Stimulus and response - Biology PDF

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Stimulus and response A stimulus is a detectable change in the internal or external environment of an organism that produces a response in the organism. The ability to respond to stimuli increases the chances of survival for an organism. For example, to be able to detect and move away from harmful stimuli such as predators, extreme temperatures, or to detect and move towards a source of food, aids survival. Those organisms that survive have a greater chance of passing their alleles to their offspring/ there is therefore a SELECTION PRESSURE favouring organisms with more appropriate responses. Stimuli are detected by organs known as RECEPTORS. They can be cells or proteins on cell surface membranes. Receptors transform energy of a stimulus into some form of energy that can be processed by the organism and leads to a response. The response is carried out by one or more of a range of different cells, tissue, organs and systems. These are known as EFFECTORS. Receptors and effectors are linked together by hormones or the nervous system. Each receptor and effector is linked to a central COORDINATOR, the coordinator acts like a switchboard, connecting information from each receptor with the appropriate effector

Stimulus  Receptor  Coordinator  Effector  Response

A TAXIS is a directional movement of an organism in response to a stimulus. As a result, a motile organism responds directly to environmental changes by moving towards a favourable stimulus, positive taxis, or away from unfavourable one, negative taxis.

Positive taxis – organism moves towards stimulus Negative taxis – organism moves away from stimulus

Examples  

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Single-celled algae will move towards light (positive phototaxis). This increases their chances of survival as they need light to manufacture their food. Earthworms will move away from light (negative phototaxis). This increases their chances of survival because it takes them into the soil, where they are better able to conserve water, find food and avoid predators. Some species of bacteria will move towards a region where glucose is more highly concentrated (positive chemotaxis). This increases their chances of survival because they use glucose as a source of food.

Some different types of Taxis are…    

Chemotaxis -chemically generated taxis (for example, phagocytic white blood cells show positive chemotaxis for a pathogen as they move towards it) Phototaxis - light sensitive taxis (for example, moths show positive chemotaxis as they move towards light) Geotaxis - taxis in response to gravity (for example, earthworms show positive geotaxis as they burrow down into the soil) Rheotaxis - taxes in response to movement (for example, salmon use positive rheotaxis to swim upstream as they are swimming with the water)

A KINESIS is an animal's non-directional response to a stimulus; it’s a form of response in which the organisms does not move away or towards a stimulus. This means that unlike taxis, kinesis cannot be positive (towards stimuli) or negative (away from stimuli) but instead is random in order to increase the organisms’ chances of survival. The more unpleasant the stimulus, the more rapidly the organisms moves and changes direction. A kinesis brings back the organism into favourable conditions. This type of response is important when a stimulus is less directional, e.g. humidity and temperature, in which there is not always a clear gradient from one extreme to the other. Example Woodlice lose water from their bodies in dry conditions. When they are in dry sea, they move more rapidly and change direction more often. This increases their chances of moving into a different area. If this different area happens to be moist, they slow down and change direction less often. This means they are likely to stay where they are. This prevent them drying out and so increase their chances of survival.

A TROPISM is a growth movement of part of a plant in response to a directional stimulus. The plant grows towards (positive response) or away from (negative response) the stimulus.   

Plant shoots growing towards light so leaves are in the most favourable position to capture light for photosynthesis (positive phototropism) Plant roots grow towards water so that root systems will develop where there is the most water in the soil (positive hydrotropism) Plant roots grow away from light (negative phototropism) and towards gravity (positive geotropism). In both cases the response increases the probability that roots will grow into the soil, where there are better able to absorb water and mineral ions

GEOTROPISM

How is tropism different from taxis? When an organism moves only a part of its body due to a stimulus, the response is called tropism. On the other hand, if the organisms moves as a whole due to a stimulus, it response is called taxis. (Phototropism is the movement of only the shoot towards light, phototaxis is the movement of the worm towards light)

What is the difference between taxis and kinesis? Taxis is a specific, directed motion in response to a stimulus, whereas kinesis is a random, undirected motion in response to a stimulus....