Plant Biology: Structure, Transport, Responses and Reproduction
What this domain tests
Plant biology questions cover structure, transport, responses and reproduction. They test whether you can link leaf and root structure to function and explain how transpiration is affected by the environment.
Structure and transport
- Root hairs increase surface area for water and mineral uptake by osmosis and active transport.
- Xylem transports water and minerals upwards; vessels are dead, lignified and continuous. Phloem transports sucrose and amino acids in sieve tubes with companion cells.
- Transpiration is water loss as vapour, mainly through stomata. It is driven by evaporation and cohesion–tension, helped by root pressure.
- Transpiration increases with light, temperature and wind and with low humidity; guard cells become turgid to open stomata.
Responses and reproduction
- Phototropism: shoots bend towards light; geotropism: roots grow downwards. Auxin accumulates on the shaded or lower side, promoting cell elongation there.
- Apical dominance is caused by auxin from the shoot tip suppressing lateral buds.
- Asexual reproduction includes vegetative propagation; sexual reproduction involves flowers, pollination, fertilisation and seed formation.
Worked example
A potometer bubble moved 30 mm in 5 minutes in bright light and 10 mm in dim light. Rate in bright light = 30 ÷ 5 = 6 mm/min; in dim light = 2 mm/min. Reasoning: light opens stomata for CO₂ uptake and warms the leaf, both increasing evaporation. Increasing wind would also raise the rate by removing the humid layer around the leaf.
Traps
Saying xylem carries food; saying phloem carries water; describing transpiration as 'breathing'; forgetting that stomata also allow gas exchange; claiming auxin always promotes growth regardless of concentration.