Survival and responseAQA A-Level Biology: Revision notes

A stimulus is a change in the internal or external environment, such as pressure, light, temperature or blood pH.

A receptor detects the stimulus. Each type of receptor is specific to one type of stimulus: light receptors do not respond to pressure. A coordinator, such as the brain, spinal cord or a hormone-producing gland, formulates a suitable response. An effector, a muscle or a gland, produces the response.

Responding to changes in the environment increases an organism's chance of survival: it helps it to avoid harm, find food and find mates, and to stay in a favourable environment.

Plants have no nervous system. Instead, specific growth factors are produced in growing regions, such as the shoot tip, and move to other tissues where they regulate growth in response to directional stimuli. One growth factor is indoleacetic acid (IAA).

In shoots, IAA stimulates cell elongation. In roots, high concentrations of IAA inhibit cell elongation. The effect of a given concentration therefore depends on the organ.

A growth response to a directional stimulus is called a tropism:

• Phototropism: in a shoot lit from one side, IAA moves to the shaded side, where it stimulates elongation, so the shoot bends towards the light.
• Gravitropism: in a horizontal shoot or root IAA accumulates on the lower side. In the shoot this stimulates elongation so the shoot bends upwards. In the root it inhibits elongation, so the root bends downwards.

Simple mobile organisms keep themselves in a favourable environment using two types of behaviour.

A taxis is a directional response: the organism moves towards a favourable stimulus (positive taxis) or away from an unfavourable one (negative taxis). Example: a unicellular alga swimming towards light.

A kinesis is a non-directional response: the organism changes the rate of movement, or how often it turns, depending on the intensity of the stimulus. It moves quickly and turns often in unfavourable conditions, and slows down in favourable ones, so it tends to end up in the favourable area. Example: woodlice move faster in dry air and slow down in humid air.

A simple reflex is a rapid, automatic response to a stimulus. The pathway is a reflex arc: receptor, sensory neurone, relay neurone, motor neurone and effector. Details of the spinal cord and dorsal and ventral roots are not needed.

The pathway has few neurones and synapses and does not involve the conscious parts of the brain, so the response is fast. Reflexes protect the body from harm, for example withdrawing the hand from a hot object before tissue damage is done.

A choice chamber or maze can be used to investigate how an environmental variable, such as humidity or light, affects the movement of a small animal such as a woodlouse.

• Make one half of the chamber dry (anhydrous calcium chloride or silica gel) and the other humid (damp filter paper), separated from the animals by gauze.
• Keep other variables the same in both halves, such as light intensity (cover the chamber) and temperature.
• Place the same number of animals in the centre, leave them to settle, and count how many are in each half at regular intervals.
• Repeat and calculate a mean. Handle animals gently and return them to their habitat afterwards.

If more animals are found in one side, they are showing a response (kinesis or taxis) that keeps them in a favourable environment.