Chapter Overview & Weightage
Plant Physiology is one of the highest-scoring chapters in NEET Biology — predictable, formula-based, and rewarding for students who put in focused effort. It covers five major topics: Photosynthesis, Respiration, Plant Growth and Development, Mineral Nutrition, and Transport in Plants.
Plant Physiology contributes 6–8 questions in NEET every year, roughly 8–10% of the Biology section. Questions cluster around photosynthesis (light reactions + Calvin cycle) and respiration (glycolysis + Krebs cycle). These two alone account for 4–5 questions consistently.
| Year | Questions | Marks | Key Topics Tested |
|---|---|---|---|
| NEET 2024 | 7 | 28 | C3/C4 plants, ATP yield, phytohormones |
| NEET 2023 | 6 | 24 | Photorespiration, ETC, mineral deficiency |
| NEET 2022 | 8 | 32 | Z-scheme, glycolysis, seed dormancy |
| NEET 2021 | 7 | 28 | Calvin cycle, Krebs cycle, plant growth regulators |
| NEET 2020 | 6 | 24 | Mineral nutrition, transpiration, fermentation |
The pattern is stable. Photosynthesis + Respiration together = 4–5 questions every single year. Do not neglect Plant Growth Regulators — 1–2 questions appear consistently from this subtopic.
Key Concepts You Must Know
Ranked by NEET frequency:
Photosynthesis (Highest Priority)
- Light reactions: Z-scheme, photosystems PS I and PS II, cyclic vs non-cyclic photophosphorylation
- Calvin cycle (C3 pathway): RuBisCO, fixation of CO₂, G3P, regeneration of RuBP
- C4 pathway (Hatch-Slack): PEP carboxylase, bundle sheath cells, Kranz anatomy
- CAM plants: temporal separation of CO₂ fixation
- Photorespiration: why it occurs, which plants lack it
Respiration (High Priority)
- Glycolysis: 10 steps, net ATP count, site (cytoplasm)
- Pyruvate oxidation: acetyl CoA formation
- Krebs cycle: 8 steps, what’s produced per turn, total turns per glucose
- Electron Transport Chain (ETC): Complex I–IV, chemiosmosis, ATP synthase
- Fermentation: alcoholic vs lactic acid, which organisms do which
- Respiratory quotient (RQ): values for carbohydrates, fats, proteins
Plant Growth Regulators (Medium-High)
- Auxins: apical dominance, root initiation, phototropism
- Gibberellins: stem elongation, seed germination, bolting
- Cytokinins: cell division, senescence delay, Richmond-Lang effect
- ABA: seed dormancy, stomatal closure — the stress hormone
- Ethylene: fruit ripening, epinasty, triple response
Mineral Nutrition (Medium)
- Macronutrients vs micronutrients: which is which, deficiency symptoms
- Nitrogen fixation: Rhizobium (symbiotic), Azotobacter (free-living)
- Hydroponics and essential element criteria
Transport in Plants (Medium)
- Apoplast vs symplast pathways
- Osmosis, water potential, plasmolysis
- Ascent of sap: cohesion-tension theory
- Pressure flow hypothesis (phloem transport)
Important Formulas
Use this as a reference — NEET often asks about reactants/products and their sources.
Breakdown: Glycolysis = 2 ATP + 2 NADH; Pyruvate oxidation = 2 NADH; Krebs cycle (×2) = 2 ATP + 6 NADH + 2 FADH₂; ETC = 32–34 ATP. NEET asks this almost every year — memorise the per-step yield, not just the total.
RQ = 1 for carbohydrates, < 1 for fats (~0.7), > 1 for organic acids. When RQ is very high (succulents, anaerobic respiration), no O₂ is consumed but CO₂ is released.
Ψₛ = solute potential (always negative), Ψₚ = pressure potential. Water moves from high Ψw to low Ψw — this direction question is very common.
Each NADH → 2.5 ATP; each FADH₂ → 1.5 ATP in modern chemiosmotic accounting. However, for NEET, use the classic values: NADH = 3 ATP, FADH₂ = 2 ATP unless the question specifies otherwise.
Solved Previous Year Questions
PYQ 1 — NEET 2023
Question: In non-cyclic photophosphorylation, what is the final electron acceptor?
(A) PS I (B) NADP⁺ (C) Ferredoxin (D) Plastoquinone
Answer: (B) NADP⁺
In non-cyclic photophosphorylation, electrons flow from water → PS II → plastoquinone → cytochrome b6f → plastocyanin → PS I → ferredoxin → NADP⁺ reductase → NADP⁺, which gets reduced to NADPH. The final electron acceptor is NADP⁺.
Students confuse ferredoxin with the final acceptor. Ferredoxin is the immediate acceptor after PS I, but it hands electrons to NADP⁺ reductase. The final acceptor is NADP⁺. This exact distinction appeared in the question — read carefully.
PYQ 2 — NEET 2022
Question: Which of the following is the correct sequence of steps in glycolysis?
(A) Glucose → Glucose-6-phosphate → Fructose-6-phosphate → Fructose-1,6-bisphosphate
(B) Glucose → Fructose-6-phosphate → Glucose-6-phosphate → Fructose-1,6-bisphosphate
(C) Glucose → Fructose-1,6-bisphosphate → Glucose-6-phosphate
(D) Fructose-6-phosphate → Glucose → Glucose-6-phosphate
Answer: (A)
Glycolysis starts with phosphorylation of glucose to Glucose-6-phosphate (by hexokinase, costs 1 ATP). This isomerises to Fructose-6-phosphate, then gets phosphorylated again to Fructose-1,6-bisphosphate (by phosphofructokinase, costs 1 ATP). This is why we say glycolysis has an “investment phase” costing 2 ATP before the payoff phase begins.
PYQ 3 — NEET 2024 (Shift 2)
Question: Which plant hormone promotes stomatal closure during water stress?
(A) Gibberellin (B) Cytokinin (C) Auxin (D) Abscisic Acid
Answer: (D) Abscisic Acid
ABA is called the “stress hormone” for a reason. During drought, ABA accumulates in leaves and triggers K⁺ efflux from guard cells → loss of turgor → stomatal closure. This reduces water loss but also limits CO₂ entry, slowing photosynthesis. NEET loves questions that link ABA to both stomata and seed dormancy.
A quick ABA memory hook: ABA = Anti-growth, Anti-transpiration, Abscission. Everything ABA does is inhibitory or stress-related.
Difficulty Distribution
For NEET, Plant Physiology questions fall into these categories:
| Difficulty | % of Questions | What It Looks Like |
|---|---|---|
| Easy | 40% | Direct recall — “Which enzyme fixes CO₂ in C3 plants?” (Answer: RuBisCO) |
| Medium | 45% | Application — sequence of steps, identification of products, RQ calculation |
| Hard | 15% | Conceptual traps — comparing C3/C4/CAM, understanding photorespiration consequences, ETC electron flow |
The good news: 85% of Plant Physiology questions are accessible if you know the material thoroughly. The hard 15% often comes from trick options in multi-step processes (Z-scheme electron flow, Krebs cycle intermediates).
Expert Strategy
Week 1: Master the flowcharts, not just the names. Draw the Z-scheme from memory until it takes less than 2 minutes. Draw glycolysis → Krebs cycle as a single connected map. Toppers who score 340+ in Biology treat these diagrams as muscle memory.
For Photosynthesis: memorise PS II before PS I. The exam always tries to confuse you — PS II acts first (absorbs 680 nm, splits water), PS I acts second (absorbs 700 nm, reduces NADP⁺). The numbering is in order of discovery, not function.
For Respiration: count in steps. Don’t just memorise “36 ATP from one glucose.” Know exactly where each ATP, NADH, and FADH₂ comes from. Glycolysis: 2 ATP (net), 2 NADH. Pyruvate oxidation: 2 NADH (one per pyruvate). Krebs cycle (2 turns): 2 GTP, 6 NADH, 2 FADH₂. ETC converts the cofactors into 32–34 ATP.
Plant Growth Regulators: use a comparison table. Make a 5×5 table: hormones as rows (Auxin, Gibberellin, Cytokinin, ABA, Ethylene), effects as columns (cell division, elongation, germination, senescence, ripening). Fill it in. This covers 90% of PGR questions.
Mineral Nutrition: focus on deficiency symptoms. NEET rarely asks which minerals are essential — it asks what happens when they’re deficient. Nitrogen deficiency → chlorosis of older leaves first (mobile nutrient). Calcium deficiency → young leaves affected first (immobile). This mobile vs immobile distinction is a favourite trap.
Mobile nutrients (N, P, K, Mg) show deficiency in older leaves first — the plant remobilises them to younger tissue. Immobile nutrients (Ca, Fe, S, B) show deficiency in younger leaves first. This single rule cracks most mineral nutrition questions.
Allocate revision time proportionally:
- Photosynthesis: 35% of your study time for this chapter
- Respiration: 30%
- Plant Growth Regulators: 20%
- Mineral Nutrition + Transport: 15%
Common Traps
Trap 1: PS I vs PS II wavelength. PS I absorbs 700 nm (P700), PS II absorbs 680 nm (P680). Students often flip these. Remember: PS II has the lower number and the lower wavelength. Or just remember P680 splits water — water-splitting requires more energy (lower wavelength = higher energy).
Trap 2: Net vs Gross ATP in glycolysis. Glycolysis produces 4 ATP but uses 2 ATP in the investment phase. Net = 2 ATP. NEET questions will say “net ATP” or “total ATP” — these are different answers. Read the question word carefully.
Trap 3: C4 plants don’t eliminate photorespiration entirely. C4 plants minimise photorespiration — they don’t eliminate it completely. The CO₂ concentrating mechanism in bundle sheath cells keeps CO₂ concentration high enough that RuBisCO rarely reacts with O₂. Options that say “C4 plants have no photorespiration” are technically wrong — but NEET has accepted this simplification. If the option says “negligible photorespiration,” that’s the correct choice.
Trap 4: Ethylene is a gas, not a liquid. Students sometimes write ethylene-related answers about “concentration in solution.” Ethylene is a gaseous hormone produced from methionine via ACC. The question context usually clarifies, but don’t confuse ethylene (gas) with ethanol (fermentation product).
Trap 5: Fermentation produces ethanol AND CO₂ — both. Alcoholic fermentation: pyruvate → acetaldehyde (decarboxylation, releases CO₂) → ethanol (NADH oxidised). Students forget that CO₂ is released in this process. Lactic acid fermentation does not release CO₂ — pyruvate is directly converted to lactate. This is why bread rises (yeast releases CO₂) but yogurt making doesn’t bubble.
Last 10 days before NEET: Solve all Plant Physiology PYQs from 2017–2024. You’ll notice questions repeat themes (sometimes near-verbatim). The Z-scheme, Calvin cycle intermediate names, and RQ values appear almost every year in some form. Past papers are the closest thing to a preview of the actual exam.