Physiology of Plants: Transport, Photosynthesis and Control
Physiology of Plants examines how vascular plants obtain water and minerals, move assimilates, fix carbon and coordinate growth. SPM questions often combine structure with function: root hair cells, xylem, phloem, stomata and photosynthetic pigments.
Transport up the stem
- Water enters root hair cells by osmosis; mineral ions by active transport, lowering water potential.
- Xylem: dead, lignified, hollow vessels/tracheids; adhesion, cohesion and tension pull water column; root pressure contributes.
- Transpiration pull driven by evaporation from spongy mesophyll through stomata; guard cells open when turgid.
- Phloem: living sieve tubes and companion cells; translocation uses pressure flow, loading sucrose at source and unloading at sink.
Photosynthesis and growth
- Light-dependent reactions in thylakoid membranes produce ATP and NADPH; light-independent Calvin cycle in stroma fixes CO2 using rubisco.
- Limiting factors: light intensity, CO2 concentration, temperature.
- Plant hormones: auxin causes cell elongation and apical dominance; gibberellin promotes stem elongation and seed germination; cytokinin promotes cell division; abscisic acid closes stomata and promotes dormancy; ethylene ripens fruit and promotes abscission.
Worked example: A potometer shows a shoot taking up 3.6 cm3 water in 6 minutes. Calculate rate per hour. 3.6/6=0.6 cm3 min-1; ×60=36 cm3 h-1. Reasoning: potometer measures water uptake, an estimate of transpiration; bubbles and leaks affect validity. If light intensity doubles and rate plateaus, CO2 or another factor is limiting.
Traps: xylem transports water and minerals, phloem transports sucrose and amino acids; transpiration is not “sucking” by roots alone; stomata close to conserve water but also restrict CO2; translocation is not transpiration.