# Photosynthesis in Higher Plants (NEET Biology Class 11): Z Scheme, Calvin Cycle & C3 vs C4 Source: https://tracknation.in/story/photosynthesis-in-higher-plants-neet-biology-class-11-z-scheme-calvin-cycle-c3-vs-c4 (TrackNation β€” free UPSC notes with PYQs) > NEET Class 11 Photosynthesis in Higher Plants, mined from NCERT: the early experiments (Priestley to van Niel) and the correct equation, photosynthetic pigments, the light reaction (PS I/PS II, P700/P680, the Z scheme, water splitting), cyclic vs non-cyclic photophosphorylation, the chemiosmotic hypothesis (CF0/CF1), the Calvin cycle with full ATP/NADPH stoichiometry, the C4 Hatch and Slack pathway with Kranz anatomy, photorespiration, a complete C3-vs-C4 comparison table, and the factors affect ## 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 β€” learn them cold.* Photosynthesis is how autotrophs use light energy to build organic food from COβ‚‚ and water. The correct overall equation β€” note **12 water molecules, not 6**, because **the Oβ‚‚ released comes from water, not from COβ‚‚**: **6COβ‚‚ + 12Hβ‚‚O β†’(light)β†’ C₆H₁₂O₆ + 6Hβ‚‚O + 6Oβ‚‚** ### The classic experiments β€” 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 β€” 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 COβ‚‚ β€” proving Oβ‚‚ comes from Hβ‚‚O, not COβ‚‚. | | Melvin Calvin | post-WWII | Used radioactive ¹⁴C to show the first COβ‚‚-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 Hβ‚‚O. In purple/green **sulphur bacteria**, Hβ‚‚S (not Hβ‚‚O) is the hydrogen donor, so the oxidation product is **sulphur or sulphate β€” not Oβ‚‚**. ## 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** β€” the **light reactions**. The **stroma** runs the enzymatic sugar synthesis β€” 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 β€” 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 β€” 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** β€” PS II acts first. - **Z scheme:** P680 electrons are excited β†’ picked up by an acceptor β†’ travel downhill through cytochromes β†’ reach PS I β†’ re-excited at P700 β†’ passed to another acceptor β†’ 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: **2Hβ‚‚O β†’ 4H⁺ + Oβ‚‚ + 4e⁻**. Critically, the water-splitting complex is associated with **PS II** and sits on the **inner side of the thylakoid membrane** β€” so protons and Oβ‚‚ 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 β†’ PS II β†’ PS I β†’ NADP⁺ (one-way) | Electron cycles back to PS I | | Products | ATP + NADPH + Oβ‚‚ | ATP only β€” no NADPH, no Oβ‚‚ | | 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 β€” 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 **CFβ‚€** 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 β€” three stages, and the numbers - **1. Carboxylation** β€” COβ‚‚ 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** β€” uses **2 ATP + 2 NADPH per COβ‚‚** fixed. **3. Regeneration** β€” regenerates RuBP, needing **1 more ATP**. - So **per COβ‚‚: 3 ATP + 2 NADPH**. For **one glucose** you need **6 turns** of the cycle: **6 COβ‚‚ + 18 ATP + 12 NADPH β†’ 1 glucose**. That ATP:NADPH mismatch (18 vs 12) is exactly **why cyclic photophosphorylation exists** β€” 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 COβ‚‚ onto **PEP** β†’ **OAA** β†’ converted to malic/aspartic acid β†’ transported to **bundle sheath cells**, where the C4 acid breaks down, **releasing COβ‚‚** and a 3-carbon molecule. The COβ‚‚ 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 β€” C3 and C4 alike. C4 is a **COβ‚‚-concentrating add-on**, not a replacement. ### Photorespiration β€” the wasteful process C4 plants avoid - RuBisCO's active site binds **both COβ‚‚ and Oβ‚‚ competitively** β€” the winner is decided by their **relative concentrations**. When Oβ‚‚ binds instead, RuBP forms one phosphoglycerate + one **phosphoglycolate (2-carbon)** β€” the photorespiratory pathway. - Photorespiration produces **no sugar, no ATP, no NADPH** β€” it merely **releases COβ‚‚ while consuming ATP**. Its biological function is **still unknown**. C4 plants avoid it entirely because pumping COβ‚‚ into the bundle sheath keeps the local COβ‚‚ concentration high, forcing RuBisCO to act as a **carboxylase**, not an oxygenase. ### C3 vs C4 β€” the full comparison NEET asks from | **Characteristic** | **C3 plants** | **C4 plants** | | Primary COβ‚‚ acceptor | RuBP (5-carbon) | PEP β€” phosphoenolpyruvate (3-carbon) | | First fixation product | 3-PGA (3-carbon) | OAA β€” oxaloacetic acid (4-carbon) | | Carboxylating enzyme (first step) | RuBisCO | PEP carboxylase (PEPcase) | | Cells that fix COβ‚‚ | One type β€” mesophyll only | Two types β€” 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 β€” large bundle sheath cells, many chloroplasts, thick walls, no intercellular spaces | | Photorespiration | Present (wasteful) | Absent/negligible | | COβ‚‚ saturation point | Only beyond 450 Β΅L/L β€” so current COβ‚‚ IS limiting | About 360 Β΅L/L | | Temperature optimum | Lower (30–40Β°C range varies by habitat) | Higher β€” 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 COβ‚‚ 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** β€” so light is **rarely limiting** in nature (except in shade/dense forest). Excess light actually breaks down chlorophyll. - **COβ‚‚ is the MAJOR limiting factor.** Atmospheric COβ‚‚ is only **0.03–0.04%**; raising it to **0.05%** increases fixation, beyond which it becomes damaging long-term. This is exploited commercially β€” greenhouse **tomatoes and bell peppers** are grown in COβ‚‚-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** β€” water stress closes stomata (cutting COβ‚‚ 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: Oβ‚‚ 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** β€” NEET swaps these constantly. Also, "dark reaction" does NOT mean it happens in the dark; and photorespiration happens in **C3 plants**, not C4 β€” 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 **Answer: (b)** β€” 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 **Answer: (b)** β€” Engelmann used a prism, the alga Cladophora, and aerobic bacteria to map where O2 was evolved β€” 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 **Answer: (a)** β€” 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 **Answer: (b)** β€” They are named in order of discovery, not function β€” 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 **Answer: (a)** β€” In cyclic flow only PS I operates and the electron cycles back, so only ATP is made β€” 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 **Answer: (d)** β€” 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 **Answer: (a)** β€” 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 **Answer: (d)** β€” 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 **Answer: (c)** β€” C4 plants have Kranz anatomy β€” 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 **Answer: (d)** β€” 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 **Answer: (a)** β€” 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 **Answer: (d)** β€” 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 **Answer: (d)** β€” Photorespiration produces neither sugars nor ATP nor NADPH β€” 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 **Answer: (b)** β€” 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 **Answer: (b)** β€” Light saturation occurs at just 10 per cent of full sunlight, which is why light is rarely limiting in nature. **Q16.** In the chloroplast shown, the Calvin cycle (dark reaction) enzymes are located in the region marked - (a) E - (b) A - (c) B - (d) C **Answer: (a)** β€” E is the stroma β€” the fluid matrix that holds the enzymes for the synthetic (Calvin) phase, along with chloroplast DNA and ribosomes. The light reaction occurs in the thylakoid membranes (grana and stroma lamellae). **Q17.** Which labelled part is made of membranes that lack photosystem II and NADP reductase, and is the site of cyclic photophosphorylation? - (a) C - (b) A - (c) E - (d) D **Answer: (d)** β€” D marks the stroma lamellae (the membrane tubes connecting the grana). NCERT notes that these membranes lack PS II and NADP reductase, so only cyclic photophosphorylation (PS I) can occur there. Grana [C] contain both photosystems. **Q18.** The stacks of disc-like thylakoids marked C are called - (a) Grana - (b) Stroma - (c) Stroma lamellae - (d) Cristae **Answer: (a)** β€” A granum (plural grana) is a stack of flattened thylakoids; grana are joined by stroma lamellae. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q19.** Which of the following is NOT an accessory pigment in photosynthesis? (NEET PYQ) - (a) Carotenoid - (b) Chlorophyll b - (c) Xanthophyll - (d) Chlorophyll a **Answer: (d)** β€” Chlorophyll a is the chief/primary pigment; chlorophyll b, xanthophylls and carotenoids are accessory pigments. **Q20.** The splitting of water during photosynthesis is associated with: (NEET PYQ) - (a) The stroma - (b) Photosystem II - (c) Photosystem I - (d) ATP synthase **Answer: (b)** β€” The water-splitting complex is associated with PS II, physically located on the inner (lumen) side of the thylakoid membrane. **Q21.** The first stable product of CO2 fixation in C4 plants is: (NEET PYQ) - (a) Glucose - (b) 3-PGA - (c) Oxaloacetic acid (OAA) - (d) RuBP **Answer: (c)** β€” C4 plants fix CO2 onto PEP via PEPcase, producing the 4-carbon oxaloacetic acid (OAA) as the first stable product. **Q22.** 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 **Answer: (c)** β€” CF0 is embedded in the thylakoid membrane as a proton channel; CF1 protrudes on the stroma-facing side. **Q23.** Which factor is considered the major limiting factor for photosynthesis? (NEET PYQ) - (a) Chlorophyll amount - (b) Carbon dioxide concentration - (c) Water - (d) Light intensity **Answer: (b)** β€” CO2 is the major limiting factor β€” atmospheric concentration is only 0.03-0.04 per cent. **Q24.** 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 **Answer: (a)** β€” In C4 plants the Calvin cycle runs only in bundle sheath cells, unlike C3 plants where it runs in mesophyll cells. **Q25.** 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 **Answer: (a)** β€” With H2S as hydrogen donor, sulphur bacteria release sulphur or sulphate instead of O2 β€” 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 β€” 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, COβ‚‚, temperature, water). Begin each fact with its topic and end it with a full stop.