Organismal Biology and Ecology
Organismal Biology and Ecology
This domain tests how whole organisms maintain homeostasis and how populations and ecosystems function. Expect questions on plant and animal transport, feedback, energy flow, population growth, species interactions, and nutrient cycles.
Core rules and concepts
- Homeostasis uses negative feedback: a deviation triggers a response that reduces it. Positive feedback amplifies a change, as in oxytocin during childbirth.
- In plants, xylem transports water and minerals upward, driven by transpiration; phloem transports sugars from sources to sinks. Stomata regulate gas exchange and water loss.
- In mammals, countercurrent exchange in the loop of Henle concentrates urine; surface area and concentration gradients govern diffusion and exchange.
- Ecology: energy flows and matter cycles. Gross primary productivity is total carbon fixed; net primary productivity is gross minus respiration. Only about 10% of energy transfers between trophic levels.
- Population growth: exponential growth occurs under unlimited resources; logistic growth includes carrying capacity (K). Mark–recapture estimates population size as (marked in first sample × total in second sample) / marked recaptures.
Worked example
A lake's producers fix 20,000 kcal of energy per year. Using the 10% rule, primary consumers receive about 2,000 kcal, secondary consumers 200 kcal, and tertiary consumers 20 kcal. The remaining energy is lost mainly as heat through respiration and in waste. Therefore a food chain rarely supports more than four or five trophic levels, and top predators require large areas to find enough energy.
Traps
Confusing gross and net primary productivity; reversing source and sink in phloem; assuming energy cycles rather than flows; using the 10% rule as exact; ignoring that logistic growth slows as N approaches K; confusing commensalism with mutualism; treating negative feedback as "bad."