IB Biology · Theme C Interaction and interdependence · Molecules
C1.3 Photosynthesis
Photosynthesis turns light energy into chemical energy stored in carbon compounds. Pigments absorb specific wavelengths; water is split for hydrogen, and oxygen is a by-product. At HL, thylakoids make ATP and reduced NADP, and the Calvin cycle in the stroma fixes carbon.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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C1.3.1 Light energy becomes chemical energy in carbon compounds
Pigments absorb light energy; photosynthesis transforms it into chemical energy.
That energy is stored in the bonds of carbon compounds such as glucose.
Carbon dioxide is reduced using hydrogen from water, so carbon is fixed and energy stored.
This one transformation supplies most of the chemical energy in ecosystems.
Students often think plants make energy. In fact they transform light energy into chemical energy; none is created.
Students often think plants get their food from soil. In fact energy comes from light and carbon from air; soil gives water and ions.
C1.3.2 Carbon dioxide plus water gives glucose, with water supplying the hydrogen
Word equation: carbon dioxide + water → glucose + oxygen, with light absorbed by chlorophyll.
Water is split to supply hydrogen, which reduces carbon dioxide to glucose.
So water is a genuine reactant, not just a solvent.
The equation shows reactants and products only, not where each atom goes.
Students often think water just dissolves things in photosynthesis. In fact it is split for the hydrogen that reduces carbon dioxide.
Students often think chlorophyll is used up. In fact it absorbs light and is left unchanged; no atom of it enters glucose.
C1.3.3 Oxygen is left over from splitting water
Oxygen is a by-product: formed, but not needed by the process.
All released oxygen comes from water, none from carbon dioxide.
Plants, algae and cyanobacteria all split water and release oxygen.
Cyanobacteria are prokaryotes: pigments in cell membranes, no chloroplasts.
Students often think oxygen is the main product. In fact carbon compounds are the product; oxygen is waste to the cell.
Students often think only plants and algae photosynthesise. In fact cyanobacteria do too, so photosynthesis is not limited to eukaryotes.
C1.3.4 Chromatography separates pigments, and Rf identifies them
A pigment spot on an origin line separates as solvent moves through paper or silica.
Each pigment travels a distance set by its solubility and attraction to the stationary phase.
Rf = distance moved by pigment ÷ distance moved by solvent front, both from the origin.
Rf is between 0 and 1, has no units, and with colour identifies the pigment.
Typical leaf extract: carotene (orange, highest Rf), xanthophylls (yellow), chlorophyll a (blue-green), chlorophyll b (yellow-green, lowest).
Students often think Rf is solvent distance over pigment distance. In fact it is pigment over solvent, giving a value below 1.
Students often think chlorophyll moves furthest because it matters most. In fact distance depends on solubility, not importance.
C1.3.5 Pigments absorb only certain wavelengths, exciting electrons
From 2028 this is supplied in the Biology data booklet — you need to recognise and interpret it, not reproduce it from memory.
An absorption spectrum plots percentage of light absorbed against wavelength and colour.
Chlorophyll absorbs mainly blue and red; green is reflected, so leaves look green.
An absorbed photon excites an electron to a higher energy level.
Only photons whose energy matches that jump are absorbed; shorter wavelength means more energy.
Students often think chlorophyll absorbs green. In fact it absorbs blue and red; green is reflected or transmitted.
Students often think absorbed light heats the pigment to drive reactions. In fact it excites an electron, which carries the energy on.
C1.3.6 Absorption spectra and action spectra look alike but measure different things
An action spectrum plots rate of photosynthesis against wavelength.
Rate is found from oxygen produced or carbon dioxide used per unit time.
Both spectra peak in blue and red.
The action spectrum stays above zero in green, because accessory pigments absorb there.
Students often think the two spectra are the same graph. In fact one plots light absorbed, the other photosynthesis rate.
Students often compare total oxygen collected. In fact totals must be divided by time before rates can be compared.
C1.3.7 Testing how limiting factors change the rate
A limiting factor is the one in shortest supply, setting the rate.
Raise it and rate rises, until another factor becomes limiting and rate plateaus.
Independent variable: what you change (lamp distance, hydrogencarbonate concentration, water bath).
Dependent variable: what you measure (bubbles per minute, gas volume in fixed time).
A hypothesis is a provisional, testable explanation; results can support it, never prove it.
Students often think a supported hypothesis is proven. In fact it survives one test and stays provisional.
Students often think a fixed temperature is the independent variable. In fact a variable kept constant is a controlled variable.
C1.3.8 Growing plants in extra carbon dioxide to predict the future
2028 guide: scope reduced — Basic CO2-enrichment set-up and major limitations only. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
Enrichment experiments grow plants above current atmospheric carbon dioxide.
Enclosed greenhouses allow tight control of variables.
FACE releases carbon dioxide from pipes around field plots; control plots stay at ambient.
Field results are more realistic but less controlled; FACE responses are smaller than in greenhouses.
A controlled variable is one kept constant so it cannot affect the result.
Students often think the controlled variable is the control group. In fact it is a variable held constant across all treatments.
Students often think uncontrolled field results are worthless. In fact control plots isolate the effect, and some questions need the field.
C1.3.9 A photosystem is an array of pigments around one reaction centre HL
A photosystem is an array of chlorophyll and accessory pigments, always in a membrane.
Found in thylakoid membranes of chloroplasts and in cyanobacterial membranes.
Pigments pass absorbed energy to a special chlorophyll, the reaction centre.
Only the reaction centre emits an excited electron to an acceptor.
Students often think a photosystem is one chlorophyll molecule. In fact it is many pigment molecules organised around one centre.
Students often think cyanobacteria have chloroplasts. In fact they are prokaryotes; photosystems sit in membranes within the cell.
C1.3.10 Why an array beats a single pigment molecule HL
Different pigments absorb a broad range of wavelengths.
Hundreds of molecules funnel energy to one reaction centre, so electrons are emitted often.
The reaction centre sits in the membrane beside its electron acceptor.
A single pigment molecule alone could perform no part of photosynthesis.
Students often think accessory pigments only shield chlorophyll. In fact they absorb wavelengths chlorophyll a misses and pass on the energy.
C1.3.11 Photosystem II splits water and releases oxygen HL
Photosystem II emits excited electrons and replaces them by pulling electrons from water.
This photolysis gives electrons, protons and oxygen.
Protons enter the thylakoid lumen; oxygen is waste and diffuses away.
Its evolution in cyanobacterial ancestors oxygenated the oceans and atmosphere.
Students often think photons hit water and split it directly. In fact photosystem II splits water to replace its lost electrons.
Students often think oxygen was good news for early life. In fact it was toxic to anaerobes and caused a mass extinction.
C1.3.12 Chemiosmosis in thylakoids makes ATP HL
Electron carriers in the thylakoid membrane pump protons from stroma into lumen.
Protons flow back into the stroma through ATP synthase, which phosphorylates ADP.
Non-cyclic: electrons from photosystem II pass to photosystem I, then NADP; oxygen released.
Cyclic: electrons from photosystem I return via the carriers; ATP only, no oxygen, no reduced NADP.
Students often think carriers make ATP directly. In fact they pump protons; ATP synthase makes ATP as protons flow back.
Students often think cyclic photophosphorylation uses both photosystems. In fact only photosystem I, and it makes only ATP.
C1.3.13 Photosystem I reduces NADP HL
Photosystem I re-excites electrons from the chain and emits them on the stroma side.
NADP accepts two electrons from photosystem I and a hydrogen ion from the stroma.
This forms reduced NADP, which carries electrons and hydrogen to the Calvin cycle.
Pair the terms consistently: NADP with reduced NADP, or NADP⁺ with NADPH.
Students often think NADP is reduced at photosystem II. In fact it is reduced on the stroma side of photosystem I.
Students often think reduced NADP feeds an electron chain for ATP. In fact it reduces glycerate 3-phosphate in the Calvin cycle.
C1.3.14 Where each light-dependent reaction happens in a thylakoid HL
A thylakoid is a membrane sac; stacks are grana; inside is the lumen.
The membrane holds photosystems, electron carriers and ATP synthase.
Photolysis is on the lumen side of photosystem II; protons accumulate in the lumen.
ATP forms as protons exit through ATP synthase; NADP is reduced on the stroma side.
Students often think the light reactions are in the stroma and the Calvin cycle inside thylakoids. In fact it is the other way round.
C1.3.15 Rubisco fixes carbon dioxide onto RuBP HL
The stroma holds Calvin cycle enzymes and receives ATP and reduced NADP.
Rubisco joins carbon dioxide to RuBP (5C); the 6C product splits at once.
Result: two molecules of glycerate 3-phosphate (3C). No ATP or reduced NADP used.
Rubisco is slow and poor at low carbon dioxide, so the stroma needs lots.
It is the most abundant enzyme on Earth for that reason.
Students often think Rubisco makes a six-carbon sugar. In fact it makes two three-carbon glycerate 3-phosphates.
Students often think Rubisco is abundant because it is efficient. In fact it is abundant because it is slow.
C1.3.16 Glycerate 3-phosphate is reduced to triose phosphate HL
Glycerate 3-phosphate is converted to triose phosphate (TP), a 3C sugar phosphate.
ATP phosphorylates it and reduced NADP reduces it.
Triose phosphate is the cycle's first carbohydrate.
Most regenerates RuBP; the rest makes glucose and other compounds.
Students often think ATP alone makes triose phosphate. In fact both ATP and reduced NADP are needed here.
Students often think another carbon dioxide is added at this step. In fact this step is a reduction, not a carboxylation.
C1.3.17 Five triose phosphates rebuild three RuBP HL
Five triose phosphate (15 carbons) become three RuBP (15 carbons), using ATP.
Without regeneration, RuBP runs out and fixation stops.
Of every six triose phosphates, five are recycled and one leaves.
If glucose is the product, five-sixths of triose phosphate goes back to RuBP.
Students often think half the triose phosphate leaves to make glucose. In fact only one sixth does.
Students often think RuBP regenerates without energy. In fact ATP from the light-dependent reactions is required.
C1.3.18 Every carbon compound in the plant traces back to the Calvin cycle HL
Triose phosphate and other intermediates start the pathways to all carbon compounds.
Glucose, sucrose, starch, cellulose, lipids, amino acids and nucleotides all trace back.
Mineral nutrients such as nitrate and phosphate supply the other elements.
Nitrogen from nitrate or ammonium joins carbon skeletons to make amino acids.
Students often think plants absorb proteins and lipids from soil. In fact they build them from Calvin cycle intermediates and mineral ions.
Students often think plants fix nitrogen from air. In fact nitrogen comes as nitrate or ammonium from the soil.
C1.3.19 Each set of reactions depends on the other HL
Light-dependent reactions supply ATP and reduced NADP; they stop at once without light.
Light-independent reactions consume them and return ADP, phosphate and NADP.
In darkness the Calvin cycle stops within minutes as its supplies run out.
Without carbon dioxide, NADP and ADP are not regenerated, so electron flow and photosystem II halt.
Students often think the dark reactions run at night. In fact they stop within minutes once ATP and reduced NADP are gone.
Students often think low carbon dioxide affects only the Calvin cycle. In fact the light-dependent reactions stall too, because NADP is not returned.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Which statement correctly describes the energy transformation that takes place in photosynthesis?
Answer and reasoning
Light energy absorbed by pigments is transformed into chemical energy in the carbon compounds made. — Photosynthesis is an energy transformation: light energy is absorbed by pigments and ends up as chemical energy in the bonds of carbon compounds such as glucose. This transformation supplies most of the chemical energy needed for life processes in ecosystems.
Energy is generated inside the chloroplast and then stored in glucose for the plant to use. — A student who thinks plants 'make energy' picks this. In fact no energy is generated: the chloroplast transforms light energy that already exists into chemical energy.
Light energy is converted into heat, which then speeds up the reactions that make glucose. — A student who can only picture light being used as heat picks this. In fact absorbed light excites electrons in pigment molecules, and the energy of those electrons is transformed into chemical energy.
Chemical energy absorbed from the soil by the roots is transferred into the glucose molecules the leaf makes. — A student who thinks plants obtain their food from the soil picks this. In fact the soil supplies only water and mineral ions; the energy in glucose comes from light.
2 Which word equation correctly summarises photosynthesis?
Answer and reasoning
glucose + oxygen → carbon dioxide + water + energy — A student who treats photosynthesis and respiration as interchangeable mirror images picks this. This is the summary of cell respiration, which releases the energy stored by photosynthesis.
carbon dioxide + light energy → glucose + oxygen — A student who does not see water as a reactant picks this. Glucose contains hydrogen, which carbon dioxide cannot supply; the hydrogen comes from water, which is split in photosynthesis.
carbon dioxide + water → glucose + oxygen — Carbon dioxide is converted to glucose using hydrogen obtained by splitting water, and the oxygen left over from the water is released as a by-product.
soil minerals + water → glucose + oxygen — A student who thinks a plant's food is built from what its roots absorb picks this. In fact the carbon in glucose comes from carbon dioxide taken in from the air, not from soil minerals.
3 On a paper chromatogram of leaf pigments, the origin line was 20 mm from the bottom edge of the paper, the centre of the carotene spot was 65 mm from the bottom edge and the solvent front was 70 mm from the bottom edge. What is the Rf value of carotene?
Answer and reasoning
0.93 — A student who measures from the bottom edge of the paper instead of the origin line divides 65 by 70. Both distances must be measured from the origin, giving 45 ÷ 50.
1.11 — A student who puts the solvent distance on top divides 50 by 45. Rf is the pigment distance divided by the solvent distance and can never exceed 1.
0.90 — Both distances are measured from the origin: the pigment moved 65 − 20 = 45 mm and the solvent front moved 70 − 20 = 50 mm. Rf = 45 ÷ 50 = 0.90.
0.10 — A student who measures the gap between the spot and the solvent front (5 mm) instead of the distance from the origin picks this. The pigment travelled 45 mm from the origin, so the Rf is high, not low.
4 Samples of an aquatic plant were exposed to light of different wavelengths and the oxygen produced was collected: at 450 nm, 12 cm³ in 10 minutes; at 550 nm, 6 cm³ in 15 minutes; at 650 nm, 15 cm³ in 15 minutes; at 700 nm, 3 cm³ in 5 minutes. Which statement about the action spectrum plotted from these data is correct?
Answer and reasoning
The highest rate is at 650 nm, because that sample produced the most oxygen. — A student who confuses amount with rate picks this. The 650 nm sample ran for 15 minutes; its rate is 15 ÷ 15 = 1.0 cm³ per minute, lower than the 1.2 cm³ per minute at 450 nm.
The rate at 550 nm should be plotted as zero, because chlorophyll reflects green. — A student who believes green light drives no photosynthesis picks this, ignoring the data. The sample at 550 nm produced 6 cm³ in 15 minutes, a rate of 0.4 cm³ per minute, which is low but not zero.
The highest rate is at 450 nm, at 1.2 cm³ of oxygen produced per minute. — Rates are amounts divided by times: 12 ÷ 10 = 1.2 cm³ per minute at 450 nm, 6 ÷ 15 = 0.4 at 550 nm, 15 ÷ 15 = 1.0 at 650 nm and 3 ÷ 5 = 0.6 at 700 nm. The action spectrum plots these rates against wavelength, with the peak at 450 nm.
The graph shows the percentage of light at each wavelength absorbed by the pigments. — A student who confuses the two spectra picks this. Percentage absorption is an absorption spectrum; these data give rates of photosynthesis, which plotted against wavelength make an action spectrum.
5 Tomato plants were grown in two enclosed greenhouses, one at 400 ppm carbon dioxide and one at 800 ppm. Both were kept at 22 °C with the same lighting and watering, and after six weeks the dry mass of the plants was measured. Which is a controlled variable in this experiment?
Answer and reasoning
The carbon dioxide concentration in the greenhouses — A student who thinks the controlled variable is the one the experimenter sets picks this. Carbon dioxide concentration is deliberately varied between the greenhouses, so it is the independent variable.
The dry mass of the plants measured at six weeks — A student who thinks 'controlled' means 'carefully measured' picks this. Dry mass is what is measured to find the effect of the carbon dioxide, so it is the dependent variable; controlled variables are those held constant.
The plants grown at 400 ppm carbon dioxide — A student who merges 'control' with 'controlled variable' picks this. The plants at 400 ppm are the control treatment for comparison; a controlled variable is a factor held constant, such as temperature.
The temperature of 22 °C in both greenhouses — Temperature could affect growth, so it is kept the same in both greenhouses to ensure that any difference in dry mass is due to carbon dioxide alone. That makes it a controlled variable; lighting and watering are also controlled.
6 Why is a structured array of many different pigment molecules, rather than a single chlorophyll molecule, needed to carry out the first step of photosynthesis? HL
Answer and reasoning
Each pigment molecule in the array carries out photosynthesis by itself, so the array simply packs more independent working units into the thylakoid membrane. — A student who thinks each chlorophyll works alone picks this. A single molecule could not perform any part of photosynthesis; the array works because the molecules cooperate, passing energy to one reaction centre.
The accessory pigments only protect the chlorophyll from damage by bright light; a single chlorophyll could absorb the light equally well by itself. — A student who knows carotenoids only as protective picks this. Accessory pigments absorb wavelengths chlorophyll a absorbs poorly and pass the energy on, broadening the light used; a lone chlorophyll would absorb photons only rarely.
Every pigment in the array emits its own excited electron to the electron carriers, so the array releases many more electrons per second than one molecule would. — A student who applies 'emits electrons' to every molecule picks this. Only the reaction-centre chlorophyll emits an electron; the advantage of the array is that it delivers energy to that one centre far more often.
Different pigments absorb a wide range of wavelengths and funnel the energy to one reaction centre, which is excited far more often than a lone molecule. — The advantages of the structured array are that pigments of different types absorb a broader range of wavelengths, and that energy from hundreds of molecules is funnelled to a single reaction centre, which therefore emits electrons far more frequently than any one molecule could. A single molecule of any pigment could not perform any part of photosynthesis.
7 Which statement correctly describes how ATP is produced in the thylakoid? HL
Answer and reasoning
The electron carriers make ATP directly as electrons pass along them, transferring the energy of each electron into a phosphate bond on ADP. — A student who expects the chain itself to make ATP picks this. The carriers use the electrons' energy to pump protons; ATP is made only by ATP synthase, using the proton gradient.
Protons are pumped out of the lumen into the stroma and flow back into the lumen through ATP synthase, releasing ATP inside it. — A student who copies the mitochondrial layout the wrong way round picks this. In chloroplasts protons accumulate in the thylakoid lumen and flow out into the stroma, where ATP is released.
Electron carriers pump protons into the thylakoid lumen, and the protons flow back into the stroma through ATP synthase, which makes ATP. — This is chemiosmosis: energy from excited electrons passing along the chain of carriers is used to pump protons into the lumen, creating a proton gradient, and ATP synthase uses the flow of protons back into the stroma to phosphorylate ADP.
Reduced NADP passes its electrons to the carriers, which pump protons for ATP synthesis, exactly as reduced NAD does in mitochondria. — A student who transfers the mitochondrial role of reduced NAD to reduced NADP picks this. In the thylakoid, electrons come from photosystem II or photosystem I, and reduced NADP is the final product of the chain, not its source.
8 Which statement correctly locates the light-dependent reactions within a thylakoid? HL
Answer and reasoning
Water is split in the stroma, and ATP and reduced NADP are made in the thylakoid lumen, where the enzymes of the Calvin cycle then use them. — A student who has swapped the compartments picks this. Photolysis is in the thylakoid membrane on the lumen side, ATP and reduced NADP are released into the stroma, and the Calvin cycle runs in the stroma.
Protons are pumped out into the stroma, and ATP synthase makes ATP inside the thylakoid lumen as the protons flow back in across the membrane. — A student who applies the mitochondrial arrangement the wrong way round picks this. In the chloroplast protons are pumped into the lumen and flow out to the stroma, where ATP is released.
The photosystems float freely in the thylakoid lumen, and ATP synthase is embedded in the outer membrane of the chloroplast. — A student who pictures photosystems as soluble picks this. Photosystems, electron carriers and ATP synthase are all embedded in the thylakoid membrane; none is in the chloroplast envelope.
Water is split on the lumen side of photosystem II, ATP synthase releases ATP into the stroma and NADP is reduced on the stroma side. — Photolysis takes place at photosystem II on the lumen side of the thylakoid membrane, releasing protons into the lumen; ATP synthase spans the membrane and releases ATP into the stroma as protons flow out; NADP is reduced on the stroma side of photosystem I.
9 What is required to convert glycerate 3-phosphate into triose phosphate in the Calvin cycle? HL
Answer and reasoning
ATP only; the reduced NADP is saved for the later regeneration of RuBP from triose phosphate. — A student who has paired the two products with the wrong steps picks this. Reducing glycerate 3-phosphate needs reduced NADP as well as ATP; regenerating RuBP uses ATP only.
Reduced NADP only; ATP from the light-dependent reactions is not used in the light-independent reactions. — A student who thinks reduced NADP supplies all the energy for the Calvin cycle picks this. ATP is used both to convert glycerate 3-phosphate to triose phosphate and to regenerate RuBP.
ATP, which phosphorylates glycerate 3-phosphate, and reduced NADP, which reduces it. — Glycerate 3-phosphate is converted into triose phosphate using both products of the light-dependent reactions: ATP supplies energy and a phosphate group, and reduced NADP supplies the hydrogen for the reduction.
A second molecule of carbon dioxide, which Rubisco attaches to glycerate 3-phosphate. — A student who expects the carbon chain to grow at each step picks this. Both compounds have three carbons; the conversion is a reduction, and carbon dioxide is added only once per turn, by Rubisco to RuBP.
10 A plant makes the amino acid alanine, which contains carbon and nitrogen. Where do these atoms come from? HL
Answer and reasoning
The whole amino acid is absorbed from the soil, where decomposers have released it from dead organisms. — A student who thinks plants absorb their proteins ready-made picks this. Plants make their own amino acids from Calvin cycle intermediates and mineral nitrogen; they do not take up amino acids as their source.
The carbon is fixed in the Calvin cycle and the nitrogen comes from nitrate or ammonium absorbed by the roots. — All the carbon in the compounds of a photosynthesizing organism is fixed in the Calvin cycle, and amino acids are made by pathways that branch from an intermediate of the cycle, using nitrogen from mineral nutrients such as nitrate.
Both are fixed in the Calvin cycle, which fixes nitrogen gas from the air as well as carbon dioxide. — A student who merges carbon fixation with nitrogen fixation picks this. The Calvin cycle fixes only carbon; plants cannot fix nitrogen gas and obtain nitrogen as nitrate or ammonium ions.
The carbon comes from humus in the soil, absorbed by the roots together with mineral ions such as nitrate. — A student who thinks plant carbon comes from the soil picks this. Roots absorb water and mineral ions, not carbon compounds; every carbon atom in the plant was fixed from carbon dioxide.
Read the ones marked not yet in Learn, then Verify.
Verify confirm before you go
28 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 A fox eats a rabbit that has fed on grass. Where did the chemical energy in the carbon compounds of the fox's body originally come from?
Answer and reasoning
New energy created by the grass in photosynthesis and passed on to the rabbit and fox. — A student who thinks plants generate energy picks this. Photosynthesis creates no energy; it transforms light energy that already exists into chemical energy in carbon compounds, which passed along the food chain.
Light energy transformed into chemical energy by photosynthesis in the grass. — The energy transformation in photosynthesis supplies most of the chemical energy needed for life processes in ecosystems: the grass transformed light energy into chemical energy in carbon compounds, which passed to the rabbit and then to the fox.
Chemical energy that the grass absorbed from the soil through its roots. — A student who thinks plants take in food from the soil picks this. In fact the soil supplies water and mineral ions only; the chemical energy of the grass came from light.
Energy in the oxygen the grass made as its main product, breathed in by the two animals. — A student who sees oxygen as the energy-rich main product of photosynthesis picks this. Oxygen is a by-product; the chemical energy is stored in the carbon compounds, not in the oxygen.
2 Glucose produced by photosynthesis contains hydrogen atoms. From which substance were these hydrogen atoms obtained?
Answer and reasoning
Carbon dioxide molecules joined together to make glucose. — A student who thinks glucose is simply carbon dioxide molecules joined up picks this. Carbon dioxide contains no hydrogen at all, so the hydrogen in glucose must come from water.
Mineral ions the roots absorbed from the soil water. — A student who thinks plants build their food from what the roots take in picks this. Mineral ions supply elements such as nitrogen and phosphorus, not the hydrogen of glucose.
Chlorophyll, which breaks down during the reaction. — A student who reads chlorophyll in the equation as a reactant picks this. Chlorophyll absorbs light and is left unchanged; it contributes no atoms to glucose.
Water molecules, which are split during photosynthesis. — Carbon dioxide is converted to glucose using hydrogen obtained by splitting water. The oxygen left over from the split water molecules is released.
3 Algae were supplied with water containing the heavy isotope oxygen-18 and ordinary carbon dioxide; the oxygen gas they released contained oxygen-18. Other algae were given ordinary water and carbon dioxide containing oxygen-18; the oxygen gas they released contained no oxygen-18. What do these results show?
Answer and reasoning
The oxygen released by photosynthesis comes from the splitting of water and not from carbon dioxide. — The label appears in the oxygen gas only when it is supplied in the water, so the oxygen produced by photosynthesis comes from the splitting of water. The oxygen atoms of carbon dioxide end up in glucose and water.
The oxygen released comes from carbon dioxide once its carbon has been removed to make glucose. — A student who pictures the plant swapping carbon dioxide for oxygen picks this. The data contradict it: labelled carbon dioxide gave unlabelled oxygen, so none of the released oxygen came from carbon dioxide.
Water is not a reactant but only dissolves the carbon dioxide before the reaction takes place. — A student who thinks of water as a mere solvent picks this. The appearance of oxygen-18 from labelled water in the oxygen gas shows that water molecules are broken apart during photosynthesis.
Producing oxygen is the main purpose of photosynthesis, using both water and carbon dioxide. — A student who sees oxygen as the product the plant is aiming for picks this. The experiment says nothing about purpose, and oxygen is a by-product formed only from water; the useful products are the carbon compounds.
4 Which list includes all of the groups of organisms that release oxygen as a by-product of photosynthesis?
Answer and reasoning
Plants, algae, cyanobacteria, fungi — A student who thinks of fungi as plants picks this. Fungi have no photosynthetic pigments and obtain carbon compounds by absorbing them; they release no oxygen.
Plants, algae and cyanobacteria — Oxygen is a by-product of photosynthesis in plants, algae and cyanobacteria. Cyanobacteria are prokaryotes, but they split water in the same way and release oxygen.
Plants and algae, no bacteria — A student who has only met bacteria as decomposers or pathogens picks this. Cyanobacteria are photosynthetic bacteria that split water and release oxygen.
Land plants and no other group — A student who associates photosynthesis only with green plants picks this. Algae and cyanobacteria also photosynthesize and release oxygen; most of the oxygen in the atmosphere first came from cyanobacteria.
5 In the solvent used, reference Rf values for leaf pigments are: carotene 0.95 (orange), xanthophyll 0.71 (yellow), chlorophyll a 0.65 (blue-green) and chlorophyll b 0.45 (yellow-green). A student's chromatogram shows three spots: orange at Rf 0.94, blue-green at Rf 0.66 and yellow-green at Rf 0.44. Which conclusion is best supported?
Answer and reasoning
The extract contains only chlorophyll, which the solvent has separated into three coloured forms. — A student who thinks a leaf contains a single pigment picks this. Chromatography does not break chlorophyll into forms; it separates the different pigments that were already present in the extract.
The Rf values differ from the reference values, so none of the three pigments can be identified reliably. — A student who does not trust Rf as a constant picks this. Differences of 0.01 are measurement uncertainty; each spot matches one reference pigment in colour and Rf and no other.
The orange spot is chlorophyll, because the most abundant pigment is the one that travels furthest. — A student who links distance moved to abundance picks this. Distance depends on solubility in the solvent and attraction to the paper; an orange spot at Rf 0.94 is carotene, whatever its amount.
The extract contains carotene, chlorophyll a and chlorophyll b; any xanthophyll was too faint to see. — Each spot matches a reference pigment in both colour and Rf value, within measurement uncertainty. No yellow spot at about 0.71 was seen, so xanthophyll was either absent or present in too small an amount to be visible.
6 A chlorophyll molecule in a leaf absorbs a photon of red light. What happens within the molecule as a result?
Answer and reasoning
An electron is raised to a higher energy level. — Absorbing a photon excites an electron within the pigment molecule, raising it to a higher energy level. This is the step in which light energy is transformed into chemical energy, carried by the excited electron.
It vibrates faster, warming the surrounding stroma. — A student who thinks light is used as heat picks this. The absorbed energy goes into exciting an electron, not into heating; the excited electron's energy is what photosynthesis captures.
It splits a molecule of water that is bound to it. — A student who takes 'photolysis' to mean that light splits water directly picks this. Absorption excites an electron; water is split by photosystem II only after the reaction centre has lost an electron.
It stores the light inside itself until it is needed. — A student who thinks plants store sunlight picks this. Light cannot be stored; its energy is transformed at once into the energy of an excited electron and then into chemical energy.
7 The absorption spectrum of chlorophyll a has peaks at about 430 nm (blue) and 660 nm (red) but a trough at about 550 nm (green). Which statement best explains this pattern?
Answer and reasoning
Green photons carry too little energy to excite an electron in chlorophyll a, whereas blue and red photons both carry more than enough energy to do so. — A student who treats absorption as a threshold picks this. Green photons (550 nm) carry more energy than red photons (660 nm), yet red is absorbed and green is not, so absorption depends on matching an energy gap, not on exceeding a minimum.
Only photons whose energy matches that needed to excite an electron in chlorophyll a are absorbed; blue and red photons match, green do not. — Electron energy levels are discrete, so a photon is absorbed only if its energy matches the gap between the ground state and an available excited state. Blue and red photons match transitions in chlorophyll a; green photons do not, and are reflected or transmitted.
Chlorophyll a absorbs green light more strongly than any other colour, which is exactly why leaves that contain it appear green. — A student who links the colour seen with the colour absorbed picks this. The trough in the spectrum shows green is absorbed least; leaves look green because green light is reflected and transmitted rather than absorbed.
Blue and red light heat the chlorophyll molecule more than green light does, so more of their energy is taken in and stored. — A student who thinks absorbed light acts by heating picks this. Absorption raises an electron to a higher energy level; the pattern of wavelengths absorbed reflects the electron energy levels of the molecule, not heating.
8 Pure chlorophyll a absorbs very little light at 550 nm (green), yet the action spectrum of a whole leaf shows a rate of photosynthesis at 550 nm of about 30% of the maximum. Which explanation is best?
Answer and reasoning
The reading must be an error, because leaves reflect green light and so cannot photosynthesize in it at all. — A student who turns 'absorbs little green' into 'absorbs none' picks this. Leaves reflect much of the green light but absorb some, so a reduced but real rate in green light is expected, not an error.
In a living leaf chlorophyll a absorbs green light strongly, which is the reason the leaf appears green to the eye. — A student who thinks a pigment absorbs the colour it shows picks this. Chlorophyll a absorbs green poorly in the leaf as in the test tube; the leaf looks green because green light is reflected.
Green light warms the leaf, and the higher temperature speeds up the enzyme-catalysed reactions of photosynthesis. — A student who thinks light acts through heating picks this. Photosynthesis is driven by photons absorbed by pigments that excite electrons; warming does not explain a rate of photosynthesis in a specific wavelength.
Accessory pigments absorb some of the green light that chlorophyll a does not, and pass the energy on. — This is a key difference between the two spectra: the action spectrum of a leaf reflects all its pigments. Accessory pigments absorb wavelengths that chlorophyll a absorbs poorly, including part of the green region, so photosynthesis continues at a reduced rate in green light.
9 A student places a lamp at different distances from a beaker of pondweed in sodium hydrogencarbonate solution kept at 25 °C, and counts the bubbles of gas released per minute at each distance. Which correctly identifies the variables?
Answer and reasoning
Independent: lamp distance; dependent: bubbles per minute — The lamp distance is what the student deliberately changes (it sets the light intensity), so it is the independent variable; the bubble count is measured to find the effect, so it is the dependent variable. Temperature and hydrogencarbonate concentration are controlled variables.
Independent: bubbles per minute; dependent: distance of lamp — A student who reasons that the result 'depends on' the light and so calls the light dependent picks this. The independent variable is the one the investigator changes; the dependent variable is the one measured.
Controlled variable: lamp distance; dependent: bubbles per minute — A student who thinks 'controlled' means 'set by the experimenter' picks this. The lamp distance is varied, so it is the independent variable; controlled variables are those held constant, such as temperature.
Independent: temperature of 25 °C; dependent: bubbles per minute — A student who picks the variable with a fixed number attached picks this. Temperature is kept constant, so it is a controlled variable; the independent variable is the one that changes between trials, the lamp distance.
10 Pondweed submerged in water at 25 °C with the carbon dioxide concentration of ordinary air was exposed to increasing light intensity. The rate of photosynthesis rose at first and then levelled off, staying constant however much the light intensity was increased. Which change is most likely to raise the rate at this plateau?
Answer and reasoning
Moving the lamp even closer, so that the light intensity becomes higher still. — A student who expects the rate to keep rising with light picks this. The plateau shows that increasing light no longer has any effect, because another factor is now limiting.
Adding sodium hydrogencarbonate to the water to raise the carbon dioxide concentration. — At the plateau light is no longer the limiting factor. With carbon dioxide at the low concentration of ordinary air and a moderate temperature, carbon dioxide is the factor most likely to be limiting, so raising it should raise the rate to a new plateau.
Nothing, because the plateau is the maximum rate of which the plant is capable. — A student who reads the plateau as an intrinsic ceiling picks this. The plateau is set by whichever factor is now limiting; raising that factor raises the plateau.
Adding more water to the beaker, since water is a reactant in photosynthesis. — A student who treats every reactant as a limiting factor picks this. The pondweed is already submerged in water; water supply is not what limits the rate, and it is not one of the factors varied in these investigations.
11 A student proposes the hypothesis that carbon dioxide concentration limits the rate of photosynthesis of pondweed in bright light. Which statement about this hypothesis is consistent with the nature of science?
Answer and reasoning
It is only valid if it was written down before any experiment was performed; a hypothesis formed after seeing results is not scientific. — A student who takes the order of a practical write-up as a rule of science picks this. Hypotheses can be based on evidence from an experiment already carried out, provided they are then tested further.
If a single experiment gives results that agree with it, the hypothesis has been proven and can be treated as an established fact. — A student who applies the mathematical idea of proof to experiments picks this. One experiment can support a hypothesis but cannot prove it; hypotheses remain provisional and require repeated testing.
It is a provisional explanation that needs repeated testing, and could have been proposed before or after a first experiment. — Hypotheses are provisional explanations that require repeated testing. They can be based on theories and then tested, or based on evidence from an experiment already carried out; students may suggest hypotheses about limiting factors before or after performing their experiments.
It is merely a guess that needs no basis in theory or evidence, because all that matters is the testing that follows. — A student who has taken 'educated guess' to mean 'guess' picks this. Hypotheses are explanations based on theory or on evidence already gathered; that basis is what makes them worth testing.
12 In an enclosed greenhouse trial, wheat grown at 550 ppm carbon dioxide gave 35% more grain than wheat at 370 ppm. In a FACE experiment with the same variety in open fields, plots at 550 ppm gave 13% more grain than plots at 370 ppm. Which is the best explanation of the difference?
Answer and reasoning
Other factors, such as water and nitrogen supply, also limit growth in the field, so the extra carbon dioxide gives a smaller gain. — In natural conditions growth is limited by several factors at once, so raising carbon dioxide alone has a smaller effect than under idealised greenhouse conditions. This is why field experiments such as FACE are needed to predict future plant growth realistically.
The FACE result is unreliable because weather and soil could not be held constant, so only the greenhouse figure should be used to predict future yields. — A student who thinks only a fully controlled experiment is valid picks this. FACE uses control plots at ambient carbon dioxide in the same field, so weather and soil affect both treatments equally; the field result is the more realistic guide to future yields.
The response of a variety to carbon dioxide is a fixed property of that variety, so the FACE plots must in fact have received less than 550 ppm carbon dioxide. — A student who assumes a greenhouse response applies anywhere picks this. The same variety responds differently in the field because other limiting factors are present; the difference does not imply an error in the treatment.
Wheat grown in the field obtains most of its extra mass from soil minerals rather than from carbon dioxide, so enrichment matters less there than in a greenhouse. — A student who thinks plant mass comes from the soil picks this. The carbon in grain comes from carbon dioxide fixed in photosynthesis in the field just as in the greenhouse; soil minerals supply no carbon.
13 Which statement correctly describes a photosystem? HL
Answer and reasoning
A single chlorophyll molecule that absorbs light by itself and carries out the whole of the light-dependent reactions on its own. — A student who has kept the GCSE picture of chlorophyll picks this. A single molecule could not perform any part of photosynthesis; a photosystem is an array of many pigment molecules.
An array of chlorophyll and accessory pigments held in a membrane, with a special chlorophyll at the reaction centre that emits an excited electron. — Photosystems are molecular arrays of chlorophyll and accessory pigments, always located in membranes, with a special chlorophyll as the reaction centre from which an excited electron is emitted.
A cluster of pigment molecules dissolved in the stroma, from which excited electrons pass across into the thylakoid space. — A student who pictures chlorophyll as free in the chloroplast fluid picks this. Photosystems are always located in membranes, in chloroplasts the thylakoid membranes.
An array of pigment molecules in the thylakoid membrane, each of which emits its own excited electron to the chain of electron carriers. — A student who applies 'emits excited electrons' to every molecule picks this. Only the special chlorophyll at the reaction centre emits an electron; the other pigments pass energy to it.
14 Cyanobacteria carry out photosynthesis using photosystems similar to those of plants. Where are the photosystems of a cyanobacterium located? HL
Answer and reasoning
In the thylakoid membranes of the chloroplasts that cyanobacteria contain, as in plant cells. — A student who thinks every photosynthesizer has chloroplasts picks this. Cyanobacteria are prokaryotes and have no membrane-bound organelles, so they have no chloroplasts.
Free in the cytoplasm, dissolved in the fluid that surrounds the DNA of the cell. — A student who pictures photosystems as soluble picks this. Photosystems function by passing electrons to neighbouring membrane proteins and pumping protons, so they are always in membranes.
In membranes inside the cell, because cyanobacteria are prokaryotes with no chloroplasts. — Photosystems are always located in membranes. Cyanobacteria are prokaryotes and have no chloroplasts, so their photosystems are held in membranes within the cell itself.
Nowhere, because only eukaryotic organisms are able to carry out photosynthesis at all. — A student who associates photosynthesis only with plants and algae picks this. Cyanobacteria are photosynthetic prokaryotes, and photosystems occur in them as well as in the chloroplasts of eukaryotes.
15 Which statement about the photolysis of water in photosystem II is correct? HL
Answer and reasoning
Its electrons replace those lost by the reaction centre, its protons join the gradient and oxygen is waste. — Photolysis in photosystem II yields electrons, which replace those emitted by the reaction-centre chlorophyll, and protons, which are released into the thylakoid lumen and add to the proton gradient. The oxygen is a waste product that diffuses away.
Light absorbed directly by water molecules in the thylakoid lumen breaks them apart into hydrogen gas and oxygen gas. — A student who takes 'photolysis' literally picks this. Light does not split water; the oxidized reaction centre of photosystem II pulls electrons from water, and the hydrogen is released as protons, not as hydrogen gas.
The hydrogen released from water is passed straight to NADP at photosystem II, reducing it before the electrons move on. — A student who links the water-splitting site directly to NADP picks this. The electrons from water pass along the chain of carriers and through photosystem I before NADP is reduced on the stroma side.
Oxygen is the useful product of photolysis, and the protons and electrons are waste that diffuses out of the chloroplast. — A student who sees oxygen as the point of photosynthesis picks this. It is the reverse: the protons and electrons are used in the light-dependent reactions, and oxygen is the waste product.
16 Photolysis of water by cyanobacteria began releasing oxygen into the oceans and atmosphere more than two billion years ago. Which statement describes a consequence of this? HL
Answer and reasoning
Oxygen benefited all the organisms living at the time, because every one of them was able to switch from anaerobic to aerobic respiration. — A student who assumes oxygen has always been good for life picks this. Early organisms were anaerobes with no protection against oxygen, and many were killed by it; aerobic respiration evolved later.
Oxygen oxidized dissolved iron, forming banded iron formations in rocks, and was toxic to many of the anaerobic organisms living at the time. — The advent of oxygen generation had immense consequences for geological processes and living organisms: dissolved iron in the oceans was oxidized and deposited as banded iron formations, and free oxygen was toxic to the anaerobes that dominated the early Earth, causing widespread extinction before aerobic life spread.
Oxygen levels did not rise until land plants evolved, because bacteria release too little oxygen to affect the atmosphere. — A student who attributes atmospheric oxygen to plants picks this. Cyanobacteria oxygenated the oceans and atmosphere more than a billion years before plants colonised the land.
The oxygen released came from carbon dioxide, so atmospheric carbon dioxide fell by exactly the amount that oxygen rose. — A student who thinks oxygen is stripped from carbon dioxide picks this. The oxygen released by photolysis comes from water; carbon dioxide levels changed through carbon fixation, not by giving up their oxygen.
17 A herbicide blocks the flow of electrons out of photosystem II. Chloroplasts treated with it are kept in bright light with a supply of ADP, phosphate and NADP. Which process can continue? HL
Answer and reasoning
Oxygen release, because light continues to split water molecules directly whether or not photosystem II passes on its electrons. — A student who thinks light splits water directly picks this. Water is split only to replace electrons lost by photosystem II; if its electrons cannot leave, it does not lose any, so no water is split and no oxygen is released.
Continuous reduction of NADP, because photosystem I generates its own electrons from light and so needs no supply from elsewhere. — A student who reads 'generate electrons' as 'create electrons' picks this. Every electron photosystem I emits must be replaced from the chain; with the supply from photosystem II blocked, non-cyclic flow and NADP reduction stop.
No ATP production at all, because both the cyclic and the non-cyclic pathways begin with electrons emitted by photosystem II. — A student who thinks cyclic photophosphorylation is a loop of the non-cyclic pathway picks this. The cyclic pathway sources its electrons from photosystem I, so it is unaffected by a block at photosystem II.
ATP production by cyclic photophosphorylation, since photosystem I can still emit electrons that return to it via the carriers. — In cyclic photophosphorylation electrons are sourced from photosystem I, passed to the chain of carriers and returned to photosystem I, pumping protons on the way. This does not need photosystem II, so ATP can still be made by chemiosmosis.
18 A chemical is added that makes the thylakoid membrane freely permeable to protons, so that no proton gradient can build up across it. The chloroplasts are illuminated with a supply of ADP, phosphate and NADP. Which prediction is correct? HL
Answer and reasoning
Oxygen and reduced NADP continue to be produced in the light, but ATP synthesis stops completely. — Electron transport, photolysis and the reduction of NADP do not need the proton gradient, so they continue in the light. ATP synthase, however, is driven by protons flowing through it down a gradient; with the membrane leaky no gradient forms and no ATP is made.
ATP is still made, because the electron carriers phosphorylate ADP directly as the electrons pass along. — A student who thinks the carriers make ATP directly picks this. The carriers only pump protons; without a gradient there is nothing for ATP synthase to use, so no ATP is made.
Everything stops, because without ATP the photosystems have no energy to emit electrons. — A student who thinks ATP powers the light-dependent reactions picks this. The photosystems are driven by light, not ATP; electron transport, photolysis and NADP reduction continue without a gradient.
More ATP is made, because the protons now reach ATP synthase in the stroma more quickly than before. — A student who thinks ATP synthase just needs protons to arrive picks this. ATP synthase is turned by protons flowing through it down a gradient; if protons leak everywhere across the membrane there is no gradient and no flow through the enzyme.
19 Which statement correctly describes the reduction of NADP in the light-dependent reactions? HL
Answer and reasoning
NADP accepts hydrogen atoms that are released directly by the splitting of water at photosystem II, forming reduced NADP. — A student who connects the hydrogen carrier straight to the water-splitting site picks this. The electrons from water travel along the whole chain and through photosystem I before they reach NADP.
NADP accepts two electrons that have come from photosystem I and a hydrogen ion from the stroma, forming reduced NADP. — NADP is reduced by accepting two electrons that have come from photosystem I, together with a hydrogen ion from the stroma. The product is reduced NADP (NADPH), which carries electrons and hydrogen to the Calvin cycle.
NADP accepts electrons that are returning to photosystem I in cyclic photophosphorylation, forming reduced NADP. — A student who thinks the cyclic pathway also makes reduced NADP picks this. In cyclic photophosphorylation the electrons go back to photosystem I instead of to NADP; NADP is reduced only in the non-cyclic pathway.
NADP accepts two electrons from photosystem I and a hydrogen ion from the thylakoid lumen, forming reduced NADP inside the thylakoid. — A student who thinks ATP and the other products are made inside the thylakoid picks this. NADP is reduced on the stroma side of photosystem I and takes its hydrogen ion from the stroma, where the Calvin cycle uses it.
20 What happens to reduced NADP after it has been formed by photosystem I? HL
Answer and reasoning
It passes its electrons to the chain of carriers so that protons are pumped, as reduced NAD does in mitochondria. — A student who transfers the role of reduced NAD in respiration picks this. In the chloroplast the chain reduces NADP as its last step; reduced NADP does not feed electrons back into it.
It supplies the phosphate and energy needed to convert triose phosphate back into RuBP in the stroma. — A student who has paired the two products with the two energy-requiring steps the wrong way round picks this. Regeneration of RuBP uses ATP only; reduced NADP is used to reduce glycerate 3-phosphate.
It carries electrons and hydrogen into the stroma, where they reduce glycerate 3-phosphate to triose phosphate. — Reduced NADP is the product of the light-dependent reactions. It is used in the stroma by the light-independent reactions, where it reduces glycerate 3-phosphate to triose phosphate, and the NADP released returns to be reduced again.
It is stored in the stroma until darkness falls, when the light-independent reactions are able to use it. — A student who thinks the light-independent reactions happen at night picks this. Reduced NADP is used as soon as it forms; the Calvin cycle runs in the light and stops soon after light is removed.
21 Which statement correctly describes carbon fixation by Rubisco? HL
Answer and reasoning
Carbon dioxide combines with RuBP, forming two molecules of glycerate 3-phosphate. — Rubisco catalyses the reaction between its substrates RuBP (five carbons) and carbon dioxide; the unstable six-carbon product immediately splits into two molecules of the product glycerate 3-phosphate.
Carbon dioxide combines with RuBP to form a six-carbon sugar, which is then released as glucose. — A student who expects the word-equation product to appear at once picks this. The six-carbon intermediate is unstable and splits into two three-carbon glycerate 3-phosphates; glucose is made much later from triose phosphate.
Rubisco uses the energy of ATP to attach carbon dioxide to RuBP, forming glycerate 3-phosphate. — A student who assumes the fixation step must be where ATP is spent picks this. Carboxylation of RuBP needs no ATP; ATP is used later, in reducing glycerate 3-phosphate and regenerating RuBP.
Rubisco adds hydrogen from reduced NADP to carbon dioxide, converting it directly into triose phosphate. — A student who thinks carbon dioxide is reduced as soon as it enters the cycle picks this. Rubisco carboxylates RuBP; reduction with reduced NADP happens in the next step, converting glycerate 3-phosphate to triose phosphate.
22 Rubisco can make up half of the soluble protein in a leaf and is the most abundant enzyme on Earth. Which explanation of this abundance is best? HL
Answer and reasoning
It is an exceptionally fast and efficient enzyme with a high affinity for carbon dioxide, so plants invest heavily in it. — A student who assumes abundance means excellence picks this. Rubisco is slow (a few reactions per second) and has a low affinity for carbon dioxide; that is precisely why so much of it is needed.
It catalyses each reaction relatively slowly and is not effective at low carbon dioxide concentrations, so many molecules are needed. — High concentrations of Rubisco are needed in the stroma because it works relatively slowly and is not effective in the low carbon dioxide concentrations found in air. The plant compensates for a slow, poorly effective enzyme by making a great deal of it.
It is used up each time it fixes a molecule of carbon dioxide, so large amounts must be made continuously to replace what is lost. — A student who thinks enzymes are consumed picks this. Rubisco is a catalyst and is released unchanged after each reaction; its abundance is needed because each molecule is slow, not because molecules are lost.
Carbon dioxide is one of the most abundant gases in the air, so a very large number of enzyme molecules is needed to fix all of it. — A student who overestimates atmospheric carbon dioxide picks this. Carbon dioxide is only about 0.04% of air; its scarcity is one reason Rubisco is not effective and must be present in such quantity.
23 In a Calvin cycle that produces one molecule of glucose, how many molecules of triose phosphate are made in total, and how many of them are used to regenerate RuBP? HL
Answer and reasoning
12 made, 6 recycled — A student who thinks one of each pair of triose phosphates leaves the cycle picks this. Six RuBP (30 carbons) need ten triose phosphates (30 carbons) to regenerate them, leaving only two for glucose.
6 made, 5 recycled — A student who counts one triose phosphate per carbon dioxide fixed picks this. Each carbon dioxide gives two glycerate 3-phosphates and so two triose phosphates, giving 12 for six carbon dioxides.
12 made, none recycled — A student who thinks RuBP is regenerated from glycerate 3-phosphate picks this. RuBP is regenerated from triose phosphate, so most of the triose phosphate produced must be recycled.
12 made, 10 recycled — Glucose has six carbons, so six carbon dioxides are fixed. Each fixation gives two glycerate 3-phosphates and hence two triose phosphates, so 12 are made. Two (six carbons) make the glucose and the other ten (30 carbons) regenerate six RuBP (30 carbons), which is five-sixths of the triose phosphate produced.
24 What is required for the conversion of five molecules of triose phosphate into three molecules of RuBP? HL
Answer and reasoning
ATP from the light-dependent reactions, but no reduced NADP — Regeneration of RuBP in the Calvin cycle uses ATP; reduced NADP is used only in the reduction of glycerate 3-phosphate to triose phosphate. Five triose phosphates (15 carbons) become three RuBP (15 carbons), so the cycle can continue.
Both ATP and reduced NADP from the light-dependent reactions — A student who thinks reduced NADP is used in regeneration picks this. Reduced NADP is used to reduce glycerate 3-phosphate; the regeneration of RuBP uses ATP alone.
No ATP and no reduced NADP, because the reactions are spontaneous — A student who thinks the rest of the cycle runs for free picks this. RuBP carries two phosphate groups, and ATP is needed to convert triose phosphate into it; without ATP the cycle stops.
Reduced NADP from the light-dependent reactions, without any ATP — A student who thinks reduced NADP powers the whole Calvin cycle picks this. Regeneration is not a reduction; it uses ATP and no reduced NADP.
25 Algae photosynthesizing steadily in light with a plentiful supply of carbon dioxide are suddenly placed in darkness. Within a minute the concentration of glycerate 3-phosphate in the cells rises sharply. Which explanation is correct? HL
Answer and reasoning
The light-independent reactions actually run faster in darkness, so glycerate 3-phosphate is produced much more quickly than it can be used up. — A student who thinks the 'dark reactions' happen in the dark picks this. The light-independent reactions slow and stop in darkness because their supply of ATP and reduced NADP has ceased.
Reduced NADP is diverted to the chain of electron carriers to make ATP, so none of it remains to reduce glycerate 3-phosphate. — A student who thinks reduced NADP feeds the chain picks this. Reduced NADP never donates electrons to the chloroplast chain; in the dark it is simply no longer produced.
No ATP or reduced NADP is being made, so glycerate 3-phosphate cannot be reduced, while Rubisco continues to fix carbon dioxide for a time. — A lack of light stops the light-dependent reactions at once. Rubisco keeps forming glycerate 3-phosphate from the remaining RuBP, but without ATP and reduced NADP it cannot be reduced to triose phosphate, so it accumulates.
Glycerate 3-phosphate is no longer converted into RuBP, because the regeneration of RuBP requires light directly. — A student who places the branch point of the cycle at glycerate 3-phosphate picks this. RuBP is regenerated from triose phosphate, and the block in darkness is at the reduction of glycerate 3-phosphate, which needs ATP and reduced NADP.
26 Illuminated chloroplasts are deprived of carbon dioxide. Photosystem II soon stops emitting electrons. Which explanation is correct? HL
Answer and reasoning
Only the Calvin cycle is affected; the light-dependent reactions continue at their full rate and store up ATP and reduced NADP for later use. — A student who treats the two stages as independent picks this. The light-dependent reactions need NADP and ADP returned by the Calvin cycle; when these run out, electron flow and photosystem II stop, as observed.
Carbon dioxide is the molecule split by photosystem II to obtain its electrons and oxygen, so without it there is no source of electrons. — A student who thinks oxygen and electrons come from carbon dioxide picks this. Photosystem II splits water, not carbon dioxide; the effect of removing carbon dioxide is indirect, through the failure to regenerate NADP.
Without carbon dioxide no ATP can be made, and ATP is what powers the emission of electrons from the reaction centre of photosystem II. — A student who thinks ATP powers the light-dependent reactions picks this. Photosystem II is driven by light; it stops because the chain of carriers beyond it is blocked when NADP is no longer regenerated, not for lack of ATP.
The Calvin cycle stops using ATP and reduced NADP, so NADP and ADP are not regenerated; electrons cannot leave photosystem I and the chain backs up to photosystem II. — The two stages are interdependent. Without carbon dioxide there is no glycerate 3-phosphate to reduce, so reduced NADP is not used and no NADP is available to accept electrons from photosystem I. Electrons accumulate along the chain, and photosystem II can no longer pass on or replace its electrons, so it stops functioning.
27 A student notices that pondweed in a tank of hydrogencarbonate solution at 30 °C releases bubbles of gas faster than identical pondweed in the same solution at 15 °C under the same lamp. Which is the most appropriate hypothesis to test in a follow-up experiment?
Answer and reasoning
Between 15 °C and 30 °C, raising the temperature increases the rate of photosynthesis because its enzyme-catalysed reactions run faster. — This is a provisional explanation based on theory (the reactions of photosynthesis are enzyme-catalysed and so temperature-dependent) and on the evidence already observed, and it names the variable that actually differed between the tanks. It can be tested by varying temperature while light and carbon dioxide concentration are held constant.
Photosynthesis is faster at 30 °C than at 15 °C, and the observation has already proven this, so no further experiment is needed. — A student who treats a single observation as proof picks this. One observation can suggest or support a hypothesis but cannot prove it; hypotheses are provisional explanations that require repeated testing.
Heat energy from the warmer water is what drives the reactions of photosynthesis, so the pondweed at 30 °C has more energy to use. — A student who thinks photosynthesis is powered by heat picks this. Light energy absorbed by pigments drives photosynthesis; temperature affects the rate because the reactions are enzyme-catalysed, not because heat is the energy source.
Warm water holds more dissolved carbon dioxide than cold water, so the pondweed at 30 °C has more carbon dioxide to fix. — A student who applies the rule for dissolving solids to gases picks this. Gas solubility falls as temperature rises, so warm water holds less carbon dioxide, and in any case both tanks contain the same hydrogencarbonate solution, so carbon dioxide supply did not differ.
28 A leaf sealed in a chamber was illuminated with light of one wavelength at a time and the fall in carbon dioxide concentration in the chamber was recorded: at 450 nm the concentration fell by 80 ppm in 4 minutes; at 550 nm by 30 ppm in 5 minutes; at 650 nm by 90 ppm in 6 minutes; at 700 nm by 20 ppm in 2 minutes. Which statement about these results is correct?
Answer and reasoning
The highest rate is at 650 nm, because the largest fall in carbon dioxide occurred there. — A student who confuses amount with rate picks this. The 650 nm run lasted 6 minutes, so its rate is 90 ÷ 6 = 15 ppm per minute, lower than the 20 ppm per minute at 450 nm.
The highest rate is at 450 nm, because carbon dioxide was consumed there at 20 ppm per minute. — Rates are amounts divided by times: 80 ÷ 4 = 20 ppm per minute at 450 nm, 30 ÷ 5 = 6 at 550 nm, 90 ÷ 6 = 15 at 650 nm and 20 ÷ 2 = 10 at 700 nm. Carbon dioxide consumption measures the rate of photosynthesis just as oxygen production does, and the peak is at 450 nm.
The rate at 550 nm should be plotted as zero, because a leaf reflects all green light. — A student who believes green light drives no photosynthesis picks this, ignoring the data. Carbon dioxide fell by 30 ppm in 5 minutes at 550 nm, a rate of 6 ppm per minute, which is low but not zero.
The results give an absorption spectrum of the leaf pigments, not an action spectrum. — A student who confuses the two spectra picks this. An absorption spectrum records the light absorbed by a pigment; these data are rates of photosynthesis, which plotted against wavelength make an action spectrum.
That was your twenty minutes. Real practice on C1.3 is past-paper questions marked against the mark scheme.
What the exam asks of C1.3
Paper 1A asks you to recognise the word equation, the source of oxygen, and pigment colours from a chromatogram. Paper 1B gives Rf measurements, absorption and action spectra or limiting-factor graphs and asks you to calculate, describe and explain; expect variables to be identified. Paper 2 uses *outline* and *explain* at SL on limiting factors and enrichment experiments. At HL, expect *explain* and *describe* on photosystems, chemiosmosis in thylakoids and the Calvin cycle: name the location, the inputs and the products of each stage.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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