8.1.1Topic 8 · Grey Matter
Co-ordination of Response — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions
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8.1.1 Co-ordination of Response — Edexcel A Level Biology A (SNAB) 9BI0 notes: Compare nervous and hormonal coordination and describe taxes, kineses and tropisms as responses to stimuli in simple organisms and plants.A stimulus is a detectable change in the internal or external e…
- coordination system
- phototropism
- geotropism or gravitropism
- hydrotropism
- thigmotropism
- transducer
Section 1 of 9Why organisms respond
A stimulus is a detectable change in the internal or external environment.
- and an effector (a muscle or gland in animals,
- or a growing region in plants) to produce the response.
Section 2 of 9Nervous and hormonal coordination compared
| Feature | Nervous system | Endocrine (hormonal) system |
|---|---|---|
| Signal | Electrical impulses (action potentials) and neurotransmitters | Chemical hormones |
| Transmission | Along neurones | In the blood plasma |
| Speed | Very fast (milliseconds) | Slower (seconds to hours) |
| Target | Specific muscles or glands | Target cells with complementary receptors, often widespread |
| Duration of effect | Short-lived | Longer-lasting |
| Response | Often localised, rapid, usually reversible | Often widespread, may be long-term (growth, puberty) |
The two systems often work together. In the fight or flight response the nervous system quickly triggers adrenaline secretion from the adrenal glands, and the hormone then sustains the changes in heart rate and blood glucose.
Section 3 of 9Simple responses in animals: taxes and kineses
Simple invertebrates show responses that bring them to a favourable environment without complex processing.
- Examples: woodlice moving away from light; maggots avoiding light; Euglena swimming towards light; bacteria moving up a glucose gradient (chemotaxis).
- (negative phototaxis)
- For example woodlice move faster and turn more often in dry conditions, and slow down and stop in damp ones,
- so that they stay in humid environments, where they lose less water.
These responses are tested using a choice chamber with different environments (light and dark halves, or humid and dry halves) and by counting the number of animals in each half after a set time. A chi-squared test can then show whether the distribution differs significantly from an even split, given a null hypothesis of no preference.
Section 4 of 9Plant responses: tropisms
Plants lack nerves, but they respond to stimuli by growth.
- whereas roots are negatively phototropic and positively gravitropic and hydrotropic.
- (in most species)
These responses are coordinated by plant growth factors, the best known being auxins such as indoleacetic acid (IAA), which are produced in the meristems and move through the plant to cause differential growth.
- and roots that grow downwards anchor the plant and find water and minerals.
Section 5 of 9Receptors and the nervous system pathway
Receptors are specific to one kind of stimulus: photoreceptors for light, chemoreceptors for chemicals, mechanoreceptors for pressure and touch, thermoreceptors for temperature and osmoreceptors for water potential.
Section 6 of 9Investigating responses: practical points
Choice chambers and similar apparatus allow responses to be tested objectively.
- and record how many are in each section at regular intervals.
- and measuring the angle of curvature with a protractor after 24 hours.
Remember that the same stimulus can produce different responses in different organisms, and the same organism can show several responses at once, such as a positive gravitropism in the root and negative gravitropism in the shoot.
Section 7 of 9Exam-style questions
Give three differences between nervous and hormonal coordination. [3]3 marks
Nervous: impulses along neurones, whereas hormonal: chemicals in the blood. Nervous is rapid whereas hormonal is slower. Nervous effects are short-lived and localised; hormonal effects are longer-lasting and may be widespread.
Woodlice were placed in a choice chamber with one humid and one dry half. After 10 minutes 17 of 20 woodlice were in the humid half. Explain this result in terms of kinesis. [3]3 marks
Woodlice move faster and turn more in dry conditions, so they are more likely to leave the dry half by chance. In humid conditions they move slowly and turn less, so they remain there. This keeps them in conditions where water loss is lower, so more end up in the humid half.
Distinguish between a taxis and a tropism. [2]2 marks
A taxis is a directional movement of a whole motile organism towards or away from a stimulus, whereas a tropism is a directional growth response of part of a plant relative to the direction of a stimulus.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Stimulus | Detectable change in the environment |
| Receptor | Cell or organ that detects a stimulus and converts it into an electrical signal |
| Effector | Muscle or gland that responds to a stimulus |
| Taxis | Directional movement of an organism in response to a stimulus |
| Kinesis | Change in rate of movement or turning that depends on stimulus intensity |
| Tropism | Directional growth response of a plant |
| Transducer | Structure that converts one form of energy into another |
| Hormone | Chemical messenger carried in the blood to target cells |
Section 9 of 9Exam technique
Marked practiceQuizzes for this lesson
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