IP Biology Upper Sec 08: Homeostasis, Co-ordination and Response

These notes align with SEAB GCE O-Level Biology (6093) content used in IP programmes (exams from 2026).

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.
• Accommodation: compare muscle/ligament/lens changes for reading a book vs looking at a distant sign.