Plant Anatomy and Physiology — NEET Biology

This is one of NEET's biggest Botany blocks — a reliable 6–8 questions covering how a plant is constructed and how it feeds, breathes, transports and grows. Photosynthesis and respiration alone are near-certain marks every year, and the plant growth regulators and transport mechanisms recur constantly. This chapter builds the plant from its tissues upward, then works through every physiological process step by step — the light and dark reactions, the respiratory pathways with their ATP tallies, and the hormones that control growth — in the ordered, factual form NEET tests.

Part A — Plant Anatomy

1. Plant tissues

A tissue is a group of cells with a common origin and function. Two broad types:

Meristematic tissue — dividing cells, responsible for growth:

• Apical meristem — at root and shoot tips; primary growth (increase in length).
• Intercalary meristem — at the base of internodes (grasses); also lengthens.
• Lateral meristem (vascular cambium, cork cambium) — secondary growth (increase in girth).

Permanent tissue — differentiated, non-dividing:

• Simple (one cell type): parenchyma (thin-walled, storage/photosynthesis), collenchyma (thickened corners, flexible support in young stems), sclerenchyma (lignified, dead, mechanical strength — fibres and sclereids).
• Complex (many cell types): xylem (water conduction upward — tracheids, vessels, fibres, parenchyma; mostly dead) and phloem (food conduction — sieve tubes, companion cells, fibres, parenchyma; mostly living).

Worked example 1.1. Which tissue provides mechanical strength in a mature stem, and is it living or dead? Sclerenchyma — its cells have thick, lignified secondary walls and are dead at maturity, giving rigid mechanical support (fibres like jute).

2. Tissue systems and anatomy of organs

Three tissue systems: the epidermal (outer protective layer + stomata, cuticle, trichomes, root hairs), the ground (all tissue except epidermis and vascular — mainly parenchyma), and the vascular (xylem + phloem).

Anatomical differences NEET tests (dicot vs monocot):

Secondary growth (in dicots) adds girth: the vascular cambium produces secondary xylem (wood) inward and secondary phloem outward; the cork cambium forms bark. Annual rings (spring/autumn wood) allow age estimation.

Worked example 2.1. Why can most monocots not undergo secondary growth? Their vascular bundles are closed — they lack a cambium between xylem and phloem — so there is no lateral meristem to produce secondary tissues and increase girth.

Part B — Transport in Plants

3. Water absorption and transpiration

Water enters root hairs by osmosis and moves across the cortex by the apoplast (through cell walls) and symplast (through cytoplasm/plasmodesmata) routes to the xylem.

Transpiration — loss of water vapour, mostly through stomata — creates the pull that draws water up. Stomata open when guard cells become turgid (K⁺ influx → water in → guard cells bow apart); they close when flaccid. Transpiration cools the plant and drives mineral uptake but risks water loss.

Ascent of sap — water rises through xylem by the cohesion–tension (transpiration-pull) theory: transpiration at the leaf creates tension, and the cohesion of water molecules (hydrogen bonding) plus adhesion to xylem walls holds the column together as it is pulled up. Root pressure (a minor push from below) causes guttation (water droplets at leaf margins at night).

Worked example 3.1. What is the main force driving the ascent of sap in a tall tree? Transpiration pull — the tension generated by water evaporating from the leaves, transmitted down an unbroken water column held together by cohesion (the cohesion–tension theory). Root pressure is too weak to raise water in tall trees.

4. Mineral nutrition and phloem translocation

Plants need macronutrients (C, H, O, N, P, K, Ca, Mg, S) in large amounts and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl) in traces. Nitrogen (from nitrate/ammonium; biologically fixed by Rhizobium and cyanobacteria) is essential for proteins and nucleic acids. Deficiency symptoms appear on older leaves for mobile nutrients (N, P, K, Mg) and young leaves for immobile ones (Ca, Fe, S).

Phloem translocation — organic food (sucrose) moves from source (leaf) to sink (root, fruit) by the pressure-flow (mass-flow) hypothesis: sugar loaded at the source raises osmotic pressure, water enters, and the resulting pressure pushes sap toward the sink where sugar is unloaded. Phloem transport is bidirectional and uses living cells (unlike one-way xylem).

Worked example 4.1. Why is xylem transport unidirectional while phloem transport is bidirectional? Xylem carries water upward only, pulled by transpiration through mostly dead cells. Phloem moves food from any source to any sink — up or down — by pressure flow through living sieve tubes, so its direction depends on where sugar is made and needed.

Part C — Photosynthesis

5. The light reactions

Photosynthesis occurs in chloroplasts; overall: $6C{O}_2+12H_{2}O\stackrel{\text{light}}{\to }{C}_6{H}_{12}{O}_6+6O_2+6H_{2}O$.

The light reactions occur on the thylakoid membranes:

• Two photosystems, PS II (P680) and PS I (P700), absorb light and pass electrons along an electron transport chain (the Z-scheme).
• Photolysis of water at PS II splits H₂O → releasing O₂, protons and electrons (the O₂ we breathe comes from water, not CO₂).
• Electron flow builds a proton gradient that drives ATP synthesis (photophosphorylation) and reduces NADP⁺ to NADPH.
• Non-cyclic photophosphorylation (both PS, makes ATP + NADPH + O₂); cyclic (PS I only, makes ATP only).

Worked example 5.1. From which molecule is the oxygen released in photosynthesis derived? From water — photolysis of H₂O at photosystem II releases the O₂ (confirmed by isotope experiments). CO₂ is not the source of the oxygen.

6. The Calvin cycle, C₄ pathway and photorespiration

The Calvin cycle (C₃, dark reaction) occurs in the stroma and fixes CO₂ using the ATP and NADPH from the light reactions. Three phases: carboxylation (CO₂ + RuBP → 2 PGA, catalysed by RuBisCO), reduction (PGA → G3P/glucose), regeneration (of RuBP). The first stable product is the 3-carbon 3-PGA (hence "C₃").

C₄ plants (maize, sugarcane) fix CO₂ first into a 4-carbon acid (oxaloacetate) in mesophyll cells, using PEP carboxylase, then release it to the Calvin cycle in bundle-sheath cells (Kranz anatomy). This concentrates CO₂, suppresses photorespiration and makes C₄ plants more efficient in hot, dry climates.

Photorespiration — in C₃ plants at high O₂/low CO₂, RuBisCO fixes O₂ instead of CO₂ (it is an oxygenase too), wastefully consuming energy with no sugar or ATP gain. C₄ plants avoid it by concentrating CO₂ around RuBisCO.

Worked example 6.1. Why are C₄ plants more productive than C₃ plants in hot climates? C₄ plants use PEP carboxylase to concentrate CO₂ in bundle-sheath cells (Kranz anatomy), so RuBisCO meets high CO₂ and little O₂ and does not photorespire. C₃ plants lose energy to photorespiration in hot, bright conditions, so C₄ plants fix carbon more efficiently.

Part D — Respiration

7. The respiratory pathways

Cellular respiration breaks glucose to release energy (ATP). Overall: $C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O+\text{ATP}$.

• Glycolysis (cytoplasm; anaerobic first step): glucose → 2 pyruvate, net 2 ATP + 2 NADH. Common to all organisms.
• Fermentation (anaerobic): pyruvate → lactic acid (muscle) or ethanol + CO₂ (yeast); little ATP.
• Aerobic respiration (mitochondria): pyruvate → acetyl-CoA → Krebs (citric acid) cycle (matrix), producing CO₂, NADH, FADH₂ and GTP; then the electron transport chain (inner membrane) uses O₂ as the final electron acceptor to make water and the bulk of the ATP by oxidative phosphorylation.
• Net yield (theoretical): ~36–38 ATP per glucose in aerobic respiration; only 2 in fermentation.

Respiratory quotient (RQ) = CO₂ released / O₂ consumed: 1.0 for carbohydrates, <1 for fats (~0.7), >1 for organic acids.

Worked example 7.1. Where does the Krebs cycle occur and how much ATP does aerobic respiration yield versus fermentation? The Krebs cycle occurs in the mitochondrial matrix. Complete aerobic respiration yields about 36–38 ATP per glucose, whereas fermentation (anaerobic) yields only 2 ATP — the reason aerobic respiration is so much more efficient.

Part E — Plant Growth and Development

8. Growth and plant growth regulators

Growth is irreversible increase in size; the growth curve is sigmoid (S-shaped) — lag, log (exponential) and stationary phases. Differentiation, dedifferentiation and redifferentiation shape the plant body.

Five plant growth regulators (PGRs) — the key table NEET tests:

Auxin and cytokinin are the two often antagonistic ones (apical dominance vs lateral growth). Ethylene is the only gaseous hormone.

Worked example 8.1. Which hormone promotes fruit ripening, and which closes stomata during water stress? Ethylene (a gas) promotes fruit ripening and senescence. Abscisic acid (ABA), the stress hormone, closes stomata during water stress and enforces seed dormancy.

9. Photoperiodism and vernalisation

• Photoperiodism — flowering response to day/night length. Short-day plants flower when the night exceeds a critical length; long-day plants when it is shorter; day-neutral plants are indifferent. The stimulus is perceived by leaves (via phytochrome) and a signal (florigen) triggers flowering.
• Vernalisation — promotion of flowering by a period of low temperature (cold treatment), as in winter wheat.

Worked example 9.1. What environmental factor does vernalisation use to induce flowering? Low temperature — a spell of cold treatment promotes subsequent flowering in certain plants (e.g. winter varieties of wheat), preventing them from flowering before winter.

10. Common traps NEET sets here

• Collenchyma living (support in young stems), sclerenchyma dead (support in mature).
• Dicot stem: open bundles (cambium, secondary growth); monocot: closed, scattered.
• O₂ in photosynthesis comes from water, not CO₂; light reactions on thylakoids, Calvin cycle in stroma.
• C₃ first product 3-PGA; C₄ first product 4-carbon OAA; Kranz anatomy in C₄.
• RuBisCO does both carboxylation and photorespiration (oxygenase).
• Glycolysis in cytoplasm (2 ATP); Krebs in matrix; ETC on inner membrane (most ATP); ~36–38 total.
• Ethylene = ripening (gas); ABA = stress/stomatal closure/dormancy.
• Auxin apical dominance; cytokinin overcomes it.
• Transpiration pull (cohesion–tension) drives ascent of sap; pressure flow drives phloem.

11. Memory aids

• "Para stores, Collen bends, Sclero strengthens (dead)" — the three simple tissues.
• "Water gives the O₂" — source of photosynthetic oxygen.
• "C₃ is 3-PGA, C₄ has Kranz" — the carbon-fixation pathways.
• "Glyco-cyto, Krebs-matrix, ETC-membrane" — respiration locations.
• "Ethylene ripens, ABA stresses" — the two easily-confused hormones.
• "Auxin bosses the tip, cytokinin frees the sides" — apical dominance antagonism.

12. Exam protocol

1. Classify tissues: meristem (apical/lateral) vs permanent (parenchyma/collenchyma/sclerenchyma; xylem/phloem); living vs dead.
2. Dicot vs monocot anatomy — open vs closed vascular bundles and secondary growth.
3. Transport: transpiration pull (cohesion–tension) for xylem; pressure flow (source→sink) for phloem; guard-cell mechanism.
4. Photosynthesis: light reactions (thylakoid, PS II/I, water gives O₂, ATP + NADPH) → Calvin cycle (stroma, RuBisCO, 3-PGA); C₄/Kranz avoids photorespiration.
5. Respiration: glycolysis (cytoplasm, 2 ATP) → Krebs (matrix) → ETC (inner membrane); ~36–38 ATP; RQ values.
6. PGRs: match hormone to role (auxin, GA, cytokinin, ethylene, ABA); photoperiodism and vernalisation.