IP Biology Notes: Homeostasis, Coordination, and Response (Upper Sec 08)
26 Nov 2025, 00:00 Z
Reviewed by
Ezekiel Tan·Academic Advisor (Biology)
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Use this as a free IP Biology notes chapter on homeostasis, coordination, and response for Year 3 to Year 4. It keeps the IP pacing while reinforcing the 6093 biology foundations most schools test through DBQs, diagrams, and practical explanations.
Status: SEAB O-Level Biology 6093 syllabus (exams from 2026) checked 2025-11-30 - scope unchanged; remains the reference for this note.
What you must know
- Homeostasis keeps internal environment constant; change detected by receptors → effectors act → negative feedback reverses change.
- Thermoregulation: hypothalamus as controller; responses include vasodilation/vasoconstriction of skin arterioles, sweating vs shivering, hairs lie flat vs stand, behavioural changes.
- Hormones: insulin from pancreas beta cells lowers blood glucose (uptake, glycogen storage); glucagon from alpha cells raises it (glycogen breakdown). Type 2 diabetes risk factors: diet, inactivity, obesity. ADH from pituitary increases water reabsorption in collecting duct.
- Nervous system: CNS (brain, spinal cord) + peripheral nerves; reflex arc = receptor → sensory neurone → relay neurone → motor neurone → effector (fast, involuntary).
- Eye: parts (cornea, lens, iris, retina, optic nerve), focusing (ciliary muscle, suspensory ligaments), pupil reflex (bright light constricts, dim dilates).
Detailed notes
- Negative feedback: change → receptor → control centre → effector → response reverses change (temperature, glucose, water).
- Nervous vs hormonal control: nervous is electrical/chemical along neurones, rapid, short-lived, specific; hormones travel in blood, slower onset, longer-lasting, broader.
- Thermoregulation: too hot → vasodilation, sweating, hairs flat, behavioural cooling; too cold → vasoconstriction, shivering, hairs erect (minor in humans), behavioural warming.
- Blood glucose (qualitative IP): insulin promotes uptake/storage as glycogen; glucagon promotes glycogen breakdown/release. Type 2 diabetes risk factors: high sugar/fat diet, inactivity, obesity.
- Eye accommodation: near-ciliary contract, suspensory ligaments slack, lens thicker; far-ciliary relax, ligaments taut, lens thinner. Pupil reflex: bright light → circular muscles contract, radial relax (constrict); dim light opposite.
- ADH: low blood water → more ADH → collecting ducts more permeable → more water reabsorbed → concentrated urine; high blood water → less ADH → dilute urine.
Worked walkthroughs
- Reflex arc: pinprick → receptor → sensory neurone → relay in spinal cord → motor neurone → effector muscle contracts; speed because it bypasses conscious processing.
- Thermoregulation scenario: stepping from air-con into sun-describe vasodilation and sweating and how they reduce body temperature.
