Homeostasis and Excretion
Negative feedback, blood glucose, temperature and water balance, kidney function and interpretation of disturbances such as diabetes.
How to study A-level Biology
Move between scales deliberately: identify the biological process, trace matter or information, use quantitative evidence, then evaluate the method and conclusion.
Core concepts
Concept 1
Homeostasis maintains internal variables within tolerable limits through receptors, coordination centres, effectors and feedback.
Exam cue: Name the deviation, receptor, coordinator, effector and response when explaining feedback.
Concept 2
Insulin and glucagon coordinate blood-glucose control through effects on uptake, storage and mobilisation.
Exam cue: Trace a substance through the glomerulus, nephron and blood, stating why it moves at each step.
Concept 3
Ultrafiltration, selective reabsorption and hormone-regulated water permeability allow kidneys to control excretion and water balance.
Exam cue: Use data to distinguish a normal regulatory response from failure of production, reception or organ function.
Risk pitfalls and guardrails
Calling homeostasis maintenance of an absolutely constant value.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Saying insulin converts glucose directly into energy.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Confusing ultrafiltration with selective reabsorption.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Memory anchors
Negative Feedback
A deviation triggers responses that oppose the original change.
Insulin
Insulin lowers blood glucose by promoting uptake and storage and reducing release.
Glucagon
Glucagon raises blood glucose by promoting glycogen breakdown and glucose production.
Ultrafiltration
High hydrostatic pressure forces small molecules from glomerular blood into Bowman's capsule.
ADH
ADH increases collecting-duct water permeability and therefore water reabsorption.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
A mammal keeps blood glucose and core temperature within tolerable ranges despite disturbance. Which process is illustrated?
How does negative feedback stabilise a variable?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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