Plant Biology and Physiology
What this domain tests
Plant Biology and Physiology covers plant structure, transport, photosynthesis, gas exchange, hormones, and responses to the environment. Exam items often ask you to connect leaf anatomy with water movement and carbon gain.
Core rules and concepts
- Xylem transports water and dissolved minerals upward, driven largely by transpiration pull and cohesion–tension. Phloem transports sugars from sources to sinks; direction depends on the location of production and use.
- Stomata open when guard cells become turgid and close when they lose water. Opening admits CO2 for photosynthesis but increases water loss, creating a trade-off.
- Light-dependent reactions occur in thylakoid membranes and produce ATP and NADPH; the Calvin cycle occurs in the stroma and fixes CO2 using RuBP carboxylase.
- Auxin promotes cell elongation and contributes to phototropism; gibberellins promote stem elongation and seed germination; abscisic acid promotes stomatal closure and seed dormancy.
Worked example
A leaf absorbs 0.60 mmol CO2 m−2 s−1 and its mitochondria release 0.10 mmol CO2 m−2 s−1. Net photosynthesis = 0.60 − 0.10 = 0.50 mmol CO2 m−2 s−1. Gross photosynthesis = 0.60 mmol CO2 m−2 s−1. Reasoning: measured CO2 uptake is net; respiration adds CO2 back, so gross carbon fixation equals net uptake plus respiratory release.
Traps
Do not say plants “breathe” through stomata; they exchange gases. Do not reverse xylem and phloem directions. In photosynthesis graphs, the light compensation point is where net CO2 uptake is zero, not where gross photosynthesis is zero. Remember that stomatal closure saves water but can limit photosynthesis.