Lesson 10: Biology: Plant Physiology and Reproduction

Plant physiology and plant reproduction reward the same habit as human physiology: learn the sequence, then the control. Photosynthesis, respiration, transport and growth are sequences of steps; hormones and environmental cues control when each step runs.

This lesson covers photosynthesis, respiration, transport of water and food, plant growth regulators, and sexual reproduction in flowering plants, including double fertilisation and seed formation. NCERT's diagrams — the chloroplast, the flower's parts, the embryo sac — are as examinable as its text.

For numerical questions, the two formulas to keep ready are the water-potential sum (Ψ = Ψs + Ψp) and the photosynthesis equation (6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O), which also fixes the stoichiometry of gas exchange.

Photosynthesis, respiration and transport

Photosynthesis has a light reaction (in thylakoids: photolysis of water releases oxygen, and ATP and NADPH are formed) and a dark reaction (in the stroma: the Calvin cycle fixes CO₂ into sugar using that ATP and NADPH). C₄ plants add a first fixation step in mesophyll cells (forming oxaloacetic acid) before the Calvin cycle in bundle-sheath cells, which reduces photorespiration in hot, bright conditions.

Respiration reverses the flow: glycolysis in the cytoplasm nets 2 ATP per glucose, and the Krebs cycle plus the electron transport chain in mitochondria complete oxidation, giving a much larger total in the standard textbook account. The respiratory quotient (CO₂ released / O₂ consumed) is 1 for carbohydrates, less than 1 for fats, and more than 1 for organic acids.

Transport: water rises through xylem by transpiration pull, cohesion and adhesion; food moves through phloem by pressure flow from source to sink. Transpiration is fastest with high temperature, low humidity, bright light and wind — and stomata close in darkness and under water stress, under the control of abscisic acid.

Plant growth regulators

Auxins promote cell elongation, apical dominance and root initiation; gibberellins promote stem elongation, seed germination and bolting. Cytokinins promote cell division and delay leaf senescence. Abscisic acid promotes dormancy and stomatal closure under stress; ethylene promotes fruit ripening and senescence.

A quick way to keep them straight: auxins and gibberellins mainly promote growth, cytokinins promote division, and abscisic acid mainly inhibits or protects. Ethylene is the odd one out as a gas. Questions often pair a regulator with its effect, so learn them as pairs, not as a list.

Sexual reproduction in flowering plants

The flower's male part (stamen: anther + filament) produces pollen grains, each containing a vegetative cell and a generative cell. The female part (pistil: stigma, style, ovary) contains ovules, each with an embryo sac holding an egg cell, two synergids, three antipodal cells and a central cell with two polar nuclei.

Double fertilisation is unique to flowering plants: one male gamete fuses with the egg to form the diploid zygote, and the second fuses with the central cell's two polar nuclei to form the triploid primary endosperm cell, which develops into endosperm that nourishes the embryo. After fertilisation the ovule becomes the seed and the ovary becomes the fruit; the zygote becomes the embryo.

Pollination may be by wind (light, non-sticky pollen, feathery stigmas), water (as in Vallisneria and Zostera), insects (bright, scented flowers with nectar) or animals. Self-pollination preserves parental characters but reduces variation; cross-pollination increases variation but needs a pollinating agent.

Common pitfalls