Botany: Photosynthesis, Water Relations, and Ratios
Botany in NEET UG covers plant diversity, morphology, anatomy, cell biology, photosynthesis, respiration, plant growth and ecology. Questions often combine a diagram-free description with a physiological calculation or a genetic ratio.
Core rules
- Photosynthesis: light reactions occur in thylakoid membranes and produce ATP, NADPH and O2 from water; the Calvin cycle occurs in the stroma. RuBisCO fixes CO2 in C3 plants; C4 plants use PEP carboxylase and Kranz anatomy, with oxaloacetate as an early 4-carbon product.
- Water potential: Ψ = Ψs + Ψp. Water moves from higher water potential to lower water potential.
- Transpiration pull depends on cohesion and adhesion in xylem.
- Hormones: auxin controls apical dominance, gibberellin promotes elongation, cytokinin promotes cell division, ABA closes stomata under stress, and ethylene ripens fruit.
- In a monohybrid cross Tt × Tt, genotypic ratio is 1 TT : 2 Tt : 1 tt and phenotypic ratio is 3 tall : 1 dwarf.
Worked example
A plant cell has solute potential -0.8 MPa and pressure potential +0.3 MPa. Its water potential is Ψ = -0.8 + 0.3 = -0.5 MPa. Pure water has Ψ = 0 MPa, so water enters the cell. If a neighbouring cell has Ψ = -0.6 MPa, water moves from the -0.5 MPa cell to the -0.6 MPa cell.
Traps
Calling malate the first stable C4 product instead of oxaloacetate; reversing the water-potential direction; confusing genotypic and phenotypic ratios; assuming ABA promotes growth; forgetting that the Calvin cycle occurs in the stroma.