Photosynthesis in Higher Plants (NEET Biology Class 11): Z Scheme, Calvin Cycle & C3 vs C4
Early Experiments & the Correct Equation
🎯 NEET priority: High-yield. 3–5 questions every year. The C3 vs C4 comparison and the Calvin cycle numbers (18 ATP + 12 NADPH per glucose) are asked almost verbatim — learn them cold.
Photosynthesis is how autotrophs use light energy to build organic food from CO₂ and water. The correct overall equation — note 12 water molecules, not 6, because the O₂ released comes from water, not from CO₂:
6CO₂ + 12H₂O →(light)→ C₆H₁₂O₆ + 6H₂O + 6O₂
The classic experiments — who proved what
Scientist | Year/Era | What they showed |
Joseph Priestley | 1770 | Plants restore to air whatever burning candles/breathing animals remove (bell jar + mint plant + mouse). He discovered oxygen in 1774. |
Jan Ingenhousz | late 1700s | Sunlight is essential, and only the GREEN parts of a plant release oxygen (aquatic plant bubble experiment). |
Julius von Sachs | 1854 | Glucose is produced when plants grow, stored as starch, and the green substance sits in special bodies (chloroplasts). |
T.W. Engelmann | late 1800s | First action spectrum of photosynthesis — using a prism + Cladophora + aerobic bacteria; bacteria gathered in blue and red light. |
Cornelius van Niel | 1930s | Photosynthesis is light-dependent, with hydrogen from an oxidisable compound reducing CO₂ — proving O₂ comes from H₂O, not CO₂. |
Melvin Calvin | post-WWII | Used radioactive ¹⁴C to show the first CO₂-fixation product is a 3-carbon acid (PGA), and worked out the whole cycle. |
Trap: van Niel's insight is why the equation needs 12 H₂O. In purple/green sulphur bacteria, H₂S (not H₂O) is the hydrogen donor, so the oxidation product is sulphur or sulphate — not O₂.
Pigments, Photosystems & the Z Scheme
Where it happens, and the four pigments
Photosynthesis occurs in chloroplasts of mesophyll cells. Division of labour inside: the membrane system (grana + stroma lamellae) traps light and makes ATP and NADPH — the light reactions. The stroma runs the enzymatic sugar synthesis — the dark reactions (a misnomer: they don't need darkness, just don't use light directly).
Paper chromatography separates four leaf pigments: chlorophyll a (blue-green), chlorophyll b (yellow-green), xanthophylls (yellow), carotenoids (yellow-orange). Chlorophyll a is the chief pigment; the rest are accessory pigments that widen the usable wavelength range AND protect chlorophyll a from photo-oxidation.
Maximum photosynthesis happens in the blue and red regions of the spectrum — the action spectrum roughly (but not exactly) overlaps chlorophyll a's absorption spectrum, and that gap is precisely what proves accessory pigments contribute.
Light reaction — the two photosystems and the Z scheme
Pigments sit in light harvesting complexes (LHC) within PS I and PS II. All pigments except one chlorophyll a form the antenna; that single chlorophyll a is the reaction centre. PS I's reaction centre absorbs at 700 nm (P700); PS II's at 680 nm (P680). They are named in order of DISCOVERY, not order of function — PS II acts first.
Z scheme: P680 electrons are excited → picked up by an acceptor → travel downhill through cytochromes → reach PS I → re-excited at P700 → passed to another acceptor → downhill to NADP⁺, reducing it to NADPH + H⁺. Plotted on a redox scale this traces a "Z" shape.
Splitting of water replenishes PS II's lost electrons: 2H₂O → 4H⁺ + O₂ + 4e⁻. Critically, the water-splitting complex is associated with PS II and sits on the inner side of the thylakoid membrane — so protons and O₂ are released into the lumen.
Photophosphorylation, Chemiosmosis & the Calvin Cycle
Cyclic vs non-cyclic photophosphorylation
Feature | Non-cyclic | Cyclic |
Photosystems used | Both PS II and PS I, in series | PS I only |
Electron path | Water → PS II → PS I → NADP⁺ (one-way) | Electron cycles back to PS I |
Products | ATP + NADPH + O₂ | ATP only — no NADPH, no O₂ |
Location | Grana lamellae (have both PS I and PS II) | Stroma lamellae (lack PS II and NADP reductase) |
When it happens | Normal conditions with both photosystems active | When only light beyond 680 nm is available, or extra ATP is needed |
Chemiosmotic hypothesis — how ATP is actually made
Chemiosmosis needs four things: a membrane, a proton pump, a proton gradient, and ATP synthase. Three processes build the gradient: (1) water splitting dumps protons into the lumen; (2) as electrons move, an H-carrier picks up a proton from the stroma and releases it into the lumen; (3) NADP reductase (on the stroma side) consumes stroma protons to make NADPH.
Net effect: stroma protons decrease, lumen protons accumulate, and lumen pH drops. Protons then flow back through the CF₀ channel (embedded in the membrane), causing a conformational change in CF₁ (protruding into the stroma) that synthesises ATP.
Key contrast with respiration: in photosynthesis, protons accumulate in the thylakoid lumen; in respiration they accumulate in the mitochondrial intermembrane space.
The Calvin cycle — three stages, and the numbers
1. Carboxylation — CO₂ is fixed onto RuBP, a 5-carbon ketose sugar (scientists wasted years hunting for a 2-carbon acceptor before finding it), catalysed by RuBisCO, forming two molecules of 3-PGA. 2. Reduction — uses 2 ATP + 2 NADPH per CO₂ fixed. 3. Regeneration — regenerates RuBP, needing 1 more ATP.
So per CO₂: 3 ATP + 2 NADPH. For one glucose you need 6 turns of the cycle: 6 CO₂ + 18 ATP + 12 NADPH → 1 glucose. That ATP:NADPH mismatch (18 vs 12) is exactly why cyclic photophosphorylation exists — to top up the extra ATP.
The C4 Pathway, Photorespiration & C3-vs-C4
The C4 pathway (Hatch and Slack pathway)
In mesophyll cells, PEPcase fixes CO₂ onto PEP → OAA → converted to malic/aspartic acid → transported to bundle sheath cells, where the C4 acid breaks down, releasing CO₂ and a 3-carbon molecule. The CO₂ enters the normal Calvin cycle; the 3-carbon molecule returns to the mesophyll to regenerate PEP.
Crucial point: the Calvin cycle is the sugar-making pathway in ALL photosynthetic plants — C3 and C4 alike. C4 is a CO₂-concentrating add-on, not a replacement.
Photorespiration — the wasteful process C4 plants avoid
RuBisCO's active site binds both CO₂ and O₂ competitively — the winner is decided by their relative concentrations. When O₂ binds instead, RuBP forms one phosphoglycerate + one phosphoglycolate (2-carbon) — the photorespiratory pathway.
Photorespiration produces no sugar, no ATP, no NADPH — it merely releases CO₂ while consuming ATP. Its biological function is still unknown. C4 plants avoid it entirely because pumping CO₂ into the bundle sheath keeps the local CO₂ concentration high, forcing RuBisCO to act as a carboxylase, not an oxygenase.
C3 vs C4 — the full comparison NEET asks from
Characteristic | C3 plants | C4 plants |
Primary CO₂ acceptor | RuBP (5-carbon) | PEP — phosphoenolpyruvate (3-carbon) |
First fixation product | 3-PGA (3-carbon) | OAA — oxaloacetic acid (4-carbon) |
Carboxylating enzyme (first step) | RuBisCO | PEP carboxylase (PEPcase) |
Cells that fix CO₂ | One type — mesophyll only | Two types — mesophyll AND bundle sheath |
Where the Calvin cycle runs | Mesophyll cells | Bundle sheath cells only |
Which cells have RuBisCO | Mesophyll | Bundle sheath only (mesophyll LACKS RuBisCO) |
Kranz anatomy | Absent | Present — large bundle sheath cells, many chloroplasts, thick walls, no intercellular spaces |
Photorespiration | Present (wasteful) | Absent/negligible |
CO₂ saturation point | Only beyond 450 µL/L — so current CO₂ IS limiting | About 360 µL/L |
Temperature optimum | Lower (30–40°C range varies by habitat) | Higher — tolerates hot, dry tropical conditions |
Biomass productivity | Lower | Greater |
Examples | Rice, wheat | Maize, sorghum, sugarcane |
Factors Affecting Photosynthesis & Why This Matters for NEET
Factors affecting photosynthesis
Blackman's (1905) Law of Limiting Factors: when a process is affected by several factors, its rate is set by the factor nearest its minimum value. A leaf with perfect light and CO₂ still won't photosynthesise if temperature is too low.
Light: rate rises linearly with light at low intensity, then plateaus. Light saturation occurs at just 10% of full sunlight — so light is rarely limiting in nature (except in shade/dense forest). Excess light actually breaks down chlorophyll.
CO₂ is the MAJOR limiting factor. Atmospheric CO₂ is only 0.03–0.04%; raising it to 0.05% increases fixation, beyond which it becomes damaging long-term. This is exploited commercially — greenhouse tomatoes and bell peppers are grown in CO₂-enriched air for higher yields.
Temperature: dark reactions are enzymatic, so strongly temperature-controlled; light reactions are far less sensitive. Tropical plants have a higher temperature optimum than temperate ones.
Water: acts indirectly — water stress closes stomata (cutting CO₂ supply) and wilts leaves (reducing surface area), rather than limiting the light reaction's water supply directly.
Why this matters for NEET
High-value one-liners: O₂ comes from water (van Niel); PS I = P700, PS II = P680, but PS II functions first; water splitting is tied to PS II on the lumen side; cyclic phosphorylation makes ATP only; RuBP is 5-carbon, PEP is 3-carbon; Hatch and Slack = the C4 pathway; 18 ATP + 12 NADPH per glucose.
Trap: C4 mesophyll cells lack RuBisCO and bundle sheath cells lack PEPcase — NEET swaps these constantly. Also, "dark reaction" does NOT mean it happens in the dark; and photorespiration happens in C3 plants, not C4 — getting that backwards costs the mark.
Test Yourself: MCQs, PYQs & Active Recall
Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.
Practice Questions
Q1. The oxygen released during photosynthesis in green plants comes from:
(a) Glucose
(b) Water
(c) Chlorophyll
(d) Carbon dioxide
Show answer
Answer: (b) — Van Niel showed, later confirmed by radioisotope studies, that the O2 evolved comes from the splitting of water, not CO2.
Q2. The first action spectrum of photosynthesis was described by:
(a) Priestley
(b) T.W. Engelmann
(c) Melvin Calvin
(d) Ingenhousz
Show answer
Answer: (b) — Engelmann used a prism, the alga Cladophora, and aerobic bacteria to map where O2 was evolved — the first action spectrum.
Q3. The reaction centre chlorophyll a of Photosystem II absorbs light at:
(a) 680 nm
(b) 450 nm
(c) 700 nm
(d) 540 nm
Show answer
Answer: (a) — PS II's reaction centre is P680 (absorbs at 680 nm); PS I's is P700.
Q4. Which of the following is true about the naming of PS I and PS II?
(a) They are named in the order they function
(b) They are named in the order of their discovery
(c) The names reflect their absorption peaks
(d) PS I always acts before PS II in the Z scheme
Show answer
Answer: (b) — They are named in order of discovery, not function — PS II actually acts first in the Z scheme.
Q5. Cyclic photophosphorylation results in the synthesis of:
(a) ATP only
(b) ATP, NADPH and O2
(c) NADPH only
(d) ATP and NADPH
Show answer
Answer: (a) — In cyclic flow only PS I operates and the electron cycles back, so only ATP is made — no NADPH and no O2.
Q6. In the chloroplast, protons accumulate during photosynthesis in the:
(a) Stroma
(b) Cytoplasm
(c) Intermembrane space
(d) Thylakoid lumen
Show answer
Answer: (d) — Protons accumulate in the thylakoid lumen (in respiration, by contrast, they accumulate in the mitochondrial intermembrane space).
Q7. The primary CO2 acceptor in the Calvin cycle is:
(a) RuBP, a 5-carbon ketose sugar
(b) PGA, a 3-carbon acid
(c) OAA, a 4-carbon acid
(d) PEP, a 3-carbon molecule
Show answer
Answer: (a) — RuBP (ribulose bisphosphate), a 5-carbon ketose sugar, accepts CO2 in the Calvin cycle.
Q8. To produce one molecule of glucose, the Calvin cycle requires:
(a) 12 ATP and 18 NADPH
(b) 6 ATP and 6 NADPH
(c) 3 ATP and 2 NADPH
(d) 18 ATP and 12 NADPH
Show answer
Answer: (d) — Per CO2: 3 ATP + 2 NADPH. Six turns for one glucose gives 18 ATP and 12 NADPH.
Q9. Kranz anatomy is characteristic of:
(a) All flowering plants
(b) C3 plants
(c) C4 plants
(d) Only aquatic plants
Show answer
Answer: (c) — C4 plants have Kranz anatomy — large bundle sheath cells with many chloroplasts, thick walls and no intercellular spaces.
Q10. In C4 plants, the enzyme RuBisCO is present in:
(a) Mesophyll cells only
(b) Both mesophyll and bundle sheath
(c) Neither cell type
(d) Bundle sheath cells only
Show answer
Answer: (d) — In C4 plants RuBisCO is confined to bundle sheath cells; mesophyll cells lack RuBisCO but have PEPcase.
Q11. The C4 pathway is also known as the:
(a) Hatch and Slack pathway
(b) Krebs cycle
(c) Calvin cycle
(d) Z scheme
Show answer
Answer: (a) — The C4 pathway is named the Hatch and Slack pathway after its discoverers.
Q12. Photorespiration occurs because RuBisCO:
(a) Is absent in C3 plants
(b) Requires darkness
(c) Can only bind CO2
(d) Can bind both CO2 and O2 competitively
Show answer
Answer: (d) — RuBisCO's active site binds both CO2 and O2 competitively; when O2 wins, the wasteful photorespiratory pathway runs.
Q13. In the photorespiratory pathway there is:
(a) Only NADPH synthesis
(b) Synthesis of sugars and ATP
(c) Higher glucose yield
(d) No synthesis of sugars, ATP or NADPH
Show answer
Answer: (d) — Photorespiration produces neither sugars nor ATP nor NADPH — it releases CO2 while consuming ATP.
Q14. According to Blackman's Law of Limiting Factors, the rate of a process is determined by:
(a) Temperature alone
(b) The factor nearest to its minimal value
(c) The factor present in the highest amount
(d) The average of all factors
Show answer
Answer: (b) — Blackman's (1905) law states the rate is set by whichever factor is nearest its minimum value.
Q15. Light saturation for photosynthesis occurs at approximately what fraction of full sunlight?
(a) 75 per cent
(b) 10 per cent
(c) 50 per cent
(d) 100 per cent
Show answer
Answer: (b) — Light saturation occurs at just 10 per cent of full sunlight, which is why light is rarely limiting in nature.
NEET Previous Year Questions (PYQs)
Real NEET previous-year questions on this chapter, with explanations in our own words.
Q16. Which of the following is NOT an accessory pigment in photosynthesis? (NEET PYQ)
(a) Carotenoid
(b) Chlorophyll b
(c) Xanthophyll
(d) Chlorophyll a
Show answer
Answer: (d) — Chlorophyll a is the chief/primary pigment; chlorophyll b, xanthophylls and carotenoids are accessory pigments.
Q17. The splitting of water during photosynthesis is associated with: (NEET PYQ)
(a) The stroma
(b) Photosystem II
(c) Photosystem I
(d) ATP synthase
Show answer
Answer: (b) — The water-splitting complex is associated with PS II, physically located on the inner (lumen) side of the thylakoid membrane.
Q18. The first stable product of CO2 fixation in C4 plants is: (NEET PYQ)
(a) Glucose
(b) 3-PGA
(c) Oxaloacetic acid (OAA)
(d) RuBP
Show answer
Answer: (c) — C4 plants fix CO2 onto PEP via PEPcase, producing the 4-carbon oxaloacetic acid (OAA) as the first stable product.
Q19. The CF1 portion of ATP synthase in the chloroplast protrudes towards the: (NEET PYQ)
(a) Cytoplasm
(b) Thylakoid lumen
(c) Stroma
(d) Outer chloroplast membrane
Show answer
Answer: (c) — CF0 is embedded in the thylakoid membrane as a proton channel; CF1 protrudes on the stroma-facing side.
Q20. Which factor is considered the major limiting factor for photosynthesis? (NEET PYQ)
(a) Chlorophyll amount
(b) Carbon dioxide concentration
(c) Water
(d) Light intensity
Show answer
Answer: (b) — CO2 is the major limiting factor — atmospheric concentration is only 0.03-0.04 per cent.
Q21. In C4 plants, the Calvin cycle occurs in: (NEET PYQ)
(a) Bundle sheath cells
(b) Both cell types equally
(c) Epidermal cells
(d) Mesophyll cells
Show answer
Answer: (a) — In C4 plants the Calvin cycle runs only in bundle sheath cells, unlike C3 plants where it runs in mesophyll cells.
Q22. Cornelius van Niel's work on purple and green sulphur bacteria showed that when H2S is the hydrogen donor, the oxidation product is: (NEET PYQ)
(a) Sulphur or sulphate
(b) Carbon dioxide
(c) Oxygen
(d) Water
Show answer
Answer: (a) — With H2S as hydrogen donor, sulphur bacteria release sulphur or sulphate instead of O2 — proving O2 in green plants comes from water.
Active Recall Prompt
Write everything you can recall about Photosynthesis in Higher Plants, naming each part first: the early experiments (Priestley, Ingenhousz, Sachs, Engelmann, van Niel — who showed what) and the correct overall equation; the site and pigments (chlorophyll a vs accessory pigments); the light reaction (PS I/PS II, P700/P680, the Z scheme, splitting of water); cyclic vs non-cyclic photophosphorylation; the chemiosmotic hypothesis (proton gradient, CF0/CF1); the Calvin cycle (three stages, RuBP, RuBisCO, and the ATP/NADPH numbers); the C4 pathway (Hatch and Slack, PEP, OAA, Kranz anatomy); photorespiration; and the factors affecting photosynthesis (Blackman's law, light, CO₂, temperature, water). Begin each fact with its topic and end it with a full stop.