Stimuli and Responses
This lesson covers:
- How stimuli and responses aid survival
- Taxes for directional responses
- Kineses for non-directional responses
- Tropisms for plant growth movements
How stimuli and responses aid survival
Organisms detect stimuli through specialised structures known as receptors, which then initiate responses from effectors to increase the organism's chances of survival.
The pathway involves:
- Stimulus - This is a detectable change in the internal or external environment of an organism.
- Receptor - This detects specific types of stimulus.
- Coordinator - This coordinates a suitable response to a stimulus.
- Effector - This produces a response to a stimulus.
- Response - This is the action produced by the effector.
Responses to stimuli are crucial for survival as they allow organisms to:
- Navigate towards beneficial resources like food.
- Evade predators or harmful conditions.
This increases the probability of survival and successful reproduction.
Taxes for directional responses
A taxis (plural: taxes) is a simple response whose direction is determined by the direction of the stimulus. They guide organisms either towards a beneficial stimulus or away from a harmful one.
A taxis may either be positive or negative:
- Positive taxes lead towards a favourable stimulus.
- Negative taxes prompt movement away from adverse stimuli.
Examples of taxes
Taxes are named for the specific stimuli they are in response to.
Some examples of taxes you need to know are:
- Positive phototaxis - This is when an organism moves towards light, e.g. algae move towards light for photosynthesis to synthesise food.
- Negative phototaxis - This is when an organism moves away from light, e.g. earthworms move away from light because the heat of the sun can dry out their skin.
- Positive chemotaxis - This is when an organism moves towards a region where a specific chemical is more concentrated, e.g. bacteria move towards regions rich in glucose to obtain food.
Kineses for non-directional responses
Kineses (singular: kinesis) adjust the movement speed and turning frequency of an organism based on the favourability of the environment.
The kinesis depends on how favourable the conditions are:
- Increased turning in unfavourable conditions.
- Accelerated movement in favourable conditions.
For instance, woodlice exhibit increased turning in dry conditions to increase the likelihood of returning to moist environments, which are more helpful to their survival.
Tropisms for plant growth movements
A tropism is the growth of a part of an organism like a plant in response to a directional stimulus.
Like taxes, tropisms are named for the specific stimuli they are in response to:
- Phototropism directs growth towards or away from light.
- Gravitropism (or geotropism) directs growth towards or away from the gravitational pull.
Different parts of plants show different tropisms:
- The shoots exhibit positive phototropism and negative gravitropism, to optimise light absorption for photosynthesis.
- The roots show negative phototropism and positive gravitropism, to increase the probability that roots will grow into the soil, where they can absorb more water and mineral ions.