Organisms and Populations
Core concepts
Organisms maintain homeostasis, keeping internal conditions stable through feedback. Feedback loops can be negative (correcting a deviation) or positive (amplifying it). Body systems include circulatory, respiratory, digestive, nervous, and endocrine systems, each with specialized organs. Plants transport water through xylem and sugars through phloem, and open stomata for gas exchange. Population size changes through births, deaths, immigration, and emigration. Exponential growth occurs with unlimited resources; logistic growth levels off at carrying capacity. Limiting factors such as food, water, and space cap population size. Population density and distribution describe how individuals are spread.
Common traps
Questions often swap xylem and phloem or confuse negative feedback with positive feedback. Do not assume logistic growth means a population stops growing forever; it stabilizes near carrying capacity. Distinguish a population from a community (multiple species). Remember that immigration adds individuals and emigration removes them.
Worked example
A population of 200 rabbits grows by 40 births and 10 deaths, with 5 immigrants and 5 emigrants. Change = 40 - 10 + 5 - 5 = 30, so the new size is 230. Rule of thumb: population change equals births minus deaths plus immigration minus emigration.