Plant Biology: Structure, Nutrition, Growth and Reproduction
Plant questions test the transport system, photosynthesis as a process with limiting factors, how growth happens at meristems, and the flower-to-fruit sequence of sexual reproduction.
Transport
Root hair cells absorb water by osmosis and mineral ions by active transport. Xylem carries water and dissolved minerals upward only, driven by transpiration: water evaporates from mesophyll cells, creating tension that pulls the water column up. Phloem carries dissolved sugars from leaves to wherever they are needed, in either direction. Transpiration rate rises with light, warmth, wind and low humidity.
Leaf and photosynthesis
Leaf: waxy cuticle, upper epidermis, palisade mesophyll (most chloroplasts), spongy mesophyll with air spaces, lower epidermis with stomata and guard cells. Photosynthesis: carbon dioxide + water → glucose + oxygen, using light energy and chlorophyll. Light, CO₂ concentration and temperature are limiting factors; the slowest one sets the rate.
Growth and reproduction
Growth occurs at meristems - root and shoot tips - by cell division and elongation; woody plants also thicken by secondary growth. Flower parts: sepals, petals, stamens (anther, filament), carpel (stigma, style, ovary). Pollination is transfer of pollen to the stigma; after fertilisation the ovule becomes the seed and the ovary the fruit. Asexual reproduction uses runners, tubers, bulbs and cuttings, giving genetically identical offspring.
Worked example
A variegated leaf (green centre, white edge) is destarched in the dark, partly covered, exposed to light, then tested with iodine. Only the green, illuminated region turns blue-black. Reasoning: iodine detects starch; starch appears only where photosynthesis occurred; the white edge has no chlorophyll, the covered part has no light. Conclusion - both light and chlorophyll are needed. Naming the control (the covered part) is part of the answer.
Traps
Reversing xylem and phloem; saying xylem carries sugars; writing that respiration and photosynthesis are the same reaction; forgetting minerals travel in the xylem as ions, not as sugars; confusing pollination with fertilisation; forgetting seeds need dispersal away from the parent.