Plant Biology
What you must know
Leaf structure in relation to gas exchange and photosynthesis: palisade mesophyll, spongy mesophyll with air spaces, stomata and guard cells, and vascular bundles containing xylem and phloem. Photosynthesis: word and balanced equation, chloroplast adaptations, light-dependent and light-independent stages at a conceptual level, and limiting factors (light intensity, carbon dioxide concentration, temperature) interpreted from graphs. Transport: transpiration pull and cohesion-tension in xylem, carrying water and mineral ions upward only; translocation in phloem, carrying sucrose and amino acids in either direction. Root hair cells and active uptake of mineral ions such as nitrate and magnesium. Plant responses: auxin, phototropism and gravitropism. Reproduction: flower parts, pollination, fertilisation, seed and fruit formation, and the conditions needed for germination. Adaptations of xerophytes and hydrophytes.
Traps and a worked example
Traps: reversing xylem and phloem; forgetting that transpiration is fastest in warm, dry, windy and bright conditions; muddling mineral deficiencies (magnesium gives yellowing because chlorophyll cannot be made; nitrate gives stunted growth and pale leaves); saying that stomata breathe.
Worked example: a graph of photosynthesis rate against light intensity rises and then plateaus. Where the line is flat, light is no longer the limiting factor - carbon dioxide concentration or temperature is now limiting. Rule of thumb: the limiting factor is whichever resource is furthest from its optimum, so read the plateau, not the slope.