1.3Unit 2 · Transport, Excretion & Coordination
Co-ordination & Response — Unit 2 — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 4 parts Lesson 4 of 4 1 quiz
Lesson summary
Explain why organisms must respond to changes and maintain homeostasis.The central nervous system (CNS) = brain + spinal cord.Reflex arcreceptor → sensory neurone → relay neurone → motor neurone → effector.Normal core body temperature is about 37 °C.Homeostasis controls body temp…
- Response to changes in the environment
- Co-ordinating a response
- Response to stimuli in plants
- Nervous and hormonal control
- Human nervous system
- Role of neurotransmitters at synapses
Section 1 of 4Co-ordination & Response — Unit 2
Response to changes in the environment:
organisms must detect and respond to changes (stimuli) to survive, e.g. finding food, avoiding predators and keeping conditions suitable for enzymes.Homeostasis:
the internal environment, including body temperature, water content and blood glucose concentration, is kept within narrow limits.- This is usually by negative feedback: a change from the set point triggers responses that reverse it.
Co-ordinating a response:
- stimulus → receptor → co-ordinator (CNS or endocrine gland) → effector → response.
- Effectors are muscles (contract) or glands (secrete).
Response to stimuli in plants:
- shoots show positive phototropism and negative geotropism;
- roots show positive geotropism.
- Auxin is made at the tip and diffuses back;
- unequal distribution of auxin causes unequal cell elongation and bending.
- shoots show positive phototropism and negative geotropism;
Nervous and hormonal control:
the nervous system uses electrical impulses along neurones: fast, short-lived and precise.- The endocrine system uses hormones carried in the blood: slower, longer-lasting and often affecting several organs (e.g. adrenaline, insulin).
Human nervous system:
- the CNS (brain and spinal cord) is linked to receptors and effectors by nerves of the peripheral nervous system.
- Sense organs contain receptors, e.g. the eye has light receptors (rods and cones) in the retina.
Role of neurotransmitters at synapses:
when an impulse reaches the end of a neurone, neurotransmitter is released into the synaptic gap, diffuses across and binds to receptor molecules on the next neurone, triggering a new impulse.- Only the first neurone releases transmitter, so impulses pass one way.
Simple reflex arc:
- reflexes are fast, automatic and protective (e.g. withdrawing a hand from a hot object, blinking).
- The pathway bypasses conscious parts of the brain, passing through a relay neurone in the spinal cord.
Structure of the eye:
- The cornea refracts light entering the eye;
- the iris controls pupil size and so how much light enters;
- the lens focuses light onto the retina,
- which contains light receptors (rods and cones).
- The fovea has many cones for detailed colour vision;
- the optic nerve carries impulses to the brain;
- the blind spot has no receptors.
- The cornea refracts light entering the eye;
Focusing on a near object:
- The ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes fatter and more curved, so light is refracted more.
- This adjustment of lens shape is called accommodation.
Focusing on a distant object:
- The ciliary muscles relax, the suspensory ligaments are pulled tight, and the lens becomes thinner, so light is refracted less.
- Remember ligaments tighten or slacken;
- they do not contract.
Pupil reflex:
In bright light the circular muscles of the iris contract and the radial muscles relax, so the pupil narrows and protects the retina.- In dim light the radial muscles contract and circular muscles relax, so the pupil widens to let in more light.
Control of body temperature:
Core temperature is kept near 37 °C.- Temperature receptors in the skin and the thermoregulatory centre in the hypothalamus (which monitors blood temperature) detect changes, and the brain coordinates responses in the skin and muscles.
Skin responses when too hot:
Vasodilationmuscles in the walls of arterioles supplying the skin relax, so the arterioles widen and more blood flows near the surface, losing heat by radiation.- Sweat glands secrete more sweat, which cools the skin as it evaporates.
Responses when too cold:
Vasoconstrictionarterioles supplying the skin narrow, so less blood flows near the surface and less heat is lost.- Sweating decreases.
- Shivering (rapid muscle contractions) releases heat from respiration, warming the blood.
Adrenaline:
Secreted by the adrenal glands in danger or stress (fight or flight).- It raises heart rate and breathing rate, diverts blood to muscles and away from the gut, and causes glycogen to be converted to glucose in liver and muscles, so muscles receive more glucose and oxygen for respiration.
Insulin and blood glucose:
When blood glucose rises, the pancreas detects it and secretes insulin into the blood.- Insulin causes liver and muscle cells to take up glucose and store it as glycogen, so blood glucose falls back to normal.
- This is an example of homeostasis by negative feedback.
Reproductive hormones:
Testosterone (testes) causes male sexual development.- Oestrogen (ovaries) causes female secondary sexual characteristics and rebuilds the uterus lining.
- Progesterone (ovaries) maintains the lining.
- FSH (pituitary) makes an egg mature and stimulates oestrogen release; LH (pituitary) triggers ovulation.
Section 2 of 4Process & sequence
- Stimulus (e.g. heat from a hot object) is detected.
- Receptor in the skin generates an impulse.
- Sensory neurone carries the impulse to the spinal cord.
- Relay neurone in the spinal cord passes the impulse on via synapses.
- Motor neurone carries the impulse to the effector.
- Effector (muscle) contracts, pulling the hand away.
The reflex arcwrite these steps in this order; each step is a marking point.
- Stimulus detected by receptor
- Sensory neurone to spinal cord
- Relay neurone across synapses
- Motor neurone to effector
- Muscle contracts to respond
Section 3 of 4Compare
| Feature | Nervous | Hormonal |
|---|---|---|
| Signal | Electrical impulses (and chemical at synapses) | Chemical hormones |
| Transmission | Along neurones | In blood plasma |
| Speed | Very fast | Slower |
| Duration | Short-lived | Longer-lasting |
| Target | Very precise area | Widespread target organs |
| Tropism | Stimulus | Shoot response | Root response |
|---|---|---|---|
| Phototropism | Light | Positive (grows towards) | Negative (grows away) |
| Geotropism (gravitropism) | Gravity | Negative (grows up) | Positive (grows down) |
- Auxin causes cell elongation in shoots but inhibits it in roots.
Section 4 of 4Worked examples
Explain how a plant shoot bends towards light coming from one side.
- Auxin is produced in the shoot tip.
- Light from one side causes auxin to accumulate on the shaded side.
- Auxin stimulates cell elongation in shoots,
- so cells on the shaded side elongate more than those on the lit side, and the shoot bends towards the light.
- This positive phototropism increases light absorbed for photosynthesis.
- Auxin is produced in the shoot tip.
Explain why a reflex action is faster than a voluntary response.
- A reflex arc involves few neurones and few synapses and passes through the spinal cord (or lower brain) without conscious decision-making,
- so the impulse reaches the effector in a shorter time.
- A reflex arc involves few neurones and few synapses and passes through the spinal cord (or lower brain) without conscious decision-making,
Marked practiceQuizzes for this lesson
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