Co-ordination & ResponseEdexcel IGCSE Biology: Revision notes

Homeostasis is the maintenance of a constant internal environment, despite changes inside or outside the organism — examples include keeping body water content and body temperature constant.

A coordinated response to a change (a stimulus) requires three components: a receptor (detects the stimulus), a coordinator (usually part of the nervous or hormonal system, which processes the information) and an effector (a muscle or gland that carries out the response).

Plants also respond to stimuli through tropisms — directional growth responses:

• Geotropism — growth response to gravity; roots grow towards gravity (positively geotropic), shoots grow away from gravity (negatively geotropic)
• Phototropism — growth response to light; shoots grow towards light (positively phototropic), roots grow away from light (negatively phototropic)

These responses are controlled by the plant hormone auxin, which accumulates on the shaded/lower side of a shoot, causing cells there to elongate more, bending the shoot towards the light (or, in roots, causing the opposite bending response away from light).

The nervous system and hormonal (endocrine) system both coordinate responses, but work differently:

The central nervous system (CNS) consists of the brain and spinal cord, linked to receptors and effectors throughout the body by nerves. When a receptor is stimulated, it sends electrical impulses along nerves into the CNS; the CNS processes this information and sends electrical impulses back out to effectors, producing a rapid response. At the junction between two neurones (a synapse), the electrical impulse triggers the release of a chemical neurotransmitter, which diffuses across the gap and triggers a new electrical impulse in the next neurone.

A reflex arc is a rapid, automatic, involuntary response that does not require conscious thought from the brain — for example, withdrawing a finger from a hot object: a receptor in the skin detects the heat (stimulus), sends an impulse via a sensory neurone to the spinal cord, which passes directly (via a relay neurone) to a motor neurone, which carries the impulse to a muscle (effector) that contracts to move the finger away — all within a fraction of a second.

The eye detects light and allows the brain to interpret images. Key structures and their roles: the cornea and lens refract (bend) light to focus it onto the retina, which contains light-sensitive receptor cells; the iris controls the size of the pupil, altering how much light enters the eye.

• Focusing on near and distant objects: the lens changes shape (using the ciliary muscles) — it becomes more rounded (thicker) to focus on near objects, and thinner/flatter to focus on distant objects
• Responding to changes in light intensity: in bright light, the iris makes the pupil constrict (smaller) to reduce the light entering; in dim light, the iris makes the pupil dilate (larger) to let in more light

The skin helps maintain a constant body temperature (part of homeostasis) through several mechanisms:

• Sweating: when the body is too hot, sweat glands produce more sweat, which evaporates from the skin surface, transferring heat energy away and cooling the body
• Vasodilation: blood vessels near the skin surface widen, increasing blood flow near the surface so more heat is lost by radiation, cooling the body when it is too hot
• Vasoconstriction: blood vessels near the skin surface narrow, reducing blood flow near the surface and reducing heat loss, helping to conserve heat when the body is too cold