Coordination and response
Nervous system and the eye
The central nervous system (brain and spinal cord) coordinates responses. Impulses travel: stimulus → receptor → sensory neurone → relay neurone (CNS) → motor neurone → effector → response. Reflexes are fast, automatic and protective. Neurones meet at synapses, where neurotransmitters diffuse across the gap. In the eye, the cornea and lens focus light on the retina (rods for dim light, cones for colour). The pupil reflex: in bright light, circular muscles of the iris contract, making the pupil smaller. Accommodation: for near objects, ciliary muscles contract, suspensory ligaments slacken, and the lens becomes fatter.
≈ 45 minHormones and homeostasis
Hormones are chemicals made by endocrine glands and carried in the blood to target organs. They act more slowly than nerves but for longer. Adrenaline (adrenal glands) prepares the body for action; insulin (pancreas) lowers blood glucose by making the liver and muscles take up glucose and store it as glycogen; glucagon raises it. Homeostasis keeps internal conditions constant using negative feedback. Body temperature is controlled by the brain: when too hot, sweating and vasodilation; when too cold, shivering and vasoconstriction. In Type 1 diabetes, the pancreas makes no insulin.
≈ 45 minTropisms
A tropism is a growth response of a plant to a directional stimulus. Phototropism: shoots grow towards light (positive), helping leaves absorb light. Gravitropism: roots grow down with gravity (positive), shoots grow up (negative). These are controlled by auxin, a hormone made in the shoot tip. In a shoot, auxin moves to the shaded side, where it makes cells elongate more, so the shoot bends towards the light.
≈ 30 min