortest supply — light intensity, carbon dioxide concentration or temperature. Light-intensity curves show a linear rise in rate at low light, a plateau at high light as another factor takes over, and a depression beyond the saturating intensity if photoinhibition sets in. Carbon-dioxide curves show a similar rise-and-plateau pattern with the plateau shifted upwards if carbon dioxide is supplemented. Temperature curves rise with a Q10 of around two across the physiological range, then collapse as the Calvin-cycle enzymes (in particular RuBisCO) begin to denature.
Light-on / light-off perturbation experiments are a reliable Paper 3 fixture: switching off the light causes GP to accumulate (because the reductive phase requires ATP and NADPH that are no longer being supplied) while RuBP is depleted (because it is still being consumed by ongoing carbon-fixation until the substrate runs out). Switching off carbon dioxide has the opposite effect: GP falls (no fixation), RuBP accumulates. The reasoning is examined explicitly and is high-yield. A common mark-loss pattern is to predict the wrong directional response to a light-off perturbation, often by overlooking that fixation can briefly continue at the expense of accumulated RuBP.
Respiration Overview and Mitochondrial Structure
The respiration overview lesson partitions aerobic respiration into four stages — glycolysis (cytoplasm), link reaction (mitochondrial matrix), Krebs cycle (mitochondrial matrix) and oxidative phosphorylation (inner mitochondrial membrane and intermembrane space) — and maps each to the corresponding mitochondrial compartment. The double envelope, the cristae folding of the inner membrane to maximise the area available for the electron transport chain and ATP synthase, the matrix housing the Krebs-cycle enzymes and the mitochondrial DNA, and the narrow intermembrane space where the proton gradient builds up are all developed here. The endosymbiotic origin of mitochondria from an alpha-proteobacterial ancestor (revisited in cell structure) supplies the historical context for the double envelope and the residual mitochondrial genome.
A common mark-loss pattern is to place the Krebs cycle "on the inner membrane" — the Krebs cycle is in the matrix; only the electron transport chain and ATP synthase are membrane-embedded. Another is to place glycolysis "in the mitochondrion" — glycolysis is in the cytoplasm, and only its pyruvate product enters the mitochondrion.
Glycolysis
The glycolysis lesson develops the cytoplasmic ten-step partial oxidation of glucose to two molecules of pyruvate. The pathway is conventionally summarised in two phases: an investment phase (two ATP consumed to phosphorylate glucose to fructose-1,6-bisphosphate, which is then cleaved to two triose phosphate molecules) and a payoff phase (each triose phosphate is oxidised by NAD⁺ and phosphorylated by inorganic phosphate, then substrate-level phosphorylation generates two ATP per triose, totalling four ATP gross and two ATP net per glucose, plus two reduced NAD).
Glycolysis is universal across aerobic and anaerobic life and is the only pathway that runs without oxygen, which is why it is the entry point for both aerobic respiration (whose pyruvate enters the mitochondrion for the link reaction) and anaerobic respiration (whose pyruvate is reduced in the cytoplasm to regenerate NAD⁺). A common pitfall is to claim that glycolysis "requires oxygen"; glycolysis is anaerobic in the sense that it does not directly consume molecular oxygen, although its NADH yield can only be passed to the electron transport chain when oxygen is the terminal electron acceptor.
Link Reaction and Krebs Cycle
The link reaction and Krebs cycle lesson develops the second and third stages of aerobic respiration, both housed in the mitochondrial matrix. In the link reaction, pyruvate (entering from glycolysis through pyruvate transporters in the inner membrane) is decarboxylated and oxidised by the pyruvate dehydrogenase complex to yield carbon dioxide, reduced NAD and an acetyl group, which is then transferred to coenzyme A to produce acetyl-CoA. Two link reactions occur per glucose (one per pyruvate), yielding two carbon dioxides and two reduced NAD per glucose.
In the Krebs cycle (the citric acid cycle, formalised by Hans Krebs in 1937), acetyl-CoA condenses with oxaloacetate (a four-carbon acid) to produce citrate (a six-carbon acid), which is then progressively decarboxylated and oxidised through eight steps to regenerate oxaloacetate. Per acetyl-CoA, the cycle releases two carbon dioxides, reduces three NAD⁺ to NADH, reduces one FAD to FADH₂ and produces one ATP by substrate-level phosphorylation. Doubling to account for both acetyl-CoA molecules per glucose gives the full Krebs accounting per hexose. A common mark-loss pattern is to attribute oxygen consumption to the Krebs cycle itself; oxygen is consumed only in oxidative phosphorylation, downstream.
Oxidative Phosphorylation and Chemiosmosis
The oxidative phosphorylation and chemiosmosis lesson is the conceptual climax of the course. Reduced NAD and reduced FAD generated by glycolysis, the link reaction and the Krebs cycle donate their electrons to the electron transport chain on the inner mitochondrial membrane. The chain consists of four complexes (I, II, III, IV) plus the mobile carriers ubiquinone and cytochrome c; as electrons fall through the chain to oxygen (the terminal electron acceptor, reduced to water), the released redox energy is used by complexes I, III and IV to pump protons from matrix to intermembrane space. The result is a proton-motive force across the inner membrane.
Protons then flow back through ATP synthase down their electrochemical gradient. ATP synthase is a rotary molecular motor: the membrane-embedded F₀ subunit rotates as protons pass through it, and the rotation drives conformational cycling of the catalytic F₁ subunit in the matrix, synthesising ATP from ADP and inorganic phosphate. The chemiosmotic hypothesis was formalised by Peter Mitchell in 1961 and elaborated mechanistically by Paul Boyer and John Walker for the ATP synthase rotary mechanism in 1997 — paraphrased here to indicate the underlying scientific work rather than to quote any source.
The same chemiosmotic principle operates in the thylakoid membrane during photophosphorylation, which is one of the most important conceptual transfers in the course. A common pitfall is to describe the proton gradient as "produced by ATP synthase"; ATP synthase consumes the proton gradient that has been established by the electron-transport-chain pumps. Another is to confuse the matrix-to-intermembrane-space direction of proton pumping with the reverse.
Anaerobic Respiration
The anaerobic respiration lesson develops the two principal cytoplasmic fates of pyruvate when oxygen is unavailable to act as the terminal electron acceptor at the end of the electron transport chain. In mammals, lactate dehydrogenase reduces pyruvate to lactate while oxidising reduced NAD back to NAD⁺ — the regenerated NAD⁺ allows glycolysis to continue, sustaining ATP production at the much-reduced anaerobic yield of two ATP per glucose. Lactate accumulates in tissue and is eventually transported to the liver for gluconeogenesis once oxygen supply is restored (the so-called oxygen debt).
In yeast, pyruvate is first decarboxylated to ethanal and then reduced to ethanol by alcohol dehydrogenase, again regenerating NAD⁺ but releasing carbon dioxide as a by-product. The industrial applications — brewing, baking, biofuel ethanol — make the yeast pathway commercially significant. A common mark-loss pattern is to claim that "anaerobic respiration produces less ATP because oxygen is missing"; the proximate reason is that the electron transport chain cannot run without oxygen as terminal acceptor, so the bulk of ATP synthesis (which is oxidative phosphorylation, not substrate-level phosphorylation) is lost.
Respiratory Substrates and the Respirometer
The respiratory substrates and respirometer lesson develops the relative energy yields per gram of carbohydrate, lipid and protein, and the respiratory quotient (RQ) — the ratio of carbon dioxide produced to oxygen consumed — as the diagnostic of substrate type. Carbohydrate gives an RQ of one (six carbon dioxides per six oxygens for glucose); lipid gives an RQ of around 0.7 (because triglycerides are more reduced and require more oxygen per carbon dioxide produced); protein gives an RQ of around 0.9, with the additional complication that nitrogen disposal requires deamination upstream of respiration (covered in communication, homeostasis and excretion).
This lesson is anchored to PAG 10 (Data logger and computer-modelling) through quantitative respirometry. A simple respirometer measures the volume change in a sealed chamber containing germinating seeds or small invertebrates and an alkali to absorb the evolved carbon dioxide; the residual decrease in gas volume tracks oxygen consumption. Switching between alkali-present and alkali-absent runs allows separate measurement of oxygen consumption and carbon dioxide production, and hence RQ calculation. A common pitfall is to forget the alkali rationale, or to confuse RQ with metabolic rate per gram of tissue.
Linking to the Other Courses
Photosynthesis and respiration is one of the most synoptic courses on H420. Chlorophyll structure, ATP structure and glucose structure all route back through biological molecules; chloroplast and mitochondrial ultrastructure (including the endosymbiotic origin of both) routes back through cell structure; RuBisCO regulation and the dehydrogenase logic of NAD-linked oxidations route back through nucleic acids and enzymes; the metabolic mobilisation of glycogen under glucagon and adrenaline routes back through neuronal and hormonal communication. The course also feeds forward into biodiversity and evolution (where C3, C4 and CAM photosynthesis are revisited as evolutionary adaptations to drought) and into cloning, biotechnology and ecosystems, where the energy-transfer efficiency between trophic levels — a separate ecological concept from the cellular bioenergetics developed here — is built on this course's accounting of ATP yield per substrate. The chemiosmotic principle also recurs in any later study of bacterial bioenergetics, photosynthetic bacteria and chloroplast-mitochondrion comparative biochemistry.
Required Practicals / PAGs
This course anchors three OCR Practical Activity Groups (PAGs):
- PAG 5 (Colorimeter) is anchored by the light-dependent stage lesson through the DCPIP Hill-reaction assay, where colour change is read against a calibration curve.
- PAG 6 (Chromatography) is anchored by the photosynthetic pigments lesson through thin-layer chromatographic separation of chloroplast pigments and Rf-value determination.
- PAG 10 (Data logger and computer-modelling) is anchored by the respiratory substrates lesson (respirometry of germinating seeds or invertebrates and RQ calculation) and reinforced by the limiting factors lesson (light-intensity dose-response curves using aquatic plant oxygen evolution).
Closing
Photosynthesis and respiration is the bioenergetic spine of H420: the Z-scheme of the light-dependent reactions, the Calvin cycle in the stroma, glycolysis in the cytoplasm, the Krebs cycle in the matrix and the chemiosmotic ATP synthase on the inner mitochondrial membrane together account for nearly all the Paper 1 mechanistic marks on Module 5 and a substantial portion of the Paper 3 synoptic marks. Start with the course landing page and work through the twelve lessons in sequence. A quick-win revision tip: sketch the Z-scheme intermediates in order (PSII to primary acceptor to plastoquinone to cytochrome b₆f to plastocyanin to PSI to primary acceptor to ferredoxin to NADP⁺ reductase to NADPH) from memory each day for a week, and you will lock in the marks examiners hand out most reliably on Paper 1. From there, the parallel architecture of the mitochondrial electron transport chain and the universality of the chemiosmotic principle make the respiratory side a series of variations on the same theme rather than a list of disconnected facts.