Homeostasis & Response

• Homeostasis regulates internal conditions to maintain optimum conditions for enzyme action and cell function.
• In humans it controls blood glucose concentration, body temperature and water levels despite internal and external changes.
• A receptor detects a stimulus; a coordination centre processes information; an effector produces the response.
• The brain, spinal cord and pancreas can act as coordination centres, while muscles and glands are effectors.

The punchline: Write the causal chain: change → receptor → coordination centre → effector → correction.

• Receptors send electrical impulses along sensory neurones to the central nervous system: the brain and spinal cord.
• The CNS coordinates a response, and motor neurones carry impulses to muscle or gland effectors.
• At a synapse, a neurotransmitter diffuses across the gap and triggers an impulse in the next neurone.
• Nervous responses are usually rapid, short-lived and directed at particular effectors.

The punchline: Impulses travel along neurones; neurotransmitters cross synapses.

• The route is stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector.
• The relay neurone lies in the CNS and links the sensory and motor routes across synapses.
• The conscious brain does not coordinate the response first, so action is faster and the chance of injury can fall.

The punchline: For a reflex, include the relay neurone and call the response rapid and automatic.

• The cerebral cortex supports consciousness, intelligence, memory and language.
• The cerebellum coordinates muscular activity and balance; the medulla controls unconscious actions such as heart and breathing rates.
• Higher Tier: functions are mapped using evidence from brain damage, electrical stimulation and MRI scans.
• Higher Tier: investigation and treatment can be difficult and risky because damage may affect interconnected functions.

The punchline: Name the region, state its function and label mapping methods Higher Tier.

• The cornea and lens refract light; the sclera protects; retinal receptors detect light intensity and colour; the optic nerve carries impulses.
• For a near object, ciliary muscles contract, suspensory ligaments loosen and the lens becomes thicker, refracting light more strongly.
• For a distant object, ciliary muscles relax, ligaments tighten and the lens becomes thinner, refracting light less strongly.
• Interpret ray diagrams: myopia focuses before the retina and uses a concave lens; hyperopia focuses behind it and uses a convex lens.

The punchline: Accommodation changes focus; the pupil reflex changes how much light enters.

• When too hot, skin blood vessels dilate and sweat glands produce sweat.
• When too cold, skin blood vessels constrict, sweating stops and skeletal muscles contract in shivering.
• Higher Tier: sweat evaporation transfers energy away, while vasodilation brings more warm blood near the skin surface.
• Higher Tier: vasoconstriction reduces energy transfer from skin, while shivering increases respiration and releases energy.

The punchline: Foundation: name the responses. Higher Tier: explain them through blood flow, evaporation, respiration or energy transfer.

• Hormones travel in the bloodstream to target organs, where they produce an effect.
• The pituitary is the master gland in the brain; several of its hormones stimulate other endocrine glands.
• Know the positions of the pituitary, thyroid, pancreas, adrenal glands, ovaries and testes.
• Compared with nervous responses, hormonal responses are usually slower and longer-lasting.

The punchline: Nerves use impulses along neurones; endocrine glands release hormones into blood.

• When blood glucose is too high, insulin causes glucose to move from blood into cells; liver and muscle cells store excess as glycogen.
• Higher Tier: when glucose is too low, glucagon causes glycogen to be converted into glucose and released into blood.
• Insulin and glucagon have opposing effects that return glucose concentration towards normal by negative feedback.

The punchline: Insulin lowers blood glucose; Higher Tier glucagon raises it.

• In Type 1 diabetes, the pancreas produces insufficient insulin; insulin injections are the usual treatment.
• In Type 2 diabetes, body cells no longer respond properly to insulin; obesity is a risk factor, not a guaranteed cause.
• Type 2 diabetes is commonly managed using a carbohydrate-controlled diet and exercise regime.
• When comparing treatments, link each one to the specific control failure.

The punchline: Do not blame insulin production for every case: Type 2 is mainly a response problem.

• Water leaves in exhaled air; water, ions and urea leave in sweat. These losses are not controlled.
• Kidneys filter blood, then reabsorb all glucose, some ions and the required amount of water; excess water, ions and urea form urine.
• Higher Tier: the liver deaminates excess amino acids, producing toxic ammonia that is converted to urea for excretion.
• Cells function poorly if they gain or lose too much water by osmosis, so body-water level must be controlled.

The punchline: Filtration removes small molecules from blood; selective reabsorption retrieves useful ones.

• Higher Tier: concentrated blood causes more ADH release from the pituitary, making kidney tubules more permeable so more water is reabsorbed.
• Across a partially permeable dialysis membrane, urea and excess ions diffuse out, excess water moves by osmosis and useful concentrations are maintained.
• Dialysis needs regular sessions but no donor; a transplant may restore long-term function but needs a donor and can be rejected.

The punchline: Evaluate kidney treatments using mechanism, time, donor need and rejection risk.

• FSH from the pituitary matures an egg; LH triggers ovulation, usually near day 14 of an approximately 28-day cycle.
• Oestrogen from the ovaries builds the uterus lining; progesterone maintains it.
• Higher Tier: FSH stimulates oestrogen; oestrogen inhibits FSH and stimulates LH; progesterone inhibits FSH and LH.
• At puberty, reproductive hormones cause secondary sexual characteristics; oestrogen is the main female hormone, while testicular testosterone stimulates sperm production.

The punchline: Link each hormone to its source and effect before explaining interactions.

• Pills inhibit FSH so eggs do not mature; slow-release progesterone from an injection, implant or skin patch inhibits egg maturation and release.
• Non-hormonal methods include condoms, diaphragms, spermicides, avoiding intercourse when an egg may be present and sterilisation.
• An intrauterine device may prevent implantation or release a hormone; barrier methods can also reduce transmission of sexually transmitted infections.
• Evaluate methods for the stated person and context rather than reciting a generic advantage list.

The punchline: Compare mechanism, reliability, reversibility, side effects and STI protection.

• FSH and LH can mature and release an egg when a woman's own FSH level is too low.
• In IVF, FSH and LH mature several eggs; eggs are collected, fertilised in a laboratory and one or two embryos are placed in the uterus.
• IVF can help people have a child, but success rates are not high and treatment can be physically and emotionally stressful.
• Implanting multiple embryos can increase the chance of multiple births, which carry health risks.

The punchline: For IVF, give the sequence and evaluate both benefit and risk.

• Adrenal glands release adrenaline during fear or stress, increasing heart rate and delivery of oxygen and glucose to brain and muscles.
• The thyroid releases thyroxine, which controls basal metabolic rate and supports growth and development.
• Rising thyroxine inhibits pituitary TSH release, so less thyroxine is produced; this is negative feedback.

The punchline: Name the gland, target effect and feedback direction.

• Auxin controls growth at shoot and root tips and becomes unevenly distributed in response to light or gravity.
• In shoots, more auxin stimulates cell elongation; shaded-side growth bends a shoot towards light.
• In roots, more auxin inhibits elongation; slower growth on the lower side bends a root downwards.
• Shoots are positively phototropic and negatively gravitropic; roots are positively gravitropic and negatively phototropic.

The punchline: State where auxin gathers, how growth changes and which way the organ bends.

• Auxins are used in selective weedkillers, rooting powders and tissue culture.
• Ethene controls cell division and fruit ripening; the food industry uses it to manage ripening during storage and transport.
• Gibberellins end seed dormancy, promote flowering and increase fruit size.

The punchline: Match the hormone to the commercial outcome; do not swap ethene and gibberellins.