How do simple and complex organisms detect and respond to changes in their environment so that they survive?
A stimulus is a detectable change in the internal or external environment of an organism that produces a response. Taxes and kineses as simple responses that maintain a mobile organism in a favourable environment; tropisms as growth responses controlled by indoleacetic acid (IAA); the role of a simple reflex arc in protecting the body from harm.
A focused answer to the AQA 3.6 dot point on stimuli and responses. Distinguishes taxes from kineses, explains tropisms and the role of IAA in phototropism and gravitropism, and traces the simple reflex arc that protects the body from harm.
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What this dot point is asking
AQA wants you to define a stimulus and a response, distinguish a taxis from a kinesis, explain how IAA (auxin) controls plant tropisms, and describe the simple reflex arc as a protective response.
The answer
A receptor detects the stimulus, a coordinator (such as the central nervous system) processes the information, and an effector (muscle or gland) produces the response.
Taxes and kineses
These are simple responses that keep a mobile organism in a favourable environment.
Taxis (plural taxes). A directional response in which the organism moves its whole body towards or away from a directional stimulus. A positive taxis moves towards the stimulus, a negative taxis moves away. Example: motile bacteria show positive chemotaxis towards glucose; maggots show negative phototaxis away from light.
Kinesis (plural kineses). A non-directional response in which the rate of movement and rate of turning change with the intensity of the stimulus. There is no movement towards or away from a particular direction. Example: woodlice move faster and turn less in dry air, so they leave dry areas and accumulate in humid areas where they slow down and turn more.
Tropisms and the role of IAA
A tropism is a directional growth response of a plant towards or away from a directional stimulus. Growth towards the stimulus is positive; away from it is negative.
- Phototropism. Shoots grow towards light (positive); roots grow away (negative).
- Gravitropism (geotropism). Roots grow towards gravity (positive); shoots grow away (negative).
These responses are controlled by the plant growth factor indoleacetic acid (IAA), an auxin. IAA is produced in the growing tips (shoot and root tips) and moves to other cells, where it controls cell elongation.
The key is that IAA causes opposite effects in shoots and roots:
- In shoots, a high concentration of IAA stimulates cell elongation.
- In roots, a high concentration of IAA inhibits cell elongation.
For a root lit or gravity-stimulated on one side, IAA also accumulates on the lower or shaded side, but because IAA inhibits root elongation, the lower side grows less and the root bends downwards (positive gravitropism), anchoring the plant and reaching water.
The simple reflex arc
A reflex is a rapid, automatic (involuntary) response to a stimulus that does not involve conscious thought. Reflexes protect the body from harm because they are fast and do not need processing by the brain.
The pathway of a simple reflex arc is:
In the withdrawal reflex (touching a sharp pin), a pain receptor in the skin is stimulated, the sensory neurone carries the impulse into the spinal cord, a relay neurone passes it to a motor neurone, and the motor neurone makes the muscle contract to pull the hand away. The impulse only later reaches the brain, which is why you feel the pain after you have already moved.
Examples in context
Example 1. Maggot dispersal and negative phototaxis. Fly larvae (maggots) move directly away from a light source, a negative phototaxis. This directional response carries them under leaf litter or into soil, away from visual predators and from the drying effect of light, illustrating how a simple directional response improves survival in a mobile organism.
Example 2. The Darwin and Boysen-Jensen experiments. Classic investigations covering the shoot tip with an opaque cap abolished phototropism, while a transparent cap did not, showing the tip detects light. Placing a permeable agar block between a cut tip and the stump still allowed bending, while an impermeable mica sheet blocked it, demonstrating that a chemical messenger (later identified as IAA) moves from the tip and controls elongation.
Try this
Q1. Distinguish between a taxis and a kinesis. [2 marks]
- Cue. A taxis is a directional response (movement towards or away from a directional stimulus). A kinesis is a non-directional response in which the rate of movement and turning change with the intensity of the stimulus.
Q2. Explain why a shoot bends towards a light source coming from one side. [4 marks]
- Cue. IAA made in the tip moves to the shaded side; in shoots IAA stimulates cell elongation; the shaded side elongates more than the lit side; so the shoot bends towards the light.
Q3. Describe the pathway of a simple reflex arc and explain why it protects the body. [4 marks]
- Cue. Receptor, sensory neurone, relay neurone, motor neurone, effector. It is fast (few synapses), automatic and does not require the brain, so the body responds before conscious awareness of the harm.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 AQA5 marksWoodlice were placed in a choice chamber that was half dry and half humid. Explain how a kinesis keeps the woodlice in the humid region.Show worked answer →
A 5-mark answer needs the definition of kinesis and the mechanism that concentrates the animals in the favourable area.
- A kinesis is a non-directional response in which the rate of movement (and rate of turning) changes with the intensity of the stimulus, not the direction of the stimulus.
- In unfavourable (dry) conditions the woodlice move quickly and turn less, so they keep moving in roughly straight lines.
- This means they soon leave the dry region by chance.
- In favourable (humid) conditions they slow down and turn more often, so they tend to stay put.
- Over time more woodlice accumulate in the humid half, keeping them in conditions that reduce water loss and the risk of desiccation. This improves survival even though no woodlouse moves directly towards the humidity.
Markers reward the idea that movement is non-directional, that speed and turning depend on intensity, and that the net effect is accumulation in the favourable area.
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