B3 Organism-level systems — OCR Gateway Biology (J257/01)
The nervous system
The nervous system allows rapid, short-lived, electrical responses to stimuli.
Key components:
- Receptors — detect stimuli (e.g. photoreceptors in retina, mechanoreceptors in skin, chemoreceptors in nose/tongue).
- Sensory neurones — carry impulses from receptor → CNS.
- CNS (brain + spinal cord) — coordinates a response; contains relay neurones.
- Motor neurones — carry impulses from CNS → effector.
- Effectors — muscles (contract) or glands (secrete hormones).
The reflex arc (involuntary, fast response): Stimulus → receptor → sensory neurone → relay neurone (in spinal cord) → motor neurone → effector. Example: touching a hot object → withdrawal reflex.
Reflexes bypass the brain (conscious thought), making them faster. They protect the body from harm.
Structure of a neurone:
- Cell body — contains nucleus.
- Dendrites — receive signals from other neurones.
- Axon — long fibre conducting impulse away from cell body.
- Myelin sheath — fatty insulating layer produced by Schwann cells; speeds up conduction (saltatory conduction).
- Synaptic terminals — ends of axon that release neurotransmitters.
Synapses:
- Gap between two neurones where one neurone's axon terminal meets another's dendrite.
- Electrical impulse arrives → vesicles release neurotransmitters (e.g. acetylcholine) into the synaptic cleft.
- Neurotransmitter diffuses across → binds to receptors on post-synaptic membrane → triggers new electrical impulse.
- Neurotransmitter is then broken down by enzymes (acetylcholinesterase) and the precursors are recycled.
The endocrine system — hormones
Hormones are chemical messengers secreted by endocrine glands directly into the bloodstream. They act on target organs and produce slower, longer-lasting effects than nerve impulses.
Key hormones:
| Hormone | Secreted by | Target | Effect |
|---|---|---|---|
| Insulin | Pancreas (β-cells) | Liver, muscle | Lowers blood glucose; stimulates glycogen storage |
| Glucagon | Pancreas (α-cells) | Liver | Raises blood glucose; stimulates glycogenolysis |
| Adrenaline | Adrenal glands | Multiple | Fight or flight; increases HR, blood glucose |
| Oestrogen | Ovaries | Multiple | Female secondary sex characteristics; uterine lining |
| Testosterone | Testes | Multiple | Male secondary sex characteristics |
| ADH (antidiuretic hormone) | Pituitary gland | Kidney tubules | Increases water reabsorption → concentrated urine |
| FSH / LH | Pituitary gland | Ovaries/testes | Control gamete production and sex hormone release |
Homeostasis
Definition: The maintenance of a stable internal environment within narrow limits, allowing enzyme-catalysed reactions to occur at optimum rates.
Homeostasis uses negative feedback loops:
Change detected → receptor → control centre (often brain/hypothalamus) → effector → corrective response → receptor detects return to normal → effector switched off.
Blood glucose regulation
Normal range: ~4–7 mmol/L.
When blood glucose rises (after a meal):
- Pancreatic β-cells detect high glucose.
- Insulin secreted into blood.
- Insulin causes liver and muscle cells to take up glucose and convert it to glycogen (glycogenesis).
- Blood glucose falls back to normal → insulin secretion decreases (negative feedback).
When blood glucose falls (after exercise/fasting):
- Pancreatic α-cells detect low glucose.
- Glucagon secreted.
- Glucagon stimulates liver to convert glycogen → glucose (glycogenolysis).
- Blood glucose rises → glucagon secretion decreases.
Diabetes mellitus:
- Type 1: autoimmune destruction of β-cells → no insulin produced. Treated with insulin injections.
- Type 2: cells become resistant to insulin (often linked to obesity). Managed by diet, exercise, metformin; sometimes insulin.
Water balance (osmoregulation)
The kidneys maintain water balance. The key hormone is ADH (produced by hypothalamus, released by pituitary):
- Blood too concentrated (dehydrated) → hypothalamus detects → more ADH released → kidney tubules more permeable → more water reabsorbed → small volume, concentrated urine.
- Blood too dilute (over-hydrated) → less ADH released → kidney tubules less permeable → less water reabsorbed → large volume, dilute urine.
Temperature regulation (thermoregulation)
Hypothalamus acts as the body's thermostat. Normal core temperature ~37°C (enzymes work at optimum).
Too hot:
- Sweat glands produce sweat → evaporates → latent heat removed from skin.
- Vasodilation: arterioles near skin surface widen → more blood near surface → more heat radiated.
- Hairs lie flat (less trapped air insulation).
Too cold:
- Sweat production stops.
- Vasoconstriction: arterioles narrow → less blood near surface → less heat radiated.
- Shivering: skeletal muscles contract rapidly → generate heat by respiration.
- Hairs erect (piloerection) → trap air as insulation.
Common OCR examiner traps
- Negative feedback does NOT mean "bad" — it means the response opposes the original change (restores equilibrium).
- Insulin lowers glucose; glucagon raises it. Students often confuse the two.
- Neurones vs hormones: nerve impulses are electrical, fast, short-lived, specific pathway. Hormones are chemical, slow, long-lasting, travel in blood to all organs but only affect target organs.
- Sweating cools by evaporation — state "evaporation" not just "sweat is produced."
- ADH increases permeability (not rate of filtration) of collecting duct and loop of Henle.
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