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Photosynthesis as an energy transfer process — Cambridge International A Level Biology 9700 Study Notes (2025-2027 syllabus)
Chloroplast structure, the light-dependent reactions in the thylakoids (cyclic and non-cyclic photophosphorylation, photolysis of water) and the Calvin cycle in the stroma (CO₂ + RuBP → GP → TP → glucose / RuBP).
What you’ll learn
Mapped to the Cambridge IGCSE 9700 syllabus (2025-2027).
13.1.1 — Describe the structure of a chloroplast as seen in electron micrographs, including the double envelope, the stroma, thylakoids, grana and lamellae, and explain how the structure is adapted for the light-dependent and light-independent reactions.
13.1.2 — State that photosynthesis is the production of complex organic molecules using energy from light and the inorganic molecules CO₂ and H₂O.
13.1.3 — Describe the role of chlorophyll a, accessory pigments (chlorophyll b, carotenoids) and the photosystems (PSI and PSII) in absorbing light energy.
13.1.4 — Describe the events of the non-cyclic light-dependent reactions, including the photolysis of water, the role of the electron transport chain, the production of ATP by chemiosmosis (photophosphorylation) and the reduction of NADP.
13.1.5 — Explain the role of cyclic photophosphorylation in producing additional ATP.
13.1.6 — Describe the events of the Calvin cycle (light-independent reactions): the role of Rubisco, the conversion of GP to TP using ATP and NADPH, and the regeneration of RuBP.
13.1.7 — Explain how products of the Calvin cycle are used to synthesise hexose sugars, sucrose, starch, lipids and amino acids.
Photosynthesis is the process by which light energy is captured by chlorophyll and used to drive the synthesis of complex organic molecules (mainly glucose) from inorganic CO₂ and water, with the release of O₂. The overall balanced equation is:
6CO2+6H2Olight, chlorophyllC6H12O6+6O2
Key points to note:
Light and chlorophyll are conditions, not reactants in the strict chemical sense — but they must be present.
The O₂ released comes entirely from the photolysis of water, not from CO₂. This was proved in 1941 by Ruben and Kamen using H₂¹⁸O (heavy water) — the released O₂ was heavy, while glucose contained only normal ¹⁶O.
The equation is the reverse of aerobic respiration: respiration releases the energy stored in glucose by oxidising it back to CO₂ + H₂O.
Energy is transferred from light into chemical bonds in glucose. This is an endergonic process — energy is stored.
Photosynthesis is the ultimate source of nearly all biological energy on Earth and of virtually all atmospheric O₂.
Double envelope; stroma + Rubisco; thylakoids stacked into grana; lamellae link grana.
The chloroplast is the organelle in which photosynthesis takes place. It is typically lens-shaped, about 5 μm long, and found in the mesophyll cells of plant leaves.
Double envelope. Two phospholipid bilayers (outer and inner membrane) enclose the chloroplast. The envelope controls the movement of CO₂, O₂, water, ATP, NADPH and metabolic intermediates between the chloroplast and the cytoplasm.
Stroma. The fluid-filled interior matrix. Contains:
Calvin cycle enzymes, especially Rubisco — the most abundant protein on Earth.
Starch grains — insoluble storage of excess carbohydrate.
Lipid droplets — for membrane synthesis.
70S ribosomes (like prokaryotes) — chloroplasts make some of their own proteins.
Circular DNA — evidence for the endosymbiotic origin of chloroplasts.
Thylakoids and grana. Within the stroma are flattened, membrane-bound sacs called thylakoids. These are stacked into piles called grana (singular: granum), each typically containing 10-100 thylakoids. The thylakoid membrane is the site of the light-dependent reactions and contains:
Photosystems I and II (chlorophyll-protein complexes).
Electron transport chain carriers.
ATP synthase.
The stacking of thylakoids gives a very large surface area for light absorption and for ATP synthesis.
Intergranal lamellae. Thin sheets of thylakoid membrane connecting one granum to another. They increase contact between thylakoids and the surrounding stroma, allowing efficient transfer of ATP and NADPH from the light reactions to the Calvin cycle.
Thylakoid space (lumen). The small enclosed compartment inside each thylakoid. H⁺ are pumped into this small volume during the light reactions, building a steep gradient that drives chemiosmosis.
Chlorophyll a + b + carotenoids absorb light; photosystems funnel energy to reaction centres P680 (PSII) and P700 (PSI).
Plants contain several photosynthetic pigments organised into clusters within the thylakoid membrane:
Chlorophyll a — the primary pigment, present at the reaction centre of every photosystem. Absorbs strongly in the red (~660 nm) and blue (~430 nm) regions; reflects green. Two forms exist:
P680 at the reaction centre of PSII (peak absorbance 680 nm).
P700 at the reaction centre of PSI (peak absorbance 700 nm).
Chlorophyll b — accessory pigment, absorbs at slightly different wavelengths (~470 / 643 nm), broadening the spectrum that the plant can use.
Carotenoids (carotenes, xanthophylls) — orange and yellow pigments, absorb mainly blue light; visible in autumn leaves once chlorophyll is broken down.
Photosystems are protein-pigment complexes embedded in the thylakoid membrane. Each contains a few hundred pigment molecules arranged as an antenna complex that absorbs light and funnels the energy by resonance transfer to a central pair of chlorophyll a molecules — the reaction centre. At the reaction centre, the energy excites an electron, which is then emitted to the electron transport chain.
Two photosystems work together:
PSII (P680) — absorbs light first in the non-cyclic pathway; donates electrons to the ETC; electrons replaced by photolysis of water.
PSI (P700) — absorbs light next; donates electrons that ultimately reduce NADP to NADPH.
The combined absorption of chlorophyll a + b + carotenoids covers most of the visible spectrum except green (which is reflected — the reason leaves look green).
Chlorophyll a = primary pigment (red + blue).
Chlorophyll b + carotenoids = accessory pigments (broader spectrum).
PSII = P680; PSI = P700.
Antenna complex funnels energy to reaction centre chlorophyll.
Thylakoid membrane: PSII → ETC (pumps H⁺) → PSI → NADP. Water photolysed → O₂. H⁺ flow through ATP synthase → ATP.
The non-cyclic light-dependent reactions take place in and across the thylakoid membrane. They require both PSII and PSI acting in series, plus water as an electron source.
Step 1 — Photoexcitation at PSII. Light absorbed by chlorophyll in PSII excites electrons in the P680 reaction centre to a higher energy level. The excited electrons are emitted from the chlorophyll molecule.
Step 2 — Electron transport chain. The excited electrons pass along a chain of electron carriers (plastoquinone → cytochrome b₆f → plastocyanin) embedded in the thylakoid membrane. At each step the electrons lose energy by sequential redox reactions.
Step 3 — H⁺ pumping. The energy released as electrons pass along the ETC is used to pump H⁺ from the stroma into the thylakoid space (lumen). This builds an electrochemical proton gradient across the thylakoid membrane.
Step 4 — Photolysis of water. PSII has lost electrons and must be replaced. The electrons come from photolysis (splitting) of water by an oxygen-evolving complex on the inner side of PSII:
2H2O→4H++4e−+O2
The 4 electrons replace those lost from PSII chlorophyll.
The 4 H⁺ are released into the thylakoid space, contributing to the proton gradient.
O₂ is released as a waste product — the source of atmospheric oxygen on Earth.
Step 5 — Photoexcitation at PSI. Light absorbed by chlorophyll in PSI excites electrons in the P700 reaction centre. These electrons are emitted from chlorophyll. They are replaced by the electrons arriving from the PSII ETC via plastocyanin.
Step 6 — Reduction of NADP. The excited electrons from PSI are picked up by ferredoxin and passed to NADP reductase, which combines them with H⁺ from the stroma to reduce NADP to NADPH (also written reduced NADP):
NADP+2H++2e−→NADPH+H+
Step 7 — Photophosphorylation (chemiosmosis). Protons accumulated in the thylakoid space flow back into the stroma down their gradient through ATP synthase. The flow drives a conformational change that condenses ADP + Pi → ATP. This is photophosphorylation — ATP synthesis driven ultimately by light energy.
Products of non-cyclic light reactions: ATP, NADPH, O₂. ATP and NADPH are then transferred to the stroma for the Calvin cycle. O₂ diffuses out through the stomata.
PSI only; electrons cycle back; ATP made; no NADPH, no O₂.
In cyclic photophosphorylation, only PSI is used. The pathway works as follows:
Light absorbed by PSI excites electrons; they are emitted from the P700 reaction centre.
The excited electrons pass along a short electron transport chain (similar carriers to the non-cyclic pathway) — but instead of flowing to NADP, they return to PSI.
As the electrons pass along the chain, energy is released and used to pump H⁺ into the thylakoid space, just as in the non-cyclic pathway.
The H⁺ flows back through ATP synthase to make ATP (photophosphorylation).
Products: ATP only.
Not produced: NADPH (NADP is not reduced), O₂ (water is not photolysed — no electrons need replacing).
Why is cyclic photophosphorylation needed? The Calvin cycle uses 3 ATP per 2 NADPH (a ratio of 1.5 : 1). Non-cyclic photophosphorylation produces ATP and NADPH in roughly equal amounts (close to 1 : 1), so there is a small ATP shortfall. Cyclic photophosphorylation supplements ATP supply without making additional NADPH, adjusting the ATP:NADPH ratio to match metabolic demand.
Cyclic activity also increases under stress conditions (e.g. low CO₂, high light) when the Calvin cycle slows but ATP is still needed.
Stroma. CO₂ + RuBP (5C) → 2 GP (3C). GP → TP (uses 2 ATP + 2 NADPH per CO₂). 5/6 TP → RuBP; 1/6 → glucose.
The Calvin cycle takes place in the stroma of the chloroplast. It uses the ATP and NADPH made in the light reactions to fix CO₂ into organic molecules (sugars). It is called the light-independent reactions because no light is directly required — but ATP and NADPH must be continuously supplied, so the cycle effectively halts in the dark.
Per CO₂ fixed:
Step 1 — Carbon fixation. CO₂ from the air diffuses through stomata and into the stroma. It combines with a 5-carbon acceptor, ribulose bisphosphate (RuBP). The enzyme Rubisco (ribulose bisphosphate carboxylase / oxygenase) catalyses this step. Rubisco is the most abundant enzyme on Earth.
Step 2 — Formation of GP. The unstable 6-carbon intermediate immediately splits into two molecules of glycerate-3-phosphate (GP), each 3 carbons long. GP is the first stable product of CO₂ fixation.
Step 3 — Reduction of GP to TP. Each GP is reduced to triose phosphate (TP) in a two-step process:
A phosphate from ATP is added to GP (forming 1,3-bisphosphoglycerate).
NADPH provides hydrogen; the molecule is reduced and the phosphate released as Pi.
Per CO₂ fixed: 2 ATP and 2 NADPH consumed in this reduction step.
Step 4 — Fate of TP. Per 6 turns of the cycle (6 CO₂ fixed), 12 TP molecules are produced. Of these:
10 TP regenerate 6 RuBP through a series of rearrangements. This regeneration requires 3 more ATP per 6 TP rearranged — so 6 extra ATP per glucose.
2 TP leave the cycle and combine to form one molecule of hexose (e.g. glucose, fructose) or are used to make sucrose, starch, lipids, amino acids and nucleotides.
So 1 glucose requires 6 turns of the Calvin cycle:
6 CO₂ fixed
18 ATP (12 for GP→TP, 6 for RuBP regeneration)
12 NADPH (for GP→TP)
12 TP made; 10 regenerate RuBP; 2 form glucose.
Per CO₂ fixed: 3 ATP + 2 NADPH consumed.
Why the cycle is so efficient. The acceptor (RuBP) is continuously regenerated, so a small pool can fix vast amounts of CO₂ over time. Rubisco operates at every step the plant is in the light.
Products from TP. Some TP combines pairwise to form hexose sugars (glucose, fructose). Glucose can be polymerised to sucrose (the transport sugar) or starch (insoluble storage in chloroplasts). TP can also be converted to glycerol and fatty acids → lipids, to amino acids (with the addition of nitrogen from nitrate), and to nucleotides.
The Calvin cycle: Rubisco fixes CO₂ onto RuBP, forming GP that is reduced to TP using ATP and NADPH; most TP regenerates RuBP while a sixth leaves the cycle to build sugars, lipids and amino acids.
Location: stroma.
CO₂ + RuBP (5C) → 2 GP (3C); enzyme = Rubisco.
GP → TP using 2 ATP + 2 NADPH per CO₂.
5/6 TP regenerates RuBP (with extra ATP).
1/6 TP leaves the cycle to form hexose / sucrose / starch / lipid / amino acids.
Per CO₂: 3 ATP + 2 NADPH. Per glucose: 18 ATP + 12 NADPH.
Uses of TP — glucose, sucrose, starch, lipids, amino acids
TP is the gateway intermediate for all photosynthetic products.
The 1 in 6 TP molecules that leaves the Calvin cycle is the gateway to all the products of photosynthesis. From TP, plants make:
Hexose sugars (glucose, fructose). Two TP molecules join to form a 6-carbon sugar. Glucose and fructose are produced by isomerisation and can be combined into sucrose.
Sucrose. The main transport sugar in phloem. Sucrose is non-reducing (so cannot interfere with metabolism in transit) and is highly soluble in water. Made in the cytoplasm from glucose-1-phosphate + fructose-6-phosphate.
Starch. The main storage carbohydrate in plants. A polymer of α-glucose, formed by glycosidic bonds between glucose-1-phosphate molecules. Stored as starch grains in the chloroplast stroma. Insoluble, so does not affect water potential.
Lipids. TP is the precursor of glycerol; acetyl-CoA (from broken-down sugars) provides the carbon skeletons for fatty acids. Together they form triglycerides, especially in seeds (e.g. sunflower, oilseed rape).
Amino acids and proteins. With the addition of nitrogen (taken up as NO₃⁻ from the soil and reduced to NH₃ / NH₄⁺), TP-derived keto-acid skeletons are converted to amino acids by transamination. These are then assembled into proteins.
Nucleic acids. TP-derived ribose, plus nitrogenous bases and phosphate, builds nucleotides for RNA and DNA.
Thus the Calvin cycle is not just a 'glucose factory' — it is the entry point for almost all biomass production on Earth.
Cambridge 9700 examines this on Paper 4 (A Level structured questions, often 7-12 marks per part). Most-tested questions: (a) chloroplast structure with adaptations (6-8 marks); (b) non-cyclic light reactions with photolysis (7-8 marks); (c) Calvin cycle stoichiometry (6-7 marks); (d) compare cyclic and non-cyclic photophosphorylation (5-6 marks); (e) trace the source of O₂ to water (3-4 marks, often via isotope labelling). Paper 5 (PAE) covers practical limiting factor experiments — see the next subtopic.
Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.
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Step-by-step worked examples — Photosynthesis as an energy transfer process
Step-by-step solutions to past-paper-style questions on photosynthesis as an energy transfer process, written exactly the way a tutor would explain them at the board.
1Chloroplast structure and function (5 marks)
Extended• Adapted from 9700/42 May/Jun 2024• chloroplast, structure-function, Paper 4
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Question
Describe the structure of a chloroplast and explain how its structure is adapted for its function in photosynthesis. (5 marks)
Step-by-step solution
Step 1
Envelope. A chloroplast is surrounded by a double membrane (inner and outer envelope) which separates the interior from the cytoplasm and controls movement of materials in and out.
Step 2
Stroma. The fluid-filled interior, the stroma, contains the enzymes of the Calvin cycle (including Rubisco), starch grains, lipid droplets, ribosomes (70S) and a circular DNA molecule. Adaptation: large volume of stroma holds many enzyme molecules for fast carbon fixation.
Step 3
Thylakoids and grana. Flattened membrane sacs called thylakoids are stacked into piles called grana (singular: granum). Thylakoids contain chlorophyll, accessory pigments, electron carriers and ATP synthase. Adaptation: stacking provides a very large surface area for light absorption and electron-transport activity.
Step 4
Inter-granal lamellae. Thylakoid sheets connecting one granum to another increase contact between grana and stroma, allowing efficient transfer of ATP and NADPH from the light reactions to the Calvin cycle.
Step 5
Photosystems. Chlorophyll molecules are organised in photosystems I and II (PSI and PSII) within the thylakoid membrane. Adaptation: precise organisation allows light energy to be efficiently funnelled to a reaction-centre chlorophyll for electron excitation.
Answer
Double envelope; stroma (Calvin cycle enzymes + Rubisco + 70S ribosomes + DNA); thylakoids stacked into grana (large SA for light absorption); lamellae link grana; photosystems I and II in thylakoid membrane.
Examiner tip
Mark scheme: (1) double envelope; (2) stroma + Rubisco; (3) thylakoids + grana + large SA; (4) lamellae / interconnection; (5) photosystems + chlorophyll in thylakoid membrane. Examiner Reports flag students who confuse grana (singular: granum) with thylakoids (the individual sacs).
2Light-dependent reactions (7 marks)
Extended• Adapted from 9700/42 Oct/Nov 2024• light-dependent, photolysis, Paper 4
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Question
Describe the main events of the non-cyclic light-dependent reactions of photosynthesis. (7 marks)
Step-by-step solution
Step 1
Location. Non-cyclic photophosphorylation takes place in the thylakoid membranes of chloroplasts. It requires photosystem II (PSII) and photosystem I (PSI) acting in series.
Step 2
Photoexcitation at PSII. Light is absorbed by chlorophyll at the reaction centre of PSII (P680). Electrons in chlorophyll are excited to a higher energy level and are emitted from the chlorophyll molecule.
Step 3
Electron transport. The high-energy electrons pass along an electron transport chain of carriers in the thylakoid membrane, losing energy at each step. The energy released is used to pump H⁺ from the stroma into the thylakoid space, creating a proton gradient.
Step 4
Photophosphorylation. H⁺ flows back through ATP synthase in the thylakoid membrane down its gradient (chemiosmosis), producing ATP from ADP + Pi. This is photophosphorylation (driven by light energy, not respiratory substrate).
Step 5
Photolysis of water. PSII has lost electrons and must replace them. This is done by splitting water (photolysis): 2H2O→4H++4e−+O2. The electrons replace those lost from PSII; the H⁺ contributes to the proton gradient; O₂ is released as a waste product.
Step 6
Photoexcitation at PSI. Meanwhile, light is absorbed by chlorophyll at PSI (P700), exciting electrons there too. These electrons leave PSI and are replaced by the electrons that travelled along the ETC from PSII.
Step 7
Reduction of NADP. The electrons from PSI, together with H⁺ from photolysis, reduce NADP to NADPH (also written reduced NADP). NADPH is the second product needed by the Calvin cycle. End products of non-cyclic light reactions: ATP, NADPH, O₂.
Answer
Thylakoid membrane. PSII excited by light → e⁻ along ETC, energy used to pump H⁺ into thylakoid space → chemiosmosis through ATP synthase → ATP. Water photolysed → 4H⁺ + 4e⁻ + O₂; e⁻ replace PSII electrons. PSI excited; e⁻ + H⁺ + NADP → NADPH.
Examiner tip
Mark scheme awards: (1) thylakoid membrane / PSII + PSI; (2) chlorophyll absorbs light / e⁻ excited; (3) ETC; (4) H⁺ pumped → ATP synthase → ATP (photophosphorylation); (5) photolysis of water → O₂ + H⁺ + e⁻; (6) PSI excited; (7) NADP + H⁺ + e⁻ → NADPH.
3Cyclic vs non-cyclic photophosphorylation (5 marks)
Extended• cyclic, non-cyclic, Paper 4
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Question
Compare cyclic and non-cyclic photophosphorylation. (5 marks)
Step-by-step solution
Step 1
Photosystems involved. Cyclic uses PSI only. Non-cyclic uses both PSII and PSI in series.
Step 2
Electron path. In cyclic photophosphorylation, electrons excited from PSI pass along an ETC and return to PSI — they cycle. In non-cyclic, electrons travel one-way: water → PSII → ETC → PSI → NADP.
Step 3
Products. Cyclic produces ATP only. Non-cyclic produces ATP, NADPH and O₂.
Step 4
Photolysis of water. Cyclic does NOT involve photolysis (no electron replacement is needed because electrons return to PSI). Non-cyclic DOES — water is split to replace electrons lost from PSII.
Step 5
Function. Cyclic provides extra ATP when the Calvin cycle's need for ATP outstrips its need for NADPH (the Calvin cycle uses 3 ATP and 2 NADPH per CO₂ fixed). Non-cyclic is the main pathway under normal conditions.
Answer
Cyclic: PSI only; e⁻ cycle back to PSI; ATP only; no photolysis. Non-cyclic: PSII + PSI; e⁻ flow water → PSII → PSI → NADP; ATP + NADPH + O₂; water photolysed. Cyclic provides extra ATP when needed.
Examiner tip
Mark scheme: (1) photosystems involved; (2) electron path; (3) products; (4) photolysis; (5) function / ratio of ATP:NADPH needed. Examiner Reports flag candidates who write 'cyclic produces NADPH' — wrong (no NADP reduced).
4Calvin cycle (6 marks)
Extended• Adapted from 9700/42 May/Jun 2024• Calvin cycle, Rubisco, Paper 4
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Question
Describe the main events of the Calvin cycle and explain the roles of ATP and NADPH from the light-dependent reactions. (6 marks)
Step-by-step solution
Step 1
Location. The Calvin cycle takes place in the stroma of the chloroplast. It is also called the light-independent reactions because it does not directly require light (although it relies on ATP and NADPH made in the light).
Step 2
Carbon fixation. CO₂ from the air diffuses into the stroma and is combined with a 5-carbon acceptor molecule, ribulose bisphosphate (RuBP). The reaction is catalysed by the enzyme Rubisco (ribulose bisphosphate carboxylase / oxygenase) — the most abundant enzyme on Earth.
Step 3
Formation of GP. The unstable 6-carbon intermediate immediately splits into two molecules of glycerate-3-phosphate (GP, 3C). This is the first stable product of CO₂ fixation.
Step 4
Reduction to TP. Each GP is reduced to triose phosphate (TP, 3C) using 2 NADPH (one per GP) and 2 ATP (one per GP). NADPH provides the hydrogen; ATP provides energy and phosphate.
Step 5
Use of TP. Most TP (5 out of every 6 molecules) is used to regenerate RuBP — this requires an additional 1 ATP per RuBP. Only 1 in 6 TP molecules leaves the cycle to form hexose (glucose, fructose), sucrose, starch, amino acids or lipids.
Step 6
Role of light products. ATP supplies energy for the reduction of GP → TP and for regeneration of RuBP. NADPH supplies the hydrogen / reducing power for the reduction of GP → TP. Per CO₂ fixed, the cycle uses 3 ATP and 2 NADPH. Both come from the light-dependent reactions.
Answer
Stroma. CO₂ + RuBP (5C) → 2 GP (3C), catalysed by Rubisco. GP → TP using 2 ATP + 2 NADPH per CO₂. 5/6 TP regenerates RuBP (1 more ATP per RuBP); 1/6 TP → hexose/sucrose/starch/lipid/amino acid. Per CO₂: 3 ATP + 2 NADPH.
Examiner tip
Mark scheme: (1) stroma; (2) CO₂ + RuBP → GP / Rubisco; (3) GP → TP using ATP + NADPH; (4) TP regenerates RuBP; (5) some TP leaves to form glucose etc; (6) 3 ATP + 2 NADPH per CO₂. Examiner reports note candidates often muddle GP (glycerate-3-phosphate) with G3P (glyceraldehyde-3-phosphate = TP).
5Overall equation (3 marks)
Extended• photosynthesis equation, Paper 4
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Question
Write the overall balanced equation for photosynthesis and identify the source of the oxygen released. (3 marks)
Step-by-step solution
Step 1
Overall equation.6CO2+6H2Olight, chlorophyllC6H12O6+6O2. The equation must be balanced and the conditions (light, chlorophyll) stated.
Step 2
Source of O₂. The oxygen released comes from the photolysis of water, not from CO₂. Isotopic labelling experiments using H₂¹⁸O showed that the heavy oxygen appeared in O₂ (not in the glucose).
Step 3
Why photosynthesis matters. This is the reverse of aerobic respiration: photosynthesis stores energy from light in glucose; respiration releases it. Photosynthesis is also the source of virtually all atmospheric O₂ on Earth.
Answer
6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂ (light, chlorophyll). O₂ comes from photolysis of water (proven by ¹⁸O isotope labelling).
Key Formulae — Photosynthesis as an energy transfer process
The formulae you need to memorise for photosynthesis as an energy transfer process on the Cambridge International A Level 9700 paper, with every variable defined in plain English and a note on when to use it.
Overall equation of photosynthesis
6CO2+6H2Olight, chlorophyllC6H12O6+6O2
6CO2
Six molecules of carbon dioxide — the carbon source for glucose
6H2O
Six molecules of water — the source of electrons and protons, AND of the O₂ released
C6H12O6
One molecule of glucose — the energy-storing product
6O2
Six molecules of oxygen — released from photolysis of water (proven by H₂¹⁸O experiments)
Light,chlorophyll
Conditions: light energy absorbed by chlorophyll drives the reaction
When to use
Always state the conditions ('light, chlorophyll' or 'light energy and chlorophyll') above the arrow. Mark schemes deduct for unbalanced equations or for omitting the conditions. Note that the oxygen comes from water (not CO₂) — this can be tested as a separate mark.
Example
The equation is the reverse of aerobic respiration: C6H12O6+6O2→6CO2+6H2O. The two processes together form the carbon cycle.
Model Answers — Photosynthesis as an energy transfer process
High-scoring sample answers for photosynthesis as an energy transfer process on the Cambridge International A Level 9700 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Question 1
9700/42 May/Jun 2024 Q6 (adapted)7 marks
Q (7 marks). Describe the structure of a chloroplast and explain how its features are adapted for photosynthesis.
Model answer
A chloroplast is a double-membrane-bound organelle found in the mesophyll cells of plant leaves. It is typically biconvex or lens-shaped, about 5 μm long, and contains the photosynthetic machinery.
Double envelope. Two phospholipid bilayers (inner and outer membranes) surround the chloroplast. The envelope controls the movement of CO₂, O₂, water, ATP and metabolic intermediates between the chloroplast and the cytoplasm.
Stroma. The fluid-filled interior contains:
the enzymes of the Calvin cycle, including Rubisco, the most abundant enzyme on Earth — large volume holds many enzyme molecules so the cycle proceeds fast;
starch grains, where excess products are stored as insoluble polysaccharide;
lipid droplets for membrane synthesis;
70S ribosomes and circular DNA, allowing the chloroplast to synthesise some of its own proteins (evidence for the endosymbiotic origin of chloroplasts from ancestral cyanobacteria).
Thylakoids and grana. Thylakoids are flattened, membrane-bound sacs. They are stacked into piles called grana (singular: granum). The thylakoid membranes are studded with chlorophyll and accessory pigments organised into photosystems I and II, electron transport chain carriers and ATP synthase. The stacking gives a very large surface area for light absorption.
Intergranal lamellae. Thin sheets of thylakoid membrane connect one granum to another. They increase the contact between thylakoids and the stroma, allowing efficient diffusion of ATP and NADPH from the light reactions to the Calvin cycle.
Stroma-thylakoid space. The thylakoid interior (thylakoid space / lumen) is a small enclosed compartment into which H⁺ are pumped during the light reactions. The small volume allows a steep proton gradient to be established quickly, maximising ATP yield from chemiosmosis.
In summary, the chloroplast is a 'two-compartment factory': the thylakoids capture light and produce ATP + NADPH; the stroma uses those products to fix CO₂ into sugars. Each compartment is exquisitely adapted to its task.
Why this scores
Why this scores 7/7. Mark scheme: (1) double envelope; (2) stroma + named contents (Rubisco / starch / 70S / DNA); (3) thylakoids stacked into grana / large SA; (4) photosystems / chlorophyll in thylakoid membrane; (5) lamellae link grana; (6) ATP synthase in thylakoid membrane; (7) small thylakoid space allows H⁺ gradient. Examiner Reports flag confusion of singular 'granum' / plural 'grana' — use the correct form.
Question 2
9700/42 Oct/Nov 2024 Q7(a) (adapted)8 marks
Q (8 marks). Describe the events of the non-cyclic light-dependent reactions of photosynthesis, including the roles of water, NADP and the proton gradient.
Model answer
The light-dependent reactions take place in and across the thylakoid membrane of chloroplasts. Non-cyclic photophosphorylation requires both photosystem II (PSII) and photosystem I (PSI), which are protein-chlorophyll complexes embedded in the membrane.
1. Photoexcitation at PSII. Light energy is absorbed by chlorophyll in PSII (peak absorption ~680 nm, hence P680). The energy is funnelled to a pair of reaction-centre chlorophyll molecules; electrons are excited to a higher energy level and emitted from the chlorophyll.
2. Electron transport chain. The high-energy electrons pass along a chain of carriers in the thylakoid membrane (plastoquinone → cytochromes → plastocyanin), losing energy at each step. The energy released is used to pump H⁺ from the stroma into the thylakoid space. This builds an electrochemical proton gradient across the thylakoid membrane.
3. Photolysis of water. PSII has lost electrons and must be replaced. The replacement comes from photolysis (splitting) of water:
2H2O→4H++4e−+O2
The electrons replace those lost from PSII. The H⁺ contribute to the proton gradient. The O₂ is released as a waste product — the source of atmospheric oxygen on Earth.
4. Photoexcitation at PSI. Meanwhile, light is also absorbed by chlorophyll in PSI (P700). Electrons in PSI are excited and emitted. They are replaced by the electrons that arrive from the PSII ETC via plastocyanin.
5. Reduction of NADP. The electrons leaving PSI are accepted by NADP (with one H⁺ from the stroma), forming NADPH (reduced NADP). NADPH carries reducing power to the Calvin cycle in the stroma.
6. Photophosphorylation (chemiosmosis). Protons accumulated in the thylakoid space flow back into the stroma down their electrochemical gradient through ATP synthase. The flow drives a conformational change in ATP synthase that condenses ADP + Pi → ATP. Because the energy ultimately comes from light, this is called photophosphorylation.
Overall products of non-cyclic light reactions: ATP, NADPH and O₂. ATP and NADPH are then transported to the stroma to drive the Calvin cycle.
Why this scores
Why this scores 8/8. Mark scheme: (1) thylakoid membrane; (2) PSII excited / e⁻ emitted; (3) ETC + H⁺ pumped to thylakoid space; (4) photolysis of water → 4H⁺ + 4e⁻ + O₂; (5) e⁻ replace PSII electrons; (6) PSI excited; (7) e⁻ + H⁺ + NADP → NADPH; (8) chemiosmosis through ATP synthase → ATP. Examiner Reports note 'NADP gains H' is correct; 'NADP gains an electron' alone is incomplete.
Question 3
9700/42 May/Jun 2024 Q6(b) (adapted)7 marks
Q (7 marks). Describe the Calvin cycle and explain how ATP and NADPH from the light-dependent reactions are used.
Model answer
The Calvin cycle (light-independent reactions) takes place in the stroma of the chloroplast. It uses ATP and NADPH from the light-dependent reactions to fix CO₂ into sugars.
1. Carbon fixation. CO₂ enters the leaf through stomata and diffuses into the chloroplast stroma. There it combines with a 5-carbon acceptor molecule, ribulose bisphosphate (RuBP). The reaction is catalysed by Rubisco (ribulose bisphosphate carboxylase / oxygenase), the most abundant enzyme on Earth.
2. Formation of GP. The unstable 6-carbon intermediate immediately splits into two molecules of glycerate-3-phosphate (GP), each 3 carbons long. GP is the first stable product of carbon fixation.
3. Reduction of GP to TP. GP is reduced to triose phosphate (TP), also 3 carbons. The reduction uses:
2 NADPH per CO₂ fixed — providing hydrogen / electrons; NADPH is oxidised back to NADP.
2 ATP per CO₂ fixed — providing energy and phosphate; ATP is hydrolysed to ADP + Pi.
4. Fate of TP. For every 6 TP molecules formed:
5 are used to regenerate RuBP (so that more CO₂ can be fixed). Regeneration of RuBP requires an additional 1 ATP per RuBP (3 ATP for every 6 TP rearranged into 3 RuBP).
1 TP leaves the cycle to be used as a building block for hexose sugars (glucose, fructose), sucrose (the transport sugar), starch (storage), amino acids, lipids and nucleotides.
5. Stoichiometry per CO₂ fixed. The cycle consumes 3 ATP and 2 NADPH per CO₂. ADP, NADP and Pi are released and returned to the thylakoid membrane for re-energising. To make one molecule of glucose, six turns of the cycle are required (each fixing 1 CO₂) — total 18 ATP and 12 NADPH per glucose.
Role of ATP. Energy for the reduction of GP → TP and for the regeneration of RuBP.
Role of NADPH. Reducing power (hydrogen donor) for the reduction of GP → TP.
Without continuous ATP and NADPH from the light reactions, the cycle would stall: GP would accumulate and TP would not be produced.
Why this scores
Why this scores 7/7. Mark scheme: (1) stroma; (2) CO₂ + RuBP → GP / Rubisco; (3) 2 GP per CO₂ fixed; (4) GP → TP using 2 NADPH + 2 ATP; (5) some TP regenerates RuBP (with extra ATP); (6) some TP leaves to form glucose etc; (7) 3 ATP + 2 NADPH per CO₂.
Question 4
9700/42 Oct/Nov 2024 Q7(b) (adapted)6 marks
Q (6 marks). Compare cyclic and non-cyclic photophosphorylation in terms of the photosystems involved, the products formed and the role of water.
Model answer
Photosystems involved. Non-cyclic photophosphorylation uses both PSII and PSI in series — electrons flow from water through PSII, along the ETC, through PSI, and finally to NADP. Cyclic photophosphorylation uses only PSI — electrons excited from PSI are returned to PSI via a short electron transport chain.
Direction of electron flow. Non-cyclic flow is one-way: water → PSII → PQ → cytochromes → PC → PSI → ferredoxin → NADP. Cyclic flow forms a closed loop back to PSI.
Products. Non-cyclic photophosphorylation produces ATP, NADPH and O₂. Cyclic photophosphorylation produces ATP only — no NADPH (because NADP is not reduced) and no O₂ (because water is not split).
Role of water. Non-cyclic depends on photolysis of water to replace the electrons that leave PSII. The H⁺ and electrons released also contribute to the proton gradient and to NADP reduction. Cyclic does not require water photolysis because no electrons are lost overall — they cycle back to PSI.
Why cells need both. The Calvin cycle uses 3 ATP per 2 NADPH (1.5:1 ratio). Non-cyclic alone produces ATP and NADPH in roughly equal amounts, so there is a small shortfall of ATP. The cell tops this up with cyclic photophosphorylation, which produces extra ATP without producing more NADPH. The proportion of cyclic activity adjusts the ATP:NADPH ratio to match metabolic demand.
ATP yield. Both processes drive ATP synthesis by the same chemiosmotic mechanism — H⁺ pumped into the thylakoid space; ATP synthase couples flow back through the membrane to ADP + Pi → ATP.
Why this scores
Why this scores 6/6. Mark scheme: (1) non-cyclic uses PSII + PSI / cyclic uses PSI only; (2) non-cyclic = one way / cyclic = loop; (3) non-cyclic products = ATP + NADPH + O₂; (4) cyclic products = ATP only; (5) water photolysed only in non-cyclic; (6) why both needed / ATP:NADPH ratio. Examiner Reports flag candidates who claim 'cyclic produces NADPH' — common error.
Question 5
Practice question — Paper 4 style4 marks
Q (4 marks). Describe the source of the oxygen released during photosynthesis and explain how this was demonstrated experimentally.
Model answer
The oxygen released in photosynthesis comes entirely from water, not from carbon dioxide. The reaction occurs during the light-dependent stage and is called photolysis of water:
2H2O→4H++4e−+O2
The reaction takes place in the thylakoid space at an oxygen-evolving complex on the inner side of photosystem II. The four electrons replace those lost from PSII chlorophyll on photoexcitation; the four protons contribute to the proton gradient; the oxygen is released as a waste product.
Isotope evidence (Ruben and Kamen, 1941). Plants were given water enriched with the heavy oxygen isotope ¹⁸O while CO₂ contained only normal ¹⁶O. The oxygen released was heavy (¹⁸O₂) — proving the source was water. In a complementary experiment with H₂¹⁶O and C¹⁸O₂, the oxygen released was light (¹⁶O₂), with the heavy oxygen ending up in glucose and water — confirming the result.
This experiment refuted the older idea that O₂ came from CO₂ (analogous to respiration in reverse) and showed that photosynthesis has a separate water-splitting half-reaction. It also explained why aquatic plants release O₂ even when CO₂ is being consumed.
Why this scores
Why this scores 4/4. Mark scheme: (1) photolysis of water; (2) at PSII / thylakoid; (3) ¹⁸O isotope-labelled water; (4) heavy O₂ released = proof. Examiner Reports note candidates often confuse the source of O₂ — make sure to state 'from photolysis of water'.
Key Definitions and Keywords — Photosynthesis as an energy transfer process
Definitions to memorise and the exact keywords mark schemes credit for photosynthesis as an energy transfer process answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9700 sitting.
Photosynthesis
Examiner keyword
The process by which light energy is absorbed by chlorophyll and used to drive the synthesis of complex organic molecules (e.g. glucose) from inorganic CO₂ and water, with the release of O₂. Overall: 6CO2+6H2O→C6H12O6+6O2.
Chloroplast
Examiner keyword
A double-membrane-bound organelle found in mesophyll cells of plant leaves. Contains the stroma (Calvin cycle enzymes) and thylakoids stacked into grana (light-dependent reactions).
Stroma
Examiner keyword
The fluid-filled matrix of a chloroplast, containing Calvin cycle enzymes (including Rubisco), 70S ribosomes, circular DNA and starch grains.
Thylakoid
Examiner keyword
A flattened, membrane-bound sac inside the chloroplast that contains chlorophyll, photosystems, electron carriers and ATP synthase. Thylakoids are stacked into grana.
Granum (plural: grana)
Examiner keyword
A stack of thylakoids in the chloroplast. The stacking increases the surface area for light absorption.
Chlorophyll
Examiner keyword
The green pigment in thylakoid membranes that absorbs light energy (mainly red and blue wavelengths) and uses it to excite electrons during photosynthesis. Chlorophyll a is the primary pigment; chlorophyll b and carotenoids are accessory pigments.
Photosystem (PSI / PSII)
Examiner keyword
A protein-pigment complex in the thylakoid membrane that absorbs light and uses the energy to emit excited electrons. PSI has a reaction-centre chlorophyll absorbing best at 700 nm (P700); PSII absorbs best at 680 nm (P680).
Photolysis of water
Examiner keyword
The light-driven splitting of water at PSII: 2H2O→4H++4e−+O2. The source of O₂ released in photosynthesis (proven by ¹⁸O labelling).
Photophosphorylation
Examiner keyword
The synthesis of ATP from ADP + Pi using energy ultimately derived from light, via chemiosmosis through ATP synthase in the thylakoid membrane.
A coenzyme that accepts H from the light-dependent reactions to become NADPH (reduced NADP). NADPH carries reducing power to the Calvin cycle to reduce GP → TP.
Calvin cycle (light-independent reactions)
Examiner keyword
The cyclic pathway in the stroma in which CO₂ is fixed onto RuBP by Rubisco to form GP, GP is reduced to TP using ATP and NADPH, and most TP is used to regenerate RuBP. Net product per turn: TP → carbohydrates / lipids / amino acids.
The enzyme that catalyses the fixation of CO₂ onto RuBP in the Calvin cycle. The most abundant protein on Earth. Optimum CO₂; can also bind O₂ (photorespiration) at low CO₂ / high temperature.
Ribulose bisphosphate (RuBP)
Examiner keyword
The 5-carbon CO₂ acceptor of the Calvin cycle. CO₂ + RuBP → 2 GP (catalysed by Rubisco).
Glycerate-3-phosphate (GP)
Examiner keyword
A 3-carbon compound; the first stable product of CO₂ fixation in the Calvin cycle. Reduced to TP using ATP and NADPH.
Triose phosphate (TP)
Examiner keyword
A 3-carbon sugar phosphate formed by reduction of GP. Most TP regenerates RuBP; some leaves the cycle to make hexose, sucrose, starch, lipids and amino acids.
Common Mistakes and Misconceptions — Photosynthesis as an energy transfer process
The traps other students keep falling into on photosynthesis as an energy transfer process questions — taken from recent Cambridge International A Level 9700 examiner reports and mark schemes — and how to avoid them.
✕Saying the O₂ released in photosynthesis comes from CO₂
9700 Examiner Reports 2023-2024
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Why it happens
Symmetry with respiration is misleading.
How to avoid it
Always state that O₂ comes from the photolysis of water at PSII. Mention the ¹⁸O isotope experiment if you have time.
✕Calling the Calvin cycle the 'dark reactions'
9700 Examiner Reports 2024
▼
Why it happens
Older textbook terminology.
How to avoid it
The Calvin cycle is called the light-independent reactions because it does not directly use light. But it requires ATP and NADPH made in the light reactions, so it cannot run for long in the dark. Avoid 'dark reactions'.
✕Writing 'NAD' instead of 'NADP' in photosynthesis
9700 Examiner Reports 2024
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Why it happens
Both are similar coenzymes.
How to avoid it
Respiration uses NAD and FAD. Photosynthesis uses NADP (with an extra phosphate). Always check which subject you are writing about.
✕Stating that cyclic photophosphorylation produces NADPH
9700 Examiner Reports 2023
▼
Why it happens
Confusion with non-cyclic pathway.
How to avoid it
Cyclic produces ATP only — no NADPH (electrons return to PSI; NADP is not reduced) and no O₂ (water is not photolysed).
✕Misusing grana / granum / thylakoid
9700 Examiner Reports 2024
▼
Why it happens
Latin singular/plural confusion.
How to avoid it
Thylakoid = a single sac. Granum = a single stack (singular). Grana = many stacks (plural). Practise: 'thylakoids are stacked into grana (singular: granum)'.
✕Misquoting the carbon count of GP, TP and RuBP
9700 Examiner Reports 2024
▼
Why it happens
Three similar 3-letter abbreviations.
How to avoid it
RuBP = 5C (the acceptor). GP = 3C (first stable product). TP = 3C (reduced product). CO₂ + RuBP (5C) → 6C unstable → 2 × GP (3C). Mark schemes credit the carbon count explicitly.
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