Organismal Biology and Ecology
Domain 3 - Organismal Biology and Ecology covers structure–function relationships in plants and animals, homeostatic control, and the population and community principles that explain distribution and abundance.
Plant systems
- Xylem contains dead tracheids and vessel elements; transpiration–cohesion–tension pulls water upward, with root pressure as a minor contributor.
- Phloem sieve tube elements, assisted by companion cells, move sugars from source to sink.
- Guard cells open stomata when turgid after K+ uptake. Hormones: auxin (apical dominance), gibberellins (stem elongation), cytokinins (cell division), ABA (stress and dormancy), ethylene (ripening).
Animal systems
- Homeostasis runs mainly on negative feedback: insulin lowers blood glucose, glucagon raises it, and thermoregulatory reflexes oppose change.
- Action potential: threshold → Na+ influx (depolarization) → K+ efflux (repolarization) → Na+/K+ pump restores gradients.
- Immunity: innate barriers and inflammation plus adaptive B cells (antibody-mediated) and T cells (cell-mediated).
Ecology
- Exponential growth occurs under unlimited resources; logistic growth slows toward carrying capacity K.
- Interactions include competition, predation, mutualism, commensalism, and parasitism; keystone species exert effects disproportionate to their abundance.
- Energy flows one way and roughly 90% is lost at each trophic level; matter cycles through carbon, nitrogen (fixation by Rhizobium and cyanobacteria, denitrification), and other biogeochemical pathways.
Worked example
Producers in a marsh fix 250,000 kcal/m²/yr. With about 10% transfer, primary consumers receive 25,000, secondary consumers 2,500, and tertiary consumers 250 kcal/m²/yr - too little to support many large predators. Apply the 10% rule successively at each step, not once overall.
Traps that cost marks
- Inverting the energy pyramid: numbers and biomass can invert, energy never does.
- Saying transpiration moves sugars, or that phloem flows only downward.
- Naming K+ as the depolarizing current; it is Na+ influx.
- Calling succession after a fire primary - soil remains, so it is secondary.