Photosynthesis
Photosynthesis is the foundation of nearly all food chains — the process by which plants, algae, and cyanobacteria capture light energy and store it as chemical bonds in organic molecules. Two coupled stages: the light-dependent reactions on the thylakoid membrane that harvest photons, split water, pump protons, and produce ATP and reduced NADP; and the light-independent Calvin cycle in the stroma that uses those products to fix atmospheric CO₂ into the organic molecules that feed the rest of the living world.
Chloroplast structure, pigments & spectra
Photosynthesis in eukaryotes occurs exclusively inside chloroplasts — organelles whose internal membranes are precisely organised to separate the two main stages of photosynthesis. Their structure directly reflects their function and is a key topic for Paper 4 structured questions. Synoptic link: chloroplast structure was introduced in Topic 1.2 (cell structure); this topic explores the functional connections in depth.
Chloroplast structure related to function
| Structure | Description | Function in photosynthesis |
|---|---|---|
| Outer membrane | Smooth phospholipid bilayer | Permeable to many small molecules including CO₂, O₂; forms the organelle boundary |
| Inner membrane | Phospholipid bilayer; selective permeability | Controls the movement of molecules into the stroma; contains specific transporter proteins (e.g. for triose phosphate export) |
| Stroma | Fluid matrix between membranes and grana; contains enzymes, ribosomes, circular DNA, starch grains | Site of the light-independent reactions (Calvin cycle); contains RuBisCO and all enzymes of the Calvin cycle; starch grains indicate stored carbohydrate product |
| Thylakoid membranes | Flattened, interconnected membrane sacs | Site of light-dependent reactions; contain photosystems I and II, electron transport chain components, and ATP synthase; high surface area for light capture |
| Thylakoid lumen | Aqueous space inside thylakoid membranes | H⁺ ions accumulate here during the light-dependent reactions, creating the proton gradient that drives photophosphorylation |
| Grana | Stacks of thylakoids (singular: granum) | Maximise surface area for light absorption; individual thylakoids within a granum are interconnected by lamellae |
| Lamellae (intergranal) | Membranes connecting adjacent grana | Allow movement of electron carriers and molecules between grana; connect the photosynthetic membrane system |
| Circular DNA & 70S ribosomes | Prokaryote-like genetic machinery in the stroma | Allow chloroplasts to synthesise some of their own proteins, including components of the photosystems; supports endosymbiotic origin theory |
The chemiosmotic mechanism of ATP synthesis is identical in principle in chloroplasts and mitochondria — but the direction of H⁺ movement is reversed. In mitochondria: H⁺ pumped from matrix → intermembrane space, flows back through ATP synthase into matrix. In chloroplasts: H⁺ pumped from stroma → thylakoid lumen, flows back through ATP synthase into stroma. The same ATP synthase enzyme is used in both; the same proton motive force principle applies.
Photosynthetic pigments
Photosynthesis does not use a single pigment. A range of pigments absorb different wavelengths of light, extending the spectrum of light that can drive photosynthesis and increasing efficiency in varying light environments:
| Pigment | Colour (in visible light) | Wavelengths absorbed | Location |
|---|---|---|---|
| Chlorophyll a | Blue-green | Red (~680 nm) and violet-blue (~430 nm); reflects green | Core of PSII and PSI reaction centres; also in antenna complexes |
| Chlorophyll b | Yellow-green | Red (~640 nm) and blue (~470 nm); reflects yellow-green | Antenna complexes only; passes energy to chlorophyll a |
| Carotenoids (carotene, xanthophyll) | Yellow-orange | Blue-violet (~400–500 nm); reflects yellow-orange | Antenna complexes; accessory pigments |
No single pigment absorbs the full visible spectrum efficiently. Having several pigments with different absorption peaks allows the plant to capture a broader range of wavelengths. Carotenoids also protect against photooxidative damage in bright light — they absorb excess energy and release it as heat or fluorescence, protecting the reaction centres.
Absorption spectrum vs action spectrum
Two spectra are commonly tested and they are NOT the same. Confusing them is one of the most frequent errors on Topic 13 questions:
Shows the wavelengths of light absorbed by each pigment (measured directly). Produced by shining different wavelengths of light through a solution of the pigment and recording how much is absorbed. Chlorophyll a shows two peaks: ~430 nm (blue-violet) and ~680 nm (red). Green (~550 nm) is absorbed least — reflected green explains why plants appear green.
Shows the rate of photosynthesis at each wavelength (measured by O₂ production or CO₂ uptake). Produced by illuminating a plant with light of different wavelengths and measuring the photosynthetic rate. The action spectrum closely matches the combined absorption spectrum of all pigments — confirming that absorbed light drives photosynthesis. Where absorption is low (green), photosynthesis rate is also low.
Absorption spectrum = wavelengths a pigment absorbs (a property of the pigment itself, measured in solution).
Action spectrum = wavelengths that drive photosynthesis (a property of the whole process, measured in live tissue).
The similarity between the two spectra provides evidence that the pigments are responsible for driving photosynthesis. The action spectrum tends to be slightly broader because all pigments combine; the individual absorption spectra of separated pigments are narrower.
Chromatography of chloroplast pigments and Rf values
Paper chromatography (or TLC — thin layer chromatography) separates the individual pigments from a leaf extract and allows their identification via Rf values.
- Grind fresh leaf material in a small volume of organic solvent (e.g. propanone, ethanol, or a petroleum ether/acetone mixture)
- Apply a small, concentrated spot of the extract near the base of a chromatography strip (paper or TLC plate) using a capillary tube — allow to dry, apply several times to concentrate the pigment
- Place the strip upright in a small volume of running solvent (do NOT submerge the pigment spot)
- The solvent moves up by capillary action, carrying pigments with it at rates depending on their solubility in the solvent and affinity for the paper
- Remove the strip before the solvent front reaches the top; mark the solvent front position and the centre of each pigment band
- Calculate the Rf value for each pigment
Rf = distance moved by pigment ÷ distance moved by solvent front
Both measured from the starting point of the pigment spot. Rf has no units (it is a ratio, always between 0 and 1). Each pigment has a characteristic Rf in a given solvent at a given temperature — this allows identification by comparison with known Rf tables.
Worked example: solvent front moves 90 mm from origin; carotene band centres at 81 mm from origin → Rf = 81 / 90 = 0.90. Chlorophyll a centres at 54 mm → Rf = 54 / 90 = 0.60.
| Pigment | Typical Rf (petroleum ether / acetone 9:1) | Band colour |
|---|---|---|
| Carotene | 0.95–0.98 | Orange |
| Xanthophyll | 0.60–0.70 | Yellow |
| Chlorophyll a | 0.55–0.65 | Blue-green |
| Chlorophyll b | 0.40–0.50 | Yellow-green |
Carotene is a non-polar, highly lipophilic hydrocarbon — it dissolves very readily in non-polar organic solvents and has little affinity for the polar cellulose paper. It is therefore carried rapidly by the running solvent, ending up near the top (high Rf). Chlorophyll b has more polar substituents — it clings more to the polar paper — and moves more slowly (lower Rf). The order from top to bottom: carotene → xanthophyll → chlorophyll a → chlorophyll b.
A student runs a chromatogram of a leaf extract. The solvent front travels 85 mm. Four bands are visible at 79, 55, 50, and 38 mm from the origin. Which band is most likely to be chlorophyll b?
- A. Band at 79 mm (Rf = 0.93)
- B. Band at 55 mm (Rf = 0.65)
- C. Band at 50 mm (Rf = 0.59)
- D. Band at 38 mm (Rf = 0.45)
Light-dependent reactions
The light-dependent reactions occur in the thylakoid membranes of the chloroplast. Light energy is absorbed by photosystems and used to drive electron flow, water splitting, proton pumping, and ultimately the synthesis of ATP and reduced NADP (NADPH) — the two products that power the Calvin cycle. Molecular oxygen is released as a by-product of water splitting.
The two photosystems
Two distinct protein complexes — Photosystem I (PSI) and Photosystem II (PSII) — act as light-harvesting and reaction-centre units. They differ in the wavelength at which their reaction-centre chlorophyll absorbs maximally:
Reaction centre absorbs maximally at 680 nm. PSII is the first to act in non-cyclic photophosphorylation. When it absorbs light, electrons are boosted to a higher energy level and passed to an electron transport chain. The electrons lost from PSII are replaced by photolysis of water. PSII is not involved in cyclic photophosphorylation.
Reaction centre absorbs maximally at 700 nm. PSI acts after PSII in non-cyclic photophosphorylation, receiving electrons that have passed through the electron transport chain from PSII. When it absorbs light, electrons are boosted to an even higher energy level and used to reduce NADP to reduced NADP. PSI is the only photosystem involved in cyclic photophosphorylation.
Non-cyclic photophosphorylation
Non-cyclic photophosphorylation produces both ATP and reduced NADP, and releases O₂ as a by-product. It is the predominant light reaction during active photosynthesis. Both PSI and PSII are involved.
- Light absorption at PSII: photons are absorbed by the PSII antenna complex (chlorophyll and carotenoids) and funnelled to the reaction centre; light energy boosts electrons in the P680 chlorophyll to a higher energy level — photoactivation
- Electron release and transfer: the excited electrons are captured by an electron acceptor and passed along the thylakoid membrane electron transport chain; as electrons fall to lower energy levels, they release energy used to pump H⁺ ions from the stroma into the thylakoid lumen
- Photolysis of water (at the oxygen-evolving complex of PSII): water molecules are split to replace the electrons lost from PSII:
2 H₂O → 4 H⁺ + 4 e⁻ + O₂
The H⁺ released adds to the proton gradient; O₂ is released as a by-product of photosynthesis - Electrons arrive at PSI: electrons from PSII, now at lower energy, reach PSI; light absorption at PSI boosts them to an even higher energy level — a second photoactivation event
- NADP reduction: the highly excited electrons from PSI are used to reduce NADP, combining with H⁺ from the stroma:
NADP⁺ + 2 H⁺ + 2 e⁻ → reduced NADP (NADPH) - Chemiosmosis and photophosphorylation: H⁺ ions (from both photolysis of water and proton pumping during electron transport) accumulate in the thylakoid lumen, building a proton gradient; H⁺ flows back into the stroma through ATP synthase, driving the phosphorylation of ADP to ATP
Net products (non-cyclic): ATP, reduced NADP, O₂ evolved
The electron flow is linear, not circular — electrons start in P680 chlorophyll at PSII, move through the ETC, pass through PSI, and end up in reduced NADP. They do not return to their starting chlorophyll. New electrons must be continuously supplied by photolysis of water. The pathway is one-directional: water → PSII → ETC → PSI → NADP.
Cyclic photophosphorylation
Cyclic photophosphorylation uses only PSI and produces only ATP — no reduced NADP, no photolysis of water, no O₂. Electrons cycle back to PSI rather than being passed to NADP.
- Light is absorbed by PSI; photoactivation of P700 chlorophyll excites electrons
- Excited electrons pass to electron carriers in the thylakoid membrane; as they lose energy, they pump H⁺ into the thylakoid lumen (generating a proton gradient)
- Electrons are passed back to PSI — they return to the chlorophyll they originated from (hence “cyclic”)
- The proton gradient drives H⁺ flow through ATP synthase → ATP is synthesised
- PSII is not involved; no water is split; no reduced NADP is produced; no O₂ evolved
Net product (cyclic): ATP only
When the cell needs more ATP relative to reduced NADP. The Calvin cycle requires both, but in a specific ratio (approximately 3 ATP per 2 reduced NADP per CO₂ fixed). If reduced NADP is already available but ATP is limiting, cyclic photophosphorylation can supply additional ATP without the cost of using more water or producing more reduced NADP. It also operates in guard cells, which need ATP for the K⁺ pump that opens stomata (Topic 7), and in other situations where extra ATP is needed.
Cyclic vs non-cyclic — comparison
| Feature | Non-cyclic photophosphorylation | Cyclic photophosphorylation |
|---|---|---|
| Photosystems involved | PSI and PSII | PSI only |
| Photoactivation occurs? | Yes (at both PSI and PSII) | Yes (at PSI only) |
| Photolysis of water? | Yes — by oxygen-evolving complex of PSII | No |
| Oxygen evolved? | Yes | No |
| ATP synthesised? | Yes | Yes |
| Reduced NADP produced? | Yes (NADP⁺ → reduced NADP) | No |
| Electron flow direction | Linear (water → PSII → ETC → PSI → NADP) | Cyclic (PSI → ETC → back to PSI) |
Chemiosmosis in the thylakoid membrane
Just as in mitochondria (Topic 12A), ATP synthesis in chloroplasts is driven by a proton gradient across a membrane — but the orientation is different:
Two sources contribute H⁺ to the thylakoid lumen: (1) electron transport — as electrons pass along the ETC embedded in the thylakoid membrane, energy is used to pump H⁺ from the stroma into the lumen; (2) photolysis of water in the lumen releases H⁺ directly into the lumen. Both sources build the proton concentration in the lumen.
The thylakoid membrane is largely impermeable to H⁺ ions except at ATP synthase — just like the inner mitochondrial membrane. This ensures the proton gradient is maintained and that H⁺ flows back only through ATP synthase (not by leaking freely).
The lumen has high H⁺ concentration; the stroma has low H⁺. H⁺ flows down its concentration gradient through ATP synthase from lumen to stroma. This flow drives ATP synthesis (ADP + Pₙ → ATP) in the stroma. This process is called photophosphorylation (ATP made using light-driven proton gradient).
Mitochondrion: H⁺ pumped from matrix → intermembrane space; flows back into matrix through ATP synthase.
Chloroplast: H⁺ pumped from stroma → thylakoid lumen; flows back into stroma through ATP synthase.
Same principle, opposite orientation. In both cases, ATP synthase sits in the membrane with its catalytic domain facing the compartment where ATP is made (matrix / stroma).
Which products are generated ONLY by non-cyclic photophosphorylation but NOT by cyclic photophosphorylation?
- A. ATP only
- B. Reduced NADP and O₂
- C. ATP and H₂O
- D. O₂ and CO₂
Photosynthesis in chloroplasts involves both cyclic and non-cyclic photophosphorylation.
(a) Compare cyclic and non-cyclic photophosphorylation, stating in each case which photosystems are involved and what products are made. [4]
(b) Explain how the proton gradient across the thylakoid membrane is established and how it is used to produce ATP. [4]
(a) Comparison [4 marks]
- Non-cyclic: involves both PSI and PSII; light causes photoactivation of both; produces ATP, reduced NADP, and O₂ (from photolysis of water); electrons flow linearly from water through PSII → ETC → PSI → reduced NADP [2]
- Cyclic: involves PSI only; photoactivation at PSI; electrons cycle back from the ETC to PSI; produces ATP only — no reduced NADP, no photolysis of water, no O₂ evolved [2]
(b) Proton gradient and ATP synthesis [4 marks]
- H⁺ accumulates in the thylakoid lumen from two sources: proton pumping by the electron transport chain (using energy from electron flow) and photolysis of water in the lumen [1]
- The thylakoid membrane is impermeable to H⁺ except at ATP synthase, so the gradient is maintained [1]
- H⁺ flows from the lumen (high concentration) back into the stroma (low concentration) through ATP synthase, down its concentration gradient [1]
- This flow drives the synthesis of ATP from ADP + Pₙ in the stroma — photophosphorylation; ATP is then used in the Calvin cycle [1]
Light-independent reactions, limiting factors & practicals
The light-independent reactions (Calvin cycle) occur in the stroma of the chloroplast. Despite the name, the Calvin cycle is not completely independent of light — it depends entirely on ATP and reduced NADP produced by the light-dependent reactions. It is more accurately described as the “light-independent” stage because it does not use light directly. It uses CO₂ to build organic molecules.
The Calvin cycle — three stages
The Calvin cycle has three main stages. The 9700 syllabus requires knowledge of the molecules at each stage; intermediate molecular names beyond those listed are NOT required.
Stage 1 — Carbon fixation
- CO₂ (1C) from the atmosphere diffuses into the stroma
- CO₂ combines with ribulose bisphosphate (RuBP, 5C) — catalysed by the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase — the most abundant enzyme on Earth)
- The unstable 6C compound immediately splits into two molecules of glycerate 3-phosphate (GP, 3C)
Stage 2 — Reduction of GP to TP
- GP is reduced to triose phosphate (TP, 3C) using energy from ATP and hydrogen from reduced NADP (both supplied by the light-dependent reactions)
- ATP is hydrolysed; reduced NADP is oxidised back to NADP⁺ — regenerated NADP⁺ can return to PSI to be re-reduced
Stage 3 — Regeneration of RuBP
- Most TP molecules (5 out of every 6) are used to regenerate RuBP via a series of reactions requiring ATP
- Only 1 out of every 6 TP molecules is available to leave the cycle and be used for synthesis of useful organic molecules
- RuBP is regenerated and is ready to accept another CO₂ molecule, allowing the cycle to continue
To fix 3 CO₂: 3 RuBP (5C) → 6 GP (3C) → 6 TP (3C). Of these 6 TP, 5 are used to regenerate 3 RuBP (using ATP); only 1 TP leaves the cycle as net output per 3 CO₂ fixed. That 1 TP (3C) is the building block for glucose (C₆H₁₂O₆), lipids, amino acids, and other organic molecules. To make one glucose (6C), 6 TP must leave the cycle, requiring 6 × 3 CO₂ = 18 CO₂ molecules to be fixed.
Uses of TP and GP — Calvin cycle intermediates
Calvin cycle intermediates feed into a range of biosynthetic pathways. The 9700 syllabus requires you to know exactly which molecules lead to which products:
| Intermediate | Used to produce |
|---|---|
| GP (glycerate 3-phosphate, 3C) | Some amino acids (GP can be converted to the carbon skeletons of certain amino acids if a nitrogen source is available) |
| TP (triose phosphate, 3C) | Carbohydrates (glucose, sucrose, starch, cellulose); lipids (glycerol backbone, and fatty acid chains via acetyl-CoA); amino acids (carbon skeletons) |
Memorise the exact split: GP → amino acids; TP → carbohydrates, lipids, amino acids. This detail is regularly tested. The key point is that TP, as the more reduced and versatile molecule, can produce a wider range of products than GP.
RuBisCO — carbon fixation enzyme
RuBisCO (ribulose bisphosphate carboxylase/oxygenase) catalyses the first step of carbon fixation — the entry point of atmospheric CO₂ into organic chemistry. Without it, the Calvin cycle cannot start and no organic carbon can be produced from CO₂.
It is thought to be the most abundant enzyme on Earth because it is catalytically slow (fixes only a few CO₂ molecules per second, vs thousands per second for typical enzymes) and plants must compensate by making enormous quantities. Large amounts of nitrogen are invested in producing it, making leaf nitrogen content a key predictor of photosynthetic capacity.
RuBisCO can use either CO₂ or O₂ as a substrate (hence the name includes “oxygenase”). When O₂ binds instead of CO₂, it initiates a process called photorespiration that wastefully consumes RuBP without fixing carbon — reducing photosynthetic efficiency. This is worse at high temperatures and low CO₂ concentrations.
(Note: C4 and CAM adaptations that reduce photorespiration are NOT required in the 9700 syllabus and should not be discussed in exam answers unless explicitly asked.)
Limiting factors of photosynthesis
The law of limiting factors (Blackman's law) states that when the rate of a process depends on several factors, the rate is limited by the one in shortest supply (the factor furthest below its optimum). For photosynthesis, three main environmental factors are assessable:
At low light, the light-dependent reactions are limiting — photosystems cannot drive enough electron flow to produce sufficient ATP and reduced NADP for the Calvin cycle.
As light intensity increases, rate of photosynthesis rises proportionally until another factor becomes limiting (CO₂ or temperature).
At very high light intensities, rate plateaus — photosystems are saturated; increasing light further gives no benefit unless CO₂ or temperature is also increased.
CO₂ is the substrate for RuBisCO in the Calvin cycle. At low CO₂, fixation slows, GP accumulation falls, and TP production decreases. ATP and reduced NADP made by the light reactions may not be used efficiently.
Rising CO₂ increases photosynthesis rate, until light intensity or temperature becomes limiting. Commercial greenhouses often enrich CO₂ to ~1000 ppm (vs ~420 ppm atmospheric) to maximise photosynthesis and crop yield.
Enzymes, especially RuBisCO, are temperature-sensitive. At low temperatures, enzyme activity is reduced (less kinetic energy), slowing the Calvin cycle.
Rate rises with temperature up to ~25–35 °C (varies by species); above the optimum, enzymes including RuBisCO start to denature, rate falls sharply.
The light reactions themselves are photochemical (not enzyme-catalysed) and less temperature-sensitive — temperature mainly affects the light-independent stage.
Paper 4 questions often ask what happens to GP and TP concentrations when a limiting factor changes suddenly:
- Remove light suddenly: ATP and reduced NADP production stops → GP cannot be reduced to TP → GP rises (accumulates), TP falls; RuBP continues to fix CO₂ briefly so GP rises before CO₂ fixation also slows.
- Reduce CO₂ suddenly: less CO₂ fixed with RuBP → less GP produced → GP falls; ATP and reduced NADP still available so TP production continues from existing GP → TP falls too after GP is depleted; RuBP accumulates (not converted to GP).
- Increase CO₂ suddenly: more CO₂ fixed → GP rises; if light is not limiting, GP is converted to TP → TP rises.
Investigating limiting factors — practical methods
A common practical uses an aquatic plant (Elodea or Cabomba) to measure the rate of photosynthesis:
- Cut a fresh sprig of Elodea and place it in water containing sodium hydrogen carbonate (NaHCO₃) as a CO₂ source — submerged in a beaker under a lamp
- Allow the plant to equilibrate for a few minutes before counting
- Count oxygen bubbles per unit time (or collect O₂ in an inverted capillary tube and measure the volume)
- Vary one factor (light intensity by moving the lamp; CO₂ by changing NaHCO₃ concentration; temperature by placing in water baths) while keeping the others constant
- Plot the appropriate graph; identify limiting factors by the plateau and slope changes
Light intensity is approximately inversely proportional to the square of the distance from the source:
I ∝ 1/d² (where d = distance in metres)
To calculate relative light intensity at different distances from a lamp, apply this relationship. Example: if light intensity at 10 cm = 100 units, at 20 cm it is 100/4 = 25 units; at 40 cm it is 100/16 = 6.25 units. Only valid when lamp size << distance.
In the Calvin cycle, which molecule accepts CO₂ directly?
- A. Triose phosphate (TP)
- B. Glycerate 3-phosphate (GP)
- C. Ribulose bisphosphate (RuBP)
- D. Reduced NADP
The light-independent stage of photosynthesis is sometimes called the Calvin cycle.
(a) Outline the three stages of the Calvin cycle, naming the key molecules involved. [5]
(b) Explain why the Calvin cycle stops when the light supply is suddenly interrupted. [2]
(c) State which Calvin cycle intermediate(s) can be used to produce lipids, and explain how this links to the importance of photosynthesis for heterotrophs. [2]
(a) Three stages [5 marks]
- Carbon fixation: CO₂ (1C) combines with RuBP (5C), catalysed by RuBisCO, to form two molecules of GP (3C) [2]
- Reduction: GP (3C) is reduced to TP (3C) using ATP and reduced NADP from the light-dependent reactions; the NADP⁺ produced is recycled back to PSI [2]
- Regeneration of RuBP: 5 out of 6 TP molecules are used to regenerate RuBP, using ATP; 1 TP per cycle leaves to form other organic molecules [1]
(b) Why cycle stops when light removed [2 marks]
- The light reactions stop; no more ATP or reduced NADP are produced [1]
- GP cannot be reduced to TP; RuBP cannot be regenerated from TP; the cycle stops (GP accumulates; TP and RuBP fall) [1]
(c) Lipid production and heterotroph importance [2 marks]
- TP (triose phosphate) can be used to produce lipids (via glycerol backbone and acetyl-CoA for fatty acids) [1]
- Heterotrophs cannot fix CO₂; they obtain their organic carbon by consuming autotrophs; the lipids, carbohydrates, and amino acids that all heterotrophs need ultimately originate from photosynthetic CO₂ fixation via Calvin cycle intermediates [1]
Topic 13 Practice — Comprehensive
Mixed practice covering all three sub-sections — chloroplast, light reactions, Calvin cycle, limiting factors.
Where in the chloroplast do the light-dependent and light-independent reactions respectively occur?
- A. Both in the stroma
- B. Light-dependent in the stroma; light-independent in the thylakoid membrane
- C. Light-dependent in the thylakoid membrane; light-independent in the stroma
- D. Both in the thylakoid membrane
In chloroplasts, where do H⁺ ions accumulate to create the proton gradient that drives photophosphorylation?
- A. The stroma
- B. The thylakoid lumen
- C. The intermembrane space between inner and outer chloroplast membranes
- D. The mitochondrial matrix
A chromatogram of a leaf extract is run with a petroleum ether / acetone solvent. The solvent front travels 92 mm. The following bands are observed (measured from the origin): Band 1: 87 mm; Band 2: 63 mm; Band 3: 53 mm; Band 4: 42 mm.
(a) Calculate the Rf value for Band 3. Show your working. [2]
(b) Identify Band 4, and explain your reasoning. [2]
(a) Rf for Band 3 [2 marks]
- Rf = distance moved by pigment ÷ distance moved by solvent front [1]
- Rf = 53 ÷ 92 = 0.58 [1] — this matches chlorophyll a (typical Rf ~0.55–0.65)
(b) Identity of Band 4 [2 marks]
- Band 4 Rf = 42 / 92 = 0.46, which falls in the range for chlorophyll b (typical Rf 0.40–0.50) [1]
- Chlorophyll b has a lower Rf than chlorophyll a because it has more polar substituents — it is more strongly attracted to the polar cellulose paper and is therefore carried less far by the organic solvent [1]
A student measures the rate of photosynthesis in Elodea at different light intensities. The rate initially increases, then plateaus. Adding extra CO₂ raises the plateau to a new higher level, which again plateaus at high light.
(a) Explain why the rate initially plateaus when light intensity is increased. [2]
(b) Explain why increasing CO₂ raises the plateau. [2]
(c) If the temperature is now raised from 20 to 30 °C, predict and explain what will happen to the rate at the new high-CO₂, high-light conditions. [2]
(d) The student then reduces CO₂ very suddenly. Predict the immediate changes in GP and RuBP concentrations in the stroma, and explain your prediction. [2]
(a) Why rate plateaus with increasing light [2 marks]
- The light-dependent reactions are no longer limiting (photosystems are saturated) [1]
- A different factor — CO₂ concentration — has become the limiting factor; no further increase in light can raise the rate [1]
(b) Why extra CO₂ raises the plateau [2 marks]
- CO₂ was the limiting factor at the plateau; increasing it provides more substrate for RuBisCO [1]
- More CO₂ allows a higher rate of carbon fixation (more GP produced), so the Calvin cycle runs faster and utilises more ATP and reduced NADP from the light reactions — raising the maximum rate [1]
(c) Temperature increase effect [2 marks]
- Rate would increase, since at 20 °C the Calvin cycle enzymes (especially RuBisCO) are not at their optimum and are temperature-limiting [1]
- At 30 °C, enzyme activity increases (more kinetic energy, faster enzyme-substrate complexes); the rate of the Calvin cycle rises; ATP and reduced NADP from the light reactions are now used more rapidly [1]
(d) Reduce CO₂ suddenly: GP and RuBP changes [2 marks]
- GP concentration falls — with less CO₂ available, less RuBP is carboxylated so less GP is produced; existing GP is still reduced to TP by available ATP and reduced NADP [1]
- RuBP concentration rises — RuBP is no longer consumed (no CO₂ to combine with) but the regeneration pathway continues producing it; RuBP accumulates [1]
Mitochondria and chloroplasts both use chemiosmosis to synthesise ATP.
(a) Describe ONE similarity and TWO differences in how ATP is synthesised by chemiosmosis in mitochondria and chloroplasts. [3]
(b) Explain how the products of the light-dependent reactions are used in the Calvin cycle. [4]
(a) Similarity and differences [3 marks]
- Similarity: in both, H⁺ ions flow through ATP synthase down their concentration gradient, driving the phosphorylation of ADP to ATP — same basic chemiosmotic mechanism [1]
- Difference 1: in mitochondria, H⁺ accumulates in the intermembrane space; in chloroplasts, H⁺ accumulates in the thylakoid lumen; the orientation of H⁺ flow is reversed [1]
- Difference 2: in mitochondria, the proton gradient is built by the electron transport chain using energy from NADH/FADH₂ (from glucose breakdown); in chloroplasts, the gradient is built by the photosynthetic ETC using energy from light (and supplemented by photolysis of water releasing H⁺ into the lumen) [1]
(b) Products of light reactions in Calvin cycle [4 marks]
- The light-dependent reactions produce ATP (by photophosphorylation) and reduced NADP (by reduction of NADP⁺ at PSI) [1]
- ATP is used in the Calvin cycle to: (i) phosphorylate GP during the reduction step (converting GP to TP) and (ii) phosphorylate TP during the regeneration of RuBP [1]
- Reduced NADP provides hydrogen (reducing power) to reduce GP to TP; NADP⁺ is regenerated and returns to PSI [1]
- Without a continuous supply of both ATP and reduced NADP from the light reactions, GP cannot be reduced to TP and RuBP cannot be regenerated — the Calvin cycle would stop [1]
Topic 13 — Common Mistakes
- 🌿Saying light-dependent reactions occur in the stroma, or Calvin cycle on the thylakoidExactly reversed. Light-dependent reactions occur on the thylakoid membranes (where PSII, PSI, ETC, and ATP synthase are embedded). Calvin cycle occurs in the stroma (where RuBisCO and all Calvin cycle enzymes are dissolved). This is the #1 Topic 13 location error on Paper 4.
- 🌞Saying "Calvin cycle does not need light at all"The Calvin cycle does not use light directly — but it is entirely dependent on ATP and reduced NADP from the light-dependent reactions. Without light, those run out and the Calvin cycle stops. “Light-independent” means it does not use photons directly, not that it is fully independent of light supply.
- 🔭Confusing absorption spectrum and action spectrumAbsorption spectrum = wavelengths absorbed by a specific pigment (measured in solution). Action spectrum = wavelengths that drive photosynthesis (measured in living tissue by O₂ production). They look similar because the pigments cause photosynthesis — but they are not the same measurement. An exam question asking you to "explain the similarity" wants this reasoning.
- ✍Calculating Rf with numerator and denominator reversedRf = distance moved by pigment ÷ distance moved by solvent front. Both measured from the origin. Reversing gives a value >1, which is impossible (Rf must be between 0 and 1). Always: pigment distance on top, solvent front distance on bottom.
- 🔄Saying cyclic photophosphorylation involves PSIICyclic involves PSI only. PSII is exclusively non-cyclic. In cyclic, electrons return from the ETC back to PSI — there is no need for PSII, no photolysis of water, no reduced NADP, and no O₂. Only ATP is produced. Make this distinction explicit when answering comparison questions.
- 💥Saying O₂ is produced in cyclic photophosphorylationNo. O₂ is produced ONLY in non-cyclic photophosphorylation, from photolysis of water at PSII. Since cyclic photophosphorylation does not involve PSII, no water is split and no O₂ is evolved. Any question about O₂ evolution in photosynthesis is about PSII and non-cyclic only.
- 🍏Confusing GP and TP products: getting the biosynthesis table wrongExact syllabus requirement: GP → some amino acids; TP → carbohydrates, lipids, amino acids. TP is the more versatile product; GP feeds only some amino acid routes. Don't say "GP makes carbohydrates" — that's TP, not GP. This exact pairing is frequently tested.
- 🧠Not explaining the RuBP/GP/TP changes when CO₂ or light is removed suddenlyThese dynamic-change questions need mechanistic reasoning, not just "everything decreases". When light removed: GP rises, TP falls, RuBP falls. When CO₂ removed: GP falls, RuBP rises. These are opposite patterns and the direction follows logically from which reaction is blocked.
- ☀Saying H⁺ accumulates in the stroma during photophosphorylationH⁺ accumulates in the thylakoid lumen (high concentration), not the stroma (low concentration). H⁺ then flows from lumen → stroma through ATP synthase. Getting the direction wrong reverses the entire chemiosmosis mechanism.
- 🥀Mentioning C4 or CAM photosynthesis in answersC4 and CAM photosynthesis are NOT in the 9700 syllabus. Do not mention them unless the question explicitly introduces them as novel contexts. Mentioning them unprompted wastes time, doesn't earn marks, and risks introducing errors. (The 9700 syllabus does cover the concept of photorespiration in passing via RuBisCO's dual function — but C4 adaptations are not required.)
- 🌳Saying photosynthesis produces O₂ from CO₂O₂ comes from the splitting of water (photolysis), NOT from CO₂. This was confirmed by isotope labelling experiments (¹&sup8;O in water appears in O₂, not in CO₂). CO₂ contributes its carbon to organic molecules via the Calvin cycle; the oxygen in CO₂ ends up in water and in organic molecules, not in the O₂ evolved.
Topic 13 mirrors Topic 12 structurally — two coupled stages, each with an input, mechanism, and outputs. The parallel with Topic 12 (especially chemiosmosis) is a gift: same principle, reversed direction. Highest-yield items: chloroplast structure with all compartments and functions, cyclic vs non-cyclic comparison table (PSI only vs PSI+PSII; ATP only vs ATP+reduced NADP+O₂), thylakoid lumen as H⁺ accumulation site, Rf calculation and pigment order on chromatogram, absorption vs action spectrum distinction, Calvin cycle three stages with RuBP/GP/TP, GP → amino acids / TP → carbohydrates+lipids+amino acids, GP and RuBP responses to sudden changes in CO₂ or light, limiting factors with mechanistic explanations, inverse-square law for light intensity. Synoptic links: Topic 1 (chloroplast structure), Topic 7 (leaf structure), Topic 12 (chemiosmosis comparison).