(j) Co-ordination and response
- Syllabus
- 2024
- Topic
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- Level
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Learning objectives
2.80Responses to environmental changeUnderstand how organisms are able to respond to changes in their environment.2.81HomeostasisUnderstand that homeostasis is the maintenance of a constant internal environment, and that body water content and body temperature are both examples of homeostasis.2.82Stimulus, receptor and effectorUnderstand that a co-ordinated response requires a stimulus, a receptor and an effector.2.83Plant responses to stimuliUnderstand that plants respond to stimuli.2.84Tropic responsesDescribe the geotropic and phototropic responses of roots and stems.2.85Auxin and phototropismUnderstand the role of auxin in the phototropic response of stems.2.86Nervous and hormonal communicationDescribe how nervous and hormonal communication control responses and understand the differences between the two systems.2.87Central nervous systemUnderstand that the central nervous system consists of the brain and spinal cord and is linked to sense organs by nerves.2.88Nerve impulses and rapid responsesUnderstand that stimulation of receptors in the sense organs sends electrical impulses along nerves into and out of the central nervous system, resulting in rapid responses.2.89Neurotransmitters at synapsesUnderstand the role of neurotransmitters at synapses.2.90Simple reflex arcDescribe the structure and functioning of a simple reflex arc illustrated by the withdrawal of a finger from a hot object.2.91Eye structure and functionDescribe the structure and function of the eye as a receptor.2.92Eye focusing and light responsesUnderstand the function of the eye in focusing on near and distant objects, and in responding to changes in light intensity.2.93Skin and temperature regulationDescribe the role of the skin in temperature regulation, with reference to sweating, vasoconstriction and vasodilation.2.94Adrenaline, insulin and reproductive hormonesUnderstand the sources, roles and effects of the following hormones: adrenaline, insulin, testosterone, progesterone and oestrogen.295B ADH, FSH and LHUnderstand the sources, roles and effects of the following hormones: ADH, FSH and LH.
Respond when the environment changes
An organism responds when it detects a change in its internal or external environment and produces an action that can improve survival.
| Stage | Role | Example |
|---|---|---|
| stimulus | detectable environmental change | increasing light, heat, sound or touch |
| detection and coordination | receptors detect the change and information is processed | sensory cells and a coordination system |
| response | an effector changes activity | muscle contracts, gland secretes or a plant changes growth direction |
Responses can move an organism toward useful conditions or away from harm. Repeated harmless stimulation can sometimes reduce a response, preventing unnecessary energy use.
A stimulus is the change, not the action it causes. A response need not involve conscious choice: plants, microorganisms and reflex pathways all respond without deliberate decision-making.
Keep the internal environment stable
Homeostasis is the maintenance of a constant internal environment within narrow limits despite internal or external change.
| Regulated condition | Why control matters | Corrective examples |
|---|---|---|
| body temperature | enzyme-controlled reactions work best in a suitable range | sweating and vasodilation when hot; reduced sweating and vasoconstriction when cold |
| body water content | cells need a suitable water balance | ADH changes kidney water reabsorption and urine concentration |
A deviation is detected, a coordination system activates effectors, and the response opposes the deviation. As the condition returns toward its set range, the corrective response is reduced: this is negative feedback.
Homeostasis does not keep every value perfectly fixed; it holds conditions within a tolerable range. It concerns the internal environment, not simply keeping the external environment unchanged.
Build a coordinated response pathway
A coordinated response links a stimulus to a receptor and then to an effector whose action produces the response.
| Component | What it does |
|---|---|
| stimulus | changes a condition, such as light intensity or temperature |
| receptor | detects the stimulus and converts it into information for the coordination system |
| coordinator | processes information and sends instructions |
| effector | carries out the response; usually a muscle or gland in animals |
Touching a hot object is the stimulus; temperature or pain receptors in skin detect it; the nervous system coordinates; arm muscles contract as effectors, withdrawing the hand.
A receptor detects; an effector acts. The organ that senses a change should not be named as the effector unless it also performs the response.
Recognise how plants respond to stimuli
Plants detect environmental stimuli and alter growth or cell activity even though they have no nervous system or muscles.
| Stimulus | Plant response and value |
|---|---|
| directional light | shoots change growth direction, improving light capture |
| gravity | roots and shoots grow in opposite directions, placing organs in useful positions |
| water shortage | stomata close, reducing water loss |
| day length | flowering can occur in a season favourable for reproduction |
Plant responses are often slower than animal movements because many depend on unequal growth, but speed does not determine whether something is a response. A stimulus can also change reversible cell activity, such as stomatal opening.
Compare root and shoot tropisms
A tropism is a directional growth response: positive growth is toward a stimulus and negative growth is away from it.
| Organ | Light response | Gravity response | Benefit |
|---|---|---|---|
| shoot | positive phototropism: toward light | negative geotropism: away from gravity | exposes leaves to light for photosynthesis |
| root | usually negative phototropism: away from light | positive geotropism: toward gravity | anchors the plant and grows into soil for water and ions |
Positive and negative describe direction relative to the stimulus, not whether the response is beneficial. A shoot growing upward is negatively geotropic because it grows opposite to gravity.
Use auxin to bend a shoot toward light
When light reaches a shoot from one side, auxin causes unequal cell elongation so the shoot bends toward the light.
| Stage | Event |
|---|---|
| 1 | auxin is produced near the shoot tip |
| 2 | one-sided light causes more auxin to accumulate on the shaded side |
| 3 | in shoots, auxin stimulates greater cell elongation on the shaded side |
| 4 | the shaded side grows faster, curving the tip toward the light |
Bending exposes developing leaves to stronger light, supporting a higher photosynthetic rate.
Auxin does not pull the shoot toward light and light does not make the illuminated side grow faster. In this shoot response, greater elongation occurs on the shaded side.
Compare nervous and hormonal communication
Nervous and hormonal systems both coordinate responses, but they carry different signals through different routes and produce different response patterns.
| Feature | Nervous communication | Hormonal communication |
|---|---|---|
| signal | electrical impulse along neurones; neurotransmitter at synapses | chemical hormone |
| route | specific nerves | blood plasma throughout the body |
| target | precise connected effector | only cells with the correct receptor respond |
| speed | usually rapid | usually slower |
| duration | often short-lived | often longer-lasting |
Hormones travel throughout the body but do not affect every cell; target cells need matching receptors. Nervous impulses travel along neurones rather than being carried in blood.
Connect sense organs to the CNS
The central nervous system consists of the brain and spinal cord; nerves connect it to receptors in sense organs and to effectors.
| Direction | Route and purpose |
|---|---|
| into CNS | receptors in sense organs detect stimuli; sensory neurones carry impulses toward brain or spinal cord |
| within CNS | relay neurones connect pathways and allow information to be processed |
| out of CNS | motor neurones carry impulses to muscles or glands |
The nerves outside the brain and spinal cord belong to the peripheral nervous system, not the CNS. A sense organ contains receptors but is not itself part of the CNS.
Send electrical impulses for a rapid response
Stimulation of receptors in a sense organ generates electrical impulses that travel along neurones into and out of the CNS to produce a rapid response.
| Stage | Information flow |
|---|---|
| 1 | a receptor detects a stimulus |
| 2 | a sensory neurone carries electrical impulses into the CNS |
| 3 | the CNS coordinates the response |
| 4 | a motor neurone carries impulses out to an effector |
| 5 | a muscle contracts or a gland secretes |
The dedicated pathway and electrical transmission along neurones allow responses to occur rapidly. Wider axons can conduct impulses faster because they offer less internal resistance to current flow.
The signal is electrical along a neurone but chemical across most synapses. Do not describe an impulse as blood-borne or say receptors themselves perform the final response.
Transmit signals across a synapse
At a synapse, a neurotransmitter carries a signal across the tiny gap from one neurone to the next cell.
| Stage | Event |
|---|---|
| 1 | an electrical impulse reaches the end of the presynaptic neurone |
| 2 | neurotransmitter is released into the synaptic cleft |
| 3 | molecules diffuse across the gap |
| 4 | neurotransmitter binds to complementary receptors on the postsynaptic membrane |
| 5 | a new electrical impulse is triggered if stimulation is sufficient |
Neurotransmitter is released on one side and receptors are concentrated on the other, making transmission one-way. Chemical diffusion also creates a small synaptic delay.
The neurotransmitter does not travel along the whole axon; the electrical impulse does. Neurotransmitter crosses only the synaptic cleft before being removed or broken down.
Withdraw a finger through a reflex arc
A withdrawal reflex is a rapid, automatic response that reduces tissue damage before conscious processing is required.
| Stage | Structure and event |
|---|---|
| 1 | heat or pain stimulates a receptor in the finger |
| 2 | a sensory neurone carries impulses to the spinal cord |
| 3 | neurotransmitters cross synapses to a relay neurone and then a motor neurone |
| 4 | the motor neurone carries impulses to an arm muscle |
| 5 | the muscle contracts as the effector and withdraws the hand |
Impulses can also travel to the brain so the person becomes aware of pain, but the protective withdrawal can begin through the spinal cord first.
A reflex is involuntary and stereotyped, not necessarily unconscious forever. The relay neurone lies inside the CNS; sensory and motor neurones connect the receptor and effector to it.
Match eye structures to visual functions
The eye is a sense organ whose structures refract light, control how much enters and convert focused light into nerve impulses.
| Structure | Function |
|---|---|
| cornea | transparent curved surface that provides most refraction |
| iris and pupil | iris muscles change pupil diameter to regulate light entry |
| lens | changes curvature to fine-focus light on the retina |
| ciliary muscles and suspensory ligaments | alter tension and therefore lens shape |
| retina | contains light-sensitive receptor cells |
| fovea | retinal region with high visual acuity |
| optic nerve | carries electrical impulses from retina to brain |
| sclera | tough outer layer that protects and supports the eye |
The pupil is a hole, not a muscle or receptor. The iris controls pupil size, while photoreceptors in the retina detect light after the cornea and lens have focused it.
Focus the eye and control incoming light
The eye focuses by changing lens curvature and responds to light intensity by changing pupil diameter; these are separate control mechanisms.
| Viewing distance | Ciliary muscles | Suspensory ligaments | Lens | Refraction |
|---|---|---|---|---|
| near object | contract | loosen | thicker and more curved | bends light more |
| distant object | relax | tighten | thinner and less curved | bends light less |
| Light condition | Iris response | Pupil outcome |
|---|---|---|
| bright | circular muscles contract; radial muscles relax | constricts, reducing light entry and retinal damage |
| dim | radial muscles contract; circular muscles relax | dilates, increasing light entry |
The lens does not move forward and backward to focus. Accommodation changes lens shape; the pupil reflex changes light quantity but does not focus the image.
Regulate temperature through the skin
The skin helps regulate body temperature by changing sweat production and blood flow near the surface.
| Condition | Sweating | Skin blood vessels | Effect on heat transfer |
|---|---|---|---|
| body too hot | increases; evaporation removes thermal energy | arterioles vasodilate, increasing blood flow through surface capillaries | more heat lost by radiation and convection |
| body too cold | decreases | arterioles vasoconstrict, reducing surface-capillary blood flow | less heat transferred to the environment |
Temperature receptors and the brain coordinate these opposing responses. When temperature returns toward its normal range, the responses reduce through negative feedback.
Capillaries do not constrict or dilate because they lack muscular walls; arterioles controlling blood supply to them do. Sweat cools only when it evaporates, not merely when it is secreted.
Link five hormones to their sources and effects
Hormones are chemical messengers carried in blood; each is released by a source gland and acts on target tissues with matching receptors.
| Hormone | Main source | Required role or effect |
|---|---|---|
| adrenaline | adrenal glands | prepares for fight or flight: increases heart rate and blood supply to muscles and raises blood glucose availability |
| insulin | pancreas | lowers high blood glucose by promoting glucose uptake and conversion of glucose to glycogen |
| testosterone | testes | stimulates male secondary sexual characteristics and supports sperm production |
| oestrogen | ovaries | stimulates female secondary sexual characteristics and rebuilds the uterus lining |
| progesterone | ovary, especially corpus luteum | maintains the uterus lining |
A hormone's source is not necessarily its target. Insulin is made by the pancreas rather than the liver; adrenaline by adrenal glands; testosterone by testes; oestrogen and progesterone mainly by ovaries.
Link ADH, FSH and LH to their control roles
ADH, FSH and LH are released from the pituitary into blood, but they act on different target organs and regulate different processes.
| Hormone | Trigger or target | Main role and effect |
|---|---|---|
| ADH | released more when blood water content is low; targets kidney collecting ducts | increases duct permeability and water reabsorption, producing a smaller volume of concentrated urine |
| FSH | targets ovaries | stimulates maturation of an ovarian follicle and secretion of oestrogen |
| LH | targets ovaries | its surge triggers ovulation and supports formation of the corpus luteum |
More ADH produces a smaller volume of more concentrated urine. FSH acts earlier in the ovarian cycle to mature a follicle; LH triggers release of the egg.
At this syllabus level, the pituitary is the required release source for all three. FSH and LH are not produced by the ovaries, and ADH does not directly add water to the blood—it changes kidney permeability.