Plant Biology
What this domain tests
Plant Biology tests photosynthesis, water and solute transport, hormones, development, and plant responses to the environment. Questions may combine a graph of photosynthesis versus light with stomatal behavior or compare C3, C4, and CAM pathways.
Core rules and concepts
- Light reactions occur in thylakoids, split water, produce O2, ATP, and NADPH; the Calvin cycle fixes CO2 in the stroma.
- Xylem moves water and minerals upward under tension through cohesion–tension; phloem transports sugars from source to sink.
- Stomata open when guard cells take up K+ and water; they close to conserve water, reducing CO2 uptake.
- C4 plants concentrate CO2 before the Calvin cycle; CAM plants separate carbon fixation temporally.
- Auxin promotes cell elongation at low concentrations but can inhibit at high concentrations; cytokinins promote cell division; gibberellins promote stem elongation.
- Phototropism and gravitropism are directional growth responses.
Worked example
A C3 plant is exposed to hot, dry conditions. Stomata close, so internal CO2 falls. Rubisco fixes O2 instead of CO2 in photorespiration, reducing net photosynthesis. Reasoning: closure saves water but limits carbon gain; C4 and CAM plants reduce photorespiration by concentrating CO2 or separating steps in time. The trade-off explains why C4 grasses dominate many hot, open habitats.
Traps that cost marks
Do not say xylem transports sugars; phloem does. Do not confuse the light reactions with the Calvin cycle: ATP and NADPH are made in the former and used in the latter. Do not assume all stomata close at night; CAM plants open them at night. Do not say auxin always promotes growth; concentration and tissue matter.