Living Systems Respond to Their Environment
Core concepts
Living systems maintain internal stability through homeostasis, using negative feedback loops: a sensor detects a change, a control centre compares it to a set point, and an effector counteracts the change. Understand positive feedback as the rarer amplifying loop (e.g., oxytocin in childbirth). Know how thermoregulation, osmoregulation, and blood glucose regulation (insulin and glucagon) illustrate these loops. Candidates must also understand neurons and synapses, hormones and their target cells, receptors (chemoreceptors, photoreceptors, thermoreceptors), and plant responses such as phototropism and geotropism. Immune response basics - first-line barriers, inflammation, and antibody production - appear in questions about responding to pathogens. Finally, understand adaptations and natural selection as long-term responses of populations to environmental pressure.
Traps to watch
- Confusing a response with a stimulus; the stimulus is the change, the response is the reaction.
- Assuming all feedback is negative - identify whether the output amplifies or reduces the original change.
- Mixing nervous (fast, short-lived, electrical plus neurotransmitter) with endocrine (slower, longer-lasting, hormonal) control.
- Forgetting that homeostasis maintains a range, not a fixed value.
- Mistaking an individual's acclimatisation for a population's evolutionary adaptation.
Worked example
A question describes body temperature rising above 37 °C. Reasoning: thermoreceptors detect the rise (stimulus); the hypothalamus acts as control centre; effectors include sweat glands (increase sweating) and skin arterioles (vasodilation). These responses increase heat loss, lowering temperature back toward the set point - a negative feedback loop. Rule of thumb: name the sensor, control centre, effector, and direction of correction before choosing an answer.