Response to Stimuli (A Level only) — AQA A Level Biology 7402 Revision Notes & Topical Questions

Detecting and responding to environmental change helps organisms survive: it lets them avoid harmful conditions, find food and escape predators. The process is: a stimulus is detected by receptor cells, information is coordinated, and effectors produce a response. Plants as well as animals depend on this ability.

Plants carry out growth responses to directional stimuli such as light and gravity. At GCSE these substances are called plant hormones, but at A Level you must always use the term growth factors. A growth response towards or away from a directional stimulus is a tropism; growth towards light is positive phototropism.

IAA is a plant growth factor and belongs to the group called auxins. It is synthesised in the growing tips of roots and shoots and then moves to other regions of the plant. IAA influences cell elongation, not cell division or 'growth' in general.

Be careful not to mix up the effect of IAA on root cells and on shoot cells. In roots the lower side, with more IAA, elongates at a slower rate than the upper side.

Interpretationroots with added IAA do not grow as long as those in group A because IAA inhibits elongation.

Taxes and kineses are simple, innate responses that enable mobile organisms to remain in a favourable environment; they respond to stimuli such as light, humidity and chemicals. Kineses (singular kinesis) are non-directional responses involving random movement: for example in bright light flatworms move faster and change direction more often, increasing the chance of leaving the unfavourable conditions, which also helps them avoid predation. Taxis involves moving towards (positive) or away from (negative) a stimulus.

Do not confuse these with plant responses: a beetle moving away from light is a taxis, not a tropism.

Simple reflexes allow rapid, automatic responses that protect an organism from harm, e.g. the pupil reflex prevents bright light from damaging the retina and the coughing reflex stops obstructions entering the airways. The pathway is: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response. Be ready to apply this to unfamiliar examples.

Environmental factors that influence animal movement can be investigated using equipment such as a choice chamber (with different humidity or light zones) or a paper maze.

Receptors are cells that detect stimuli and are specific: each type detects a different type of stimulus.

A generator potential is a change in the potential difference across the membrane of a nerve cell in response to a stimulus. Its size depends on the size of the stimulus.

When pressure is applied, the layers of membrane deform and the stretch-mediated sodium ion channels in the sensory neurone ending open.

It is possible to investigate receptors in human skin, for example the sensitivity of temperature receptors using warm and cool objects, and the effect of distance between two points of contact on the accuracy of touch perception, using a blunt pair of scissors or an unfolded paperclip.

Areas such as the fingertips and soles of the feet have much higher resolution: two points can be identified when very close together, because of a higher density of touch receptors. A limitation is that only one subject is used; ideally the experiment would be repeated with a number of subjects to achieve more reliable results.

Over time touch receptors in the skin can become habituated to certain stimuli: the response to a stimulus decreases after repeated or prolonged exposure. This is why you stop noticing clothing on your skin.

The eye is a sense organ containing receptors sensitive to light intensity and wavelength.

When red-, blue- and green-sensitive cones are stimulated in different numbers and locations, colour images are generated. A stimulated receptor generates an impulse in the bipolar cell.

Each cone cell connects to its own bipolar neurone and produces its own impulse when stimulated, so information from each cone is interpreted by the brain as a separate point and detailed images are generated. Because of the type of connection each cell type forms with its bipolar neurones, and the pigments they contain, rod cells are highly sensitive to light (many rods share one bipolar neurone, giving summation), can only generate black and white images and give low visual acuity; cone cells are less sensitive to light but give colour vision and high visual acuity.

Heart muscle contraction is myogenic: the heart beats at a baseline rate without any input from the nervous system.

Although initiation is myogenic, heart rate has to change from baseline at times.

Which nerve the medulla uses depends on information from receptors in blood vessels: chemoreceptors detect blood pH/CO₂ and pressure receptors detect blood pressure.

The effect of a named variable on an organism's heart rate can be investigated, for example caffeine on human heart rate.

Investigations on living organisms, such as the effect of caffeine on heart rate in humans, are affected by natural variation between individuals.

A single measurement may be affected by error.

When evaluating, comment on sources of error and improvement: a small number of volunteers, volunteers who differ in habitual caffeine intake, different times since last meal, and counting pulse by hand (use a heart rate monitor for greater precision). Improvements include a larger sample, repeats of each measurement, a placebo control and matching volunteers.