Plant Physiology

Plant physiology is the "how it works" half of botany, and in NEET it is the single most productive area of Class 11 Biology — questions on water potential, C₄ pathway, respiratory quotient, and plant hormones appear almost every year. This lesson stitches the whole unit together so you can see how one functional theme (moving matter and energy through a plant that cannot move itself) links transport, nutrition, photosynthesis, respiration and growth.

Movement of Water, Minerals and Food

A plant has no heart, so all long-distance movement depends on physical gradients and on the anatomy of xylem and phloem. Three mechanisms of short-distance movement matter:

• Diffusion – passive, slow, no energy or membrane protein needed; the only route for gases moving in and out of leaves and for movement inside cells.
• Facilitated diffusion – still passive and still down a gradient, but a channel or carrier protein is required; it is selective and saturable. Symports, antiports and uniports fall here.
• Active transport – uphill movement against the gradient using ATP-driven pumps; saturable, selective and inhibited by chemicals that block the protein.

Water movement is described by water potential (Ψw), measured in pascals (usually MPa). Pure water at standard conditions is given Ψw = 0, so all solutions have a negative water potential. Two components determine it:

Ψw = Ψs (solute or osmotic potential, always negative) + Ψp (pressure potential, positive in a turgid cell, negative in transpiring xylem)

Water always moves from higher (less negative) to lower (more negative) Ψw. In osmosis water crosses a semipermeable membrane along this gradient; the pressure that must be applied to just stop it is the osmotic pressure. A cell in a hypertonic solution loses water and undergoes plasmolysis (protoplast shrinks away from the wall; the space fills with the external solution); returning it to a hypotonic medium causes deplasmolysis. Imbibition is a special case of diffusion in which water is adsorbed by hydrophilic colloids such as cellulose or seed proteins — it needs both a water-potential gradient and an affinity between adsorbent and liquid, and it generates enormous imbibition pressure (as when swelling seeds crack the soil).

Water entering a root moves radially to the xylem by two routes: the apoplast pathway (through walls and intercellular spaces, no membrane crossing, faster) and the symplast pathway (from cell to cell through plasmodesmata and cytoplasm). At the endodermis the Casparian strip blocks the apoplast, forcing water and ions through the plasma membranes of endodermal cells — this is where the plant exerts control over what enters. Long-distance ascent of sap is explained by the cohesion–tension/transpiration-pull theory: evaporation from mesophyll cell walls creates a strongly negative Ψw at the leaf surface, and because water molecules cohere to one another and adhere to xylem walls, an unbroken column is dragged upward under tension. Root pressure (positive push generated by active ion uptake in the root, visible as guttation) contributes only modestly and mainly in small plants at night.

Transpiration occurs largely through stomata, whose opening depends on guard-cell turgor: K⁺ influx lowers Ψw, water enters, the thin outer wall bulges and the inelastic inner wall is pulled apart, opening the pore. Transpiration cools leaves, supplies the pull for water and mineral ascent, and maintains cell turgidity. Plants with the C₄ pathway are more water-use efficient, losing far less water per gram of dry matter fixed.

Food (mainly sucrose) travels in phloem by the pressure-flow (mass-flow) hypothesis: sucrose is actively loaded at the source, water follows osmotically from adjacent xylem, and the resulting high hydrostatic pressure drives bulk flow to the sink where sugar is unloaded. Phloem transport is bidirectional overall (source-to-sink, and sinks change with season), whereas xylem transport is essentially unidirectional.

Mineral Nutrition

Hydroponic (soil-less) culture experiments established which elements are essential and what each does. An element is essential if the plant cannot complete its life cycle without it, if its role is specific and irreplaceable, and if it is directly involved in metabolism.

• Macronutrients (needed in concentrations above about 10 mmol per kg dry weight): C, H, O, N, P, S, K, Ca, Mg.
• Micronutrients / trace elements: Fe, Mn, Cu, Zn, B, Cl, Mo, Ni.
• Beneficial but not universally essential: Na, Si, Co, Se.

Functionally, elements act as (i) structural components of biomolecules, (ii) components of energy-related compounds (Mg in chlorophyll, P in ATP), (iii) enzyme activators or inhibitors, and (iv) osmotic regulators (K⁺ in guard cells).

Deficiency symptoms are the exam favourites. Because mobile elements (N, P, K, Mg) are exported from old leaves to young tissue, their deficiency shows first in older leaves; immobile elements (Ca, S, Fe, B) show symptoms first in young leaves.

• Chlorosis (loss of chlorophyll): N, K, Mg, S, Fe, Mn, Zn, Mo deficiency
• Necrosis (tissue death): Ca, Mg, Cu, K deficiency
• Inhibited cell division / stunting: N, K, S, Mo deficiency
• Delayed flowering: N, S, Mo deficiency

Excess of a micronutrient becomes toxic — the classic case is excess Mn, which competes with Fe and Mg uptake and mimics their deficiency. Mineral absorption itself has a passive apoplastic phase and an active, ATP-requiring phase across membranes; ions are then loaded into xylem and carried up with the transpiration stream, and are actively remobilised from senescing leaves.

Nitrogen deserves separate attention. Atmospheric N₂ is unavailable to plants and must be fixed. Biological nitrogen fixation is carried out only by prokaryotes: free-living forms (Azotobacter, Rhodospirillum, Nostoc, Anabaena) and symbionts (Rhizobium in legume root nodules, Frankia in Alnus). The enzyme nitrogenase is Mo–Fe-containing and extremely oxygen-sensitive, so nodules make leghaemoglobin as an oxygen scavenger; the reaction reduces N₂ to 2NH₃ at a high ATP cost. Ammonia is then assimilated by reductive amination (into glutamate) or transamination, and amide formation (asparagine, glutamine) allows nitrogen-rich transport in xylem sap. Nodule formation involves recognition and attachment of the bacterium to root hairs, curling of the hair, infection thread formation, cortical cell division and finally establishment of vascular connection with the host.

Photosynthesis in Higher Plants

Photosynthesis converts light energy into chemical energy in chloroplasts, whose thylakoid membranes host the light reactions and whose stroma hosts the carbon-fixing enzymatic reactions. Pigments are of four kinds — chlorophyll a (chief pigment), chlorophyll b, xanthophylls and carotenoids; the accessory pigments broaden the absorbed spectrum and protect chlorophyll a from photo-oxidation. Maximum photosynthesis occurs in blue and red regions of the spectrum.

The light reaction proceeds as follows:

1. Light harvested by antenna pigments is funnelled to the reaction centre of PS II (P680), which raises electrons to a higher energy level.
2. Electrons pass through the electron transport chain — plastoquinone, cytochrome b₆f complex, plastocyanin — to PS I (P700), pumping protons into the thylakoid lumen and building a proton gradient.
3. From PS I electrons reach ferredoxin and NADP⁺ reductase on the stromal side, reducing NADP⁺ to NADPH.
4. The electron hole in PS II is filled by photolysis of water by the oxygen-evolving complex on the inner (lumen) side, releasing O₂ and protons: this is the source of atmospheric oxygen.
5. Protons flow back through ATP synthase (chemiosmosis), producing ATP. This linear, two-photosystem route is non-cyclic photophosphorylation, giving ATP + NADPH + O₂; when only PS I operates in isolated stroma lamellae, cyclic photophosphorylation yields ATP alone.

In the Calvin cycle (C₃ pathway) CO₂ is accepted by the 5-carbon RuBP under the enzyme RuBisCO, giving two molecules of 3-PGA. The cycle has three phases — carboxylation, reduction and regeneration — and to make one glucose it consumes 6 CO₂, 18 ATP and 12 NADPH; three ATP and two NADPH are used per CO₂ fixed.

Photorespiration occurs because RuBisCO also binds O₂; at high O₂/low CO₂ the oxygenation of RuBP produces one PGA and one phosphoglycolate, and the salvage pathway releases CO₂ without making sugar, ATP or NADPH. It is therefore wasteful, and C₄ plants avoid it. In C₄ plants (maize, sugarcane, sorghum, Amaranthus) leaves show Kranz anatomy: mesophyll cells fix CO₂ using PEP carboxylase into the 4-carbon oxaloacetic acid, which is converted to malate and shuttled to thick-walled bundle-sheath cells; there malate is decarboxylated, raising CO₂ concentration around RuBisCO so much that photorespiration is suppressed. The Calvin cycle still occurs — in the bundle sheath. C₄ plants tolerate higher temperatures, show higher productivity and lack a CO₂ compensation point.

Rate-limiting factors follow Blackman's law of limiting factors: the rate is set by the factor closest to its minimum. Light saturation occurs at about 10% of full sunlight (so light is rarely limiting in nature), while CO₂ is usually the limiting factor in field conditions; C₃ plants respond strongly to CO₂ enrichment above current atmospheric levels, C₄ plants saturate early.

Respiration in Plants

Respiration releases the energy locked in photosynthates. Since plants lack specialised respiratory organs, every living cell handles its own gas exchange by diffusion through stomata and lenticels, aided by loosely packed tissue with abundant intercellular spaces.

The stages:

1. Glycolysis (EMP pathway) — in the cytoplasm, common to aerobes and anaerobes. One glucose → 2 pyruvate, with a net gain of 2 ATP (substrate-level phosphorylation) and 2 NADH, using no oxygen at any step — this is why glycolysis is common to both aerobic and anaerobic respiration.

2. Fermentation — under anaerobic conditions pyruvate is not oxidised further. In alcoholic fermentation (yeast) pyruvate is decarboxylated to acetaldehyde and then reduced to ethanol, regenerating NAD⁺; in lactic acid fermentation (some bacteria, and animal muscle under oxygen debt) pyruvate is directly reduced to lactic acid. Both pathways release only a fraction of the energy in glucose and accumulate toxic end products.

3. Aerobic respiration takes over inside the mitochondrion when oxygen is available. Pyruvate is transported into the matrix and oxidatively decarboxylated by the pyruvate dehydrogenase complex to acetyl-CoA, releasing CO₂ and generating one NADH per pyruvate (the link reaction). Acetyl-CoA then enters the Krebs (citric acid) cycle in the matrix: condensation with oxaloacetate gives citrate, and a series of oxidative steps regenerates oxaloacetate while releasing 2 CO₂, 3 NADH, 1 FADH₂ and 1 GTP (≈ATP) per acetyl-CoA, i.e. per turn of the cycle.

4. Electron transport chain and oxidative phosphorylation. NADH and FADH₂ from glycolysis, the link reaction and the Krebs cycle donate electrons to carriers in the inner mitochondrial membrane, pumping protons into the intermembrane space; electrons finally reduce O₂ to water, which is why aerobic respiration needs oxygen as the terminal electron acceptor. The proton gradient drives ATP synthase (chemiosmosis, the same principle as in photophosphorylation): each NADH oxidised yields about 3 ATP and each FADH₂ about 2 ATP in the classical textbook accounting. Totalling glycolysis, the link reaction, the Krebs cycle and the ETC for one glucose gives a widely quoted maximum of 36–38 ATP, the exact figure depending on the shuttle used to ferry cytoplasmic NADH into the mitochondrion.

The respiratory quotient (RQ) = volume of CO₂ evolved / volume of O₂ consumed, and it reveals the respiratory substrate:

• RQ = 1 for carbohydrates (fully oxidised, CO₂ and O₂ in equal volumes)
• RQ < 1 for fats and proteins (more O₂ needed relative to CO₂ released, since these substrates are more reduced)
• RQ > 1 for organic acids (already partly oxidised, so relatively less O₂ is needed)

Respiratory pathways are also amphibolic: intermediates of glycolysis and the Krebs cycle are drawn off for biosynthesis of fatty acids and amino acids, so respiration is not purely catabolic but also feeds anabolism.

Plant Growth, Development and Hormones

Plant growth is, in principle, indeterminate — meristems keep adding cells throughout life — and passes through three phases at the cellular level: the meristematic phase (small, thin-walled, densely protoplasmic, actively dividing cells at root/shoot tips), the elongation phase (increased vacuolation and cell size, just behind the meristem) and the maturation phase (cells attain final size, shape and wall thickening for a specific function). Growth rate can be arithmetic (constant absolute increase per unit time, a straight-line graph) or, much more commonly, geometrical/exponential (rate itself increases as cells accumulate, giving the characteristic sigmoid curve as resources eventually become limiting).

Five classical groups of plant growth regulators coordinate development, each discovered through a distinct experimental story:

• Auxins — discovered through the Went coleoptile assay following Darwin's phototropism experiments; promote cell elongation, apical dominance (suppressing lateral bud growth) and root initiation; used commercially to root cuttings, prevent fruit/leaf drop, and (as 2,4-D) as a selective weedicide against dicot weeds.
• Gibberellins (GA) — discovered from the fungus Gibberella fujikuroi, which causes the "foolish seedling" (bakanae) disease of rice by over-elongating the stem; promote stem elongation, cause bolting in rosette plants, and can induce parthenocarpic fruit and hasten seed germination by triggering α-amylase synthesis in cereal aleurone layers.
• Cytokinins — first isolated from coconut milk and later as kinetin from herring sperm DNA; promote cell division, delay leaf senescence, and, together with auxin, control the root/shoot balance in tissue culture (high auxin:cytokinin favours roots, the reverse favours shoots).
• Ethylene — the only gaseous hormone; promotes fruit ripening, senescence and abscission of leaves/flowers, and the "triple response" in seedlings; used commercially to ripen fruit and induce flowering in pineapple.
• Abscisic acid (ABA) — the general growth inhibitor and stress hormone; induces seed dormancy and closes stomata under water stress by promoting K⁺ efflux from guard cells.

Two environmentally cued responses complete the picture: photoperiodism, the flowering response to relative day/night length (short-day, long-day and day-neutral plants, mediated by the phytochrome pigment system, with the dark period being the truly critical interval); and vernalisation, the requirement of a period of low temperature to induce or hasten flowering, ensuring plants like winter wheat do not flower before winter has passed.

Common Mistakes and Exam Traps

1. RQ direction confusion. RQ is CO₂ evolved divided by O₂ consumed, not the reverse. RQ = 1 only for carbohydrate substrates; fats give RQ < 1 because they are more reduced and need extra oxygen to burn completely.
2. Water potential sign errors. Ψs and Ψp are signed quantities, not magnitudes — a common error is assuming water always moves toward "more concentrated" solutions without accounting for how Ψp (pressure potential) can reverse the expected direction.
3. Treating photorespiration as ordinary respiration. Photorespiration occurs only in the light, involves chloroplast–peroxisome–mitochondrion cooperation on the oxygenation product of RuBisCO, and releases CO₂ without producing any ATP — it is not a form of "night-time respiration".
4. Assigning growth roles to the wrong hormone. Auxin causes apical dominance, not ethylene; ABA is the stress/dormancy hormone, not gibberellin — mixing these up under exam pressure is extremely common.