NEET Biology · Plant Physiology

Photosynthesis

Light reactions, Calvin cycle, C3 vs C4 — visualise every step with interactive controls and live molecule animations.

Section 1 of 5 · Foundations

The full picture before the details

Photosynthesis converts light energy into chemical energy stored as glucose. The process has two major stages — light-dependent reactions in the thylakoid membrane, and the Calvin cycle (light-independent) in the stroma. Every NEET question traces back to one of these two stages.

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Light Reactions LabChange light intensity and wavelength. Watch photosystems I & II, the Z-scheme, electron transport, and ATP/NADPH production animate live.

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Calvin Cycle LabAdjust CO₂ concentration. See carbon fixation, RuBiSCO action, G3P production, and RuBP regeneration as an animated cycle.

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C3 / C4 / CAMCompare the three photosynthetic pathways. Understand Kranz anatomy, Hatch-Slack pathway, and why C4 plants dominate in tropical heat.

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6 NEET trapsPS I vs PS II order confusion, correct wavelengths, Z-scheme, quantum yield, compensation point — the questions that trip students every year.

OVERALL EQUATION OF PHOTOSYNTHESIS

    6CO₂ + 12H₂O + light energy
     ──chlorophyll──▶ 
    C₆H₁₂O₆ + 6O₂ + 6H₂O
  

ΔG = +2870 kJ/mol · 6CO₂ fixed per glucose · O₂ released from water splitting (not CO₂)

Light / Photons

Electrons / Water

CO₂ / Carbon

ATP / Glucose

O₂ / NADPH

NADP⁺ / NADPH

Light Reactions (Thylakoid)

H₂O + light → ATP + NADPH + O₂

Occur in thylakoid membranes. Photosystems II and I capture photons. Water is split (photolysis), electrons flow through ETC, ATP is synthesised by photophosphorylation, NADPH is produced. O₂ is released as byproduct.

Calvin Cycle (Stroma)

CO₂ + ATP + NADPH → G3P

Occurs in stroma. CO₂ is fixed by RuBiSCO onto RuBP (5C) making 2× PGA (3C). PGA is reduced using ATP and NADPH to form G3P. RuBP is regenerated. For 1 glucose: 3 turns needed (3CO₂ → 1 G3P net).

Z-Scheme (Non-cyclic)

PS II (P680) → ETC → PS I (P700)

Non-cyclic photophosphorylation: PSII absorbs 680nm, electrons flow to PSI via ETC (making ATP), PSI absorbs 700nm, electrons reduce NADP⁺ to NADPH. Water donates electrons to PSII. Produces O₂, ATP, NADPH.

Pigments & Absorption — 4 Types

Chl a · Chl b · Xanthophylls · Carotenoids

Chlorophyll a (bright/blue-green) — primary pigment, reaction centre. Absorbs 430 nm (violet-blue) & 662 nm (red).
Chlorophyll b (yellow-green) — accessory. Absorbs 453 nm & 642 nm.
Xanthophylls (yellow) — accessory. Absorbs 400–530 nm.
Carotenoids (yellow to yellow-orange) — accessory. Absorbs 400–500 nm. All accessory pigments transfer energy to Chl a and protect it from photo-oxidation.

Chloroplast Anatomy

Section 1 of 5

Section 2 of 5 · Light Reactions

Light Reactions & Z-Scheme

Adjust light intensity and CO₂ — watch photons hit PSII, electrons travel the Z-scheme through the ETC to PSI, water split releasing O₂, and ATP + NADPH produced. The animation shows electron flow in real time.

Set light intensityHigher intensity → more photons → faster electron flow → more ATP and NADPH per second.

Watch the Z-schemeElectrons flow: H₂O → PSII (P680) → PQ → Cyt b₆f → PC → PSI (P700) → Fd → NADP⁺ reductase → NADPH.

See the outputsATP (from photophosphorylation), NADPH (from PSI), and O₂ (from water splitting) all update live with rates.

Light Reactions Lab

Z-Scheme — Thylakoid Membrane

Electron flow animated in real time

Light Intensity

50%

Wavelength (nm)

680 nm

400nm violet680nm red

ATP rate

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rel. units/s

NADPH rate

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rel. units/s

O₂ released

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rel. units/s

Efficiency

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% absorbed

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Non-Cyclic Photophosphorylation — Z-Scheme step by step

☀️Step 1 — PSII absorbs light (680 nm): The sun indicator above PSII shows absorption efficiency at the current wavelength. Chlorophyll a (P680) in PSII absorbs a photon — electrons are excited to a higher energy level and passed to the first electron acceptor, pheophytin.

💧Step 2 — Water splitting (photolysis): The oxidised PSII (P680⁺) is a strong oxidising agent — it pulls electrons from water: 2H₂O → 4H⁺ + 4e⁻ + O₂. This replenishes PSII with electrons. All photosynthetic O₂ comes from here, not from CO₂.

⚡Step 3 — Plastoquinone (PQ) — the mobile H-carrier: PQ (Plastoquinone) is a small, lipid-soluble molecule that diffuses freely within the thylakoid membrane. It is unique because it carries both electrons AND protons (H⁺) — unlike other electron carriers that carry only electrons. PQ picks up 2e⁻ from PSII + 2H⁺ from the stroma → becomes PQH₂ (plastoquinol) → moves to Cyt b₆f → releases 2H⁺ into the lumen (building the proton gradient) → passes electrons to Cyt b₆f → continues to Plastocyanin (PC) → PSI. This H⁺ pumping into the lumen is what drives ATP synthesis (chemiosmosis).

☀️Step 4 — PSI absorbs light (700 nm): The sun indicator above PSI shows absorption at this wavelength. P700 chlorophyll a re-energises the arriving electrons. Electrons pass through Fd (ferredoxin) → NADP⁺ reductase → NADP⁺ + H⁺ + 2e⁻ → NADPH. NADPH is the reducing power for the Calvin cycle.

🔋Step 5 — ATP synthesis (chemiosmosis): H⁺ accumulated in the lumen (from water splitting at PSII + PQ pumping) flows back to the stroma through CF₀-CF₁ ATP synthase. This proton gradient drives phosphorylation: ADP + Pᵢ → ATP. Requires: membrane + proton pump + proton gradient + ATP synthase.

Cyclic Photophosphorylation — PSI Only (Fig. 11.6)

Cyclic flow — ATP only

PSI → Fd → Cyt b₆f → PC → back to PSI

🔄What happens: Only PSI (P700) is involved. After PSI excites electrons → Fd, instead of going to NADP⁺ reductase, electrons cycle back through Cyt b₆f → PC → and return to PSI. Electrons loop continuously around PSI.

✅Products: ATP only. No NADPH produced. No O₂ released (no PSII, no water splitting). ATP is made because the cyclic electron flow still pumps H⁺ through Cyt b₆f into the lumen → drives ATP synthase.

📍Where it occurs: Stroma lamellae — membranes that lack PS II and NADP reductase. Also occurs when only wavelengths >680 nm are available (PSI absorbs 700 nm; PSII needs 680 nm).

🎯Why it matters: Calvin cycle needs 3 ATP per CO₂ but only 2 NADPH. Cyclic photophosphorylation tops up the extra ATP without making unwanted NADPH — balancing the ratio the stroma needs.

Non-Cyclic

✅ PS II + PS I both active
✅ Water splitting → O₂ released
✅ Produces ATP + NADPH + O₂
✅ Linear electron flow H₂O → NADPH
✅ Z-scheme shape on energy diagram

Cyclic

❌ PS II not involved
❌ No water splitting → no O₂
✅ Produces ATP only (no NADPH)
✅ Cyclic flow loops back to PSI
✅ Location: stroma lamellae

NEET TIPPSII comes before PSI in the electron flow — the numbering is historical, not sequential. O₂ is released from water splitting at PSII, NOT at PSI. P680 absorbs 680nm (red), P700 absorbs 700nm (far red). The "Z" in Z-scheme refers to the shape of the energy diagram, not the letter Z.

Section 2 of 5

Section 3 of 5 · Calvin Cycle

Calvin Cycle — Carbon Fixation in the Stroma

The Calvin cycle uses ATP and NADPH from the light reactions to convert CO₂ into G3P (glyceraldehyde-3-phosphate). For each turn: 1 CO₂ fixed, 2 ATP and 2 NADPH used. Glucose needs 6 turns (net 3 G3P).

Adjust CO₂ & ATP supplyCO₂ drives carbon fixation rate. ATP and NADPH from the light reactions fuel G3P production. Both sliders affect cycle speed.

Watch the 3 phasesPhase 1: CO₂ + RuBP → PGA (carboxylation by RuBiSCO). Phase 2: PGA → G3P (reduction). Phase 3: G3P → RuBP (regeneration).

Count the moleculesPer 3 CO₂ fixed: 6 G3P made, 5 used for RuBP regeneration = 1 G3P net output. 2 G3P → 1 molecule glucose (requires 6 turns).

Calvin Cycle Lab

Calvin Cycle — Stroma

Carbon molecules animated through 3 phases

CO₂ concentration

50%

ATP supply

50%

G3P output

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rel. units/s

RuBP regen

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rel. units/s

PGA pool

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rel. units

Cycle speed

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turns/min

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Calvin cycle — three phases

🔵Phase 1 — Carbon fixation (carboxylation): RuBiSCO (ribulose bisphosphate carboxylase-oxygenase) catalyses: CO₂ + RuBP (5C) → 2× PGA (3C). This is the key step — PGA is the first stable product.

⚡Phase 2 — Reduction: PGA is phosphorylated by ATP → 1,3-bisPGA, then reduced by NADPH → G3P (3C). Per CO₂: costs 2 ATP + 2 NADPH. G3P is the sugar that eventually becomes glucose, starch, etc.

🔄Phase 3 — Regeneration of RuBP: 5 of every 6 G3P molecules are used to regenerate 3 RuBP (5C) using ATP. This costs 3 ATP. Net per 3 turns: 9 ATP + 6 NADPH used, 1 G3P net produced.

📌Stoichiometry (NEET critical): Per glucose: 18 ATP + 12 NADPH + 12 H⁺ consumed. 6 CO₂ fixed, 6 turns of cycle. C3 of PGA = first product. RuBiSCO is the most abundant enzyme on Earth.

NEET TIPPGA (3-phosphoglycerate) is the FIRST stable product of CO₂ fixation in C3 plants — not G3P. G3P is produced after reduction. Also: RuBiSCO can act as an oxygenase (photorespiration) — it adds O₂ instead of CO₂ to RuBP. This wasteful process occurs in C3 plants at high temperature, which is why C4 plants evolved.

Section 3 of 5

Section 4 of 5 · Pathways

C3, C4, and CAM Plants

Three evolutionary strategies for fixing CO₂. C3 is the ancestral form. C4 (Hatch-Slack) concentrates CO₂ around RuBiSCO. CAM separates fixation and Calvin cycle in time (day vs night) to survive drought.

Compare side by sideAll three canvases update together when you move the temperature/light slider. See how each strategy responds differently.

Spot the key differencesFirst CO₂ product: OAA (4C) in C4 vs PGA (3C) in C3. Kranz anatomy in C4. Stomata open at night in CAM. Photorespiration in C3 but not C4/CAM.

Check the examplesC3: wheat, rice, potato, spinach. C4: sugarcane, maize, sorghum, Amaranthus. CAM: cacti, pineapple, Agave, Aloe.

Pathway Comparison

Temperature

30°C

15°C cool45°C hot

Light Intensity

50%

C3 Plants

Wheat · Rice · Spinach · Potato

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C4 Plants

Sugarcane · Maize · Sorghum

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CAM Plants

Cactus · Pineapple · Aloe

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C3 vs C4 vs CAM — key differences

🌾C3 — simple but vulnerable: CO₂ is fixed directly by RuBiSCO in mesophyll cells. First product is PGA (3C). Susceptible to photorespiration at high temperature (RuBiSCO fixes O₂ instead of CO₂). Optimum ≈ 25°C.

🌽C4 — CO₂ pump (Hatch-Slack): CO₂ is first fixed in mesophyll cells by PEP carboxylase to form OAA (4C). OAA → malate → transported to bundle sheath cells → decarboxylated → CO₂ released near RuBiSCO. Suppresses photorespiration. Kranz anatomy essential.

🌵CAM — time-separated: Stomata open at night to fix CO₂ as malate (stored in vacuole). During day, stomata close (water conservation), stored malate releases CO₂ for Calvin cycle. Extremely water-efficient.

📌NEET key facts: PEP carboxylase (C4/CAM) has higher CO₂ affinity than RuBiSCO. C4 plants have no photorespiration. Compensation point: C3 at 25–100 ppm CO₂; C4 near 0–10 ppm. Light saturation: C4 > C3.

NEET TIPThe first CO₂ acceptor in C4 is PEP (phosphoenolpyruvate) not RuBP. The first product is OAA (oxaloacetate, 4C) not PGA. C4 plants have TWO carboxylation steps — one in mesophyll (PEP → OAA) and one in bundle sheath (Calvin cycle). Kranz = German for "wreath/halo" — bundle sheath cells form a ring around the vascular bundle.

Section 4 of 5

Section 5 of 5 · Revision Sheet

Quick Reference — All Photosynthesis Facts

Every equation, number, name, and NEET trap for photosynthesis — one page.

Key Formulae & Stoichiometry

Reaction / Event	Equation / Fact	Location	Products
Overall	6CO₂ + 12H₂O + light → C₆H₁₂O₆ + 6O₂ + 6H₂O	Chloroplast	Glucose, O₂
Water splitting	2H₂O → 4H⁺ + 4e⁻ + O₂	PSII (thylakoid lumen)	O₂ (byproduct!)
Non-cyclic ETP	H₂O → PSII → PQ → Cyt b₆f → PC → PSI → Fd → NADP⁺R	Thylakoid membrane	ATP + NADPH + O₂
Cyclic ETP	PSI → Fd → Cyt b₆f → PC → PSI (loop)	Thylakoid membrane	ATP only
C3 fixation	CO₂ + RuBP (5C) → 2 PGA (3C) [by RuBiSCO]	Stroma (mesophyll)	PGA = first product
C4 fixation (step 1)	CO₂ + PEP (3C) → OAA (4C) [by PEP carboxylase]	Mesophyll cytoplasm	OAA = first product
Per glucose (Calvin)	18 ATP + 12 NADPH + 12 H⁺ consumed	Stroma	1 glucose
Per turn (1 CO₂)	3 ATP + 2 NADPH used	Stroma	1/3 net G3P

Photosystems & Pigments

Component	Absorption / Detail	Function
PS II (P680)	680 nm (red) — reaction centre Chl a	Water splitting, electron excitation, O₂ release
PS I (P700)	700 nm (far red) — reaction centre Chl a	NADPH production, cyclic/non-cyclic flow
Chlorophyll a	430 nm (violet-blue) + 662 nm (red) — bright/blue-green colour — primary pigment	Direct photochemistry at reaction centres (P680, P700)
Chlorophyll b	453 nm + 642 nm — yellow-green colour — accessory pigment	Broadens absorption, transfers energy to Chl a
Xanthophylls	400–530 nm (blue-violet to green) — yellow colour — accessory pigment	Accessory light absorption, photoprotection
Carotenoids	400–500 nm (blue-violet) — yellow to yellow-orange colour — accessory pigment	Accessory absorption + photoprotection of Chl a
PEP carboxylase	C4/CAM plants only — higher CO₂ affinity than RuBiSCO	Primary CO₂ fixation in mesophyll
RuBiSCO	All plants — most abundant enzyme on Earth	CO₂ fixation (carboxylase) or O₂ fixation (oxygenase)

NEET Traps

TRAP 1PSII is excited first, not PSI. Despite the numbering, PSII (P680) acts before PSI (P700) in the Z-scheme electron flow. PSI was discovered first — hence the name.

TRAP 2O₂ comes from water, not CO₂. ¹⁸O labelling experiments proved photosynthetic O₂ is released from H₂O splitting at PSII, not from CO₂ reduction in the Calvin cycle.

TRAP 3PGA is the first stable C3 product, not G3P. CO₂ + RuBP → 2 PGA (3C) → then G3P after ATP/NADPH reduction. Many students incorrectly identify G3P as the first product.

TRAP 4C4 first product is OAA (4C), not malate or aspartate. OAA is immediately formed when CO₂ is added to PEP. OAA is then converted to malate/aspartate for transport — these are NOT the first products.

TRAP 5Cyclic photophosphorylation makes only ATP — no NADPH, no O₂. Only PSI is involved. The compensation point for C4 plants is near zero (no photorespiration). Light saturation point of C4 > C3.

TRAP 6CAM plants open stomata at night, not day. This is the opposite of normal plants. By night CO₂ fixation → malate. By day Calvin cycle runs on stored CO₂. This minimises water loss in arid conditions.

Section 5 of 5 · Complete!