GCSE Biology Brain, eye & heat revision

The brain is the coordination centre of the nervous system, and the GCSE splits it into three regions worth memorising. The cerebral cortex is the wrinkled outer layer: consciousness, intelligence, memory and language live here. The cerebellum, tucked under the back, handles muscle coordination and balance — every smooth step is cerebellum at work. The medulla, at the base, runs the unconscious jobs you never think about: breathing and heart rate.

This precious equipment is well guarded: the skull surrounds the brain and the vertebral column surrounds the spinal cord, with protective membranes and fluid cushioning both against knocks.

Unlike the heart or skin, the brain cannot be prodded safely — damage is often permanent, and the skull that protects it also blocks access. Scientists have three main routes. First, patients with brain damage: mapping lost abilities to damaged regions reveals what each region does. Second, electrical stimulation of exposed brain tissue during surgery, observing which sensation or movement each zap provokes. Third, MRI scanning, which images brain activity without cutting anything.

Each method has limits: brain-damage patients are rare and every injury is unique; stimulation is invasive and risky; scans show activity but not always its meaning. Treatments are equally hard — drugs struggle to reach the brain across its protective barriers, and surgery risks destroying precious tissue. That is why progress in treating brain disease and damage is slow, a point the exam asks you to explain.

Light enters the eye and is focused onto a carpet of receptors. The journey: through the transparent, curved cornea (which does most of the light bending), through the pupil — its size set by the iris, adjusting for bright or dim light — then through the lens, which fine-tunes the focus, onto the retina, where light-sensitive receptors fire. Impulses leave along the optic nerve to the brain. The whole ball is protected by the tough white sclera.

Accommodation is the lens changing shape. For distant objects, the incoming rays are nearly parallel: ciliary muscles relax, the suspensory ligaments are pulled tight, and the lens is stretched thin — light is refracted less, focusing on the retina. For near objects the rays diverge: ciliary muscles contract, the ligaments slacken, and the elastic lens springs thick — more refraction, focused again on the retina.

Myopia — short sight. Distant objects look blurry because the rays are focused in front of the retina: the lens is too strong or the eyeball too long. The fix is a diverging (concave) lens, which spreads the rays slightly before the eye, so focus lands back on the retina.

Hyperopia — long sight. Near objects blur because rays are focused behind the retina: the lens is too weak or the eyeball too short. The fix is a converging (convex) lens, which pre-bends the rays inward so they meet on the retina.

Beyond glasses, three treatments exist: hard contact lenses sitting on the eye (same optics, invisible); laser surgery, reshaping the cornea so it refracts correctly without any external lens; and replacement lenses, where the natural lens is swapped for an artificial one. Each carries risks — infection, over- or under-correction — which evaluation answers should weigh.

Enzymes demand a core temperature near 37 °C. The thermoregulatory centre in the brain monitors the temperature of blood flowing through it, while receptors in the skin report the outside temperature — the brain compares both and commands effectors.

1. 1Too hot

   Vasodilation — blood vessels in the skin widen, so more blood flows near the surface and radiates heat; sweat glands pour out sweat, and its evaporation transfers energy away from the skin; hairs lie flat so the insulating air layer is minimised.

2. 2Too cold

   Vasoconstriction — skin vessels narrow, keeping warm blood away from the surface; sweating stops; hairs are pulled erect to trap an insulating layer of air; and shivering — rapid muscle contraction — releases energy from respiration as heat.

Two details earn regular marks: evaporation is why sweating cools (energy is transferred from the skin to evaporate the water), and shivering warms you because respiration in the contracting muscles is exothermic. On a hot day you may also see the classic data question: skin temperature creeping up a degree or two while core temperature stays almost flat — the system working.