Edexcel International GCSE Biology 4BI1: Co-ordination and response – Study Guide

This study guide teaches sub-topic 2(j), Co-ordination and response (specification points 2.80–2.95B), of the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3. The qualification is untiered. Point 2.95B is printed in bold with a B reference, so it is examined on Paper 2 only; every other point can appear on either paper. It applies to the June and November series examined under Issue 3.

Use it with the revision notes and the practice questions. The rest of Topic 2 is on the Edexcel IGCSE Biology hub, and the printable checklist lists every point. The cell-level basics are in the cell structure study guide.

What this sub-topic covers

Spec point	What you must be able to do	Paper
2.80	Understand how organisms respond to changes in their environment	1 and 2
2.81	Define homeostasis; body water content and body temperature as examples	1 and 2
2.82	A co-ordinated response needs a stimulus, a receptor and an effector	1 and 2
2.83–2.84	Plants respond to stimuli; geotropic and phototropic responses of roots and stems	1 and 2
2.85	Role of auxin in the phototropic response of stems	1 and 2
2.86	Nervous and hormonal control, and the differences between them	1 and 2
2.87–2.88	CNS (brain and spinal cord), nerves, electrical impulses	1 and 2
2.89	Role of neurotransmitters at synapses	1 and 2
2.90	Simple reflex arc: withdrawing a finger from a hot object	1 and 2
2.91–2.92	Eye structure and function; focusing; response to light intensity	1 and 2
2.93	Skin and temperature: sweating, vasoconstriction, vasodilation	1 and 2
2.94	Adrenaline, insulin, testosterone, progesterone, oestrogen	1 and 2
2.95B	ADH, FSH and LH	Paper 2 only

1. Responding to change (2.80–2.82)

Organisms survive because they detect changes and respond to them. A change in the internal or external environment that an organism can detect is a stimulus. A receptor detects the stimulus. An effector carries out the response. In animals, effectors are muscles (which contract) and glands (which secrete). Something must link receptor to effector: nerves, hormones, or in plants, plant growth substances such as auxin.

Homeostasis is the maintenance of a constant internal environment. The specification names two examples you must know:

• Body water content – water is gained from food and drink and lost in urine, sweat, faeces and breath. The kidneys adjust how much water leaves in urine so that the water content of the blood stays steady.
• Body temperature – kept close to 37 °C, which keeps enzymes working near their best. The skin carries out much of this control (section 6).

Homeostasis matters because cells work properly only within narrow limits. Too little water makes blood plasma too concentrated, so cells lose water by osmosis. Too high a temperature denatures enzymes.

2. Plant responses (2.83–2.85)

Plants respond to stimuli by growing in a particular direction. A growth response to a directional stimulus is a tropism. It is positive if growth is towards the stimulus and negative if away from it.

Response	Stimulus	Stem (shoot)	Root
Phototropism	Light from one direction	Positive – grows towards light	Negative – grows away from light
Geotropism	Gravity	Negative – grows upwards	Positive – grows downwards

These responses help survival. A shoot that grows towards light absorbs more light for photosynthesis. A root that grows down anchors the plant and reaches water and mineral ions.

How auxin causes phototropism in a stem

1. Auxin is made in the shoot tip and moves down the stem.
2. When light comes from one side, auxin moves to the shaded side.
3. Auxin stimulates cell elongation, so cells on the shaded side elongate more than cells on the lit side.
4. The shaded side grows longer, so the stem bends towards the light.

Note the word “elongation”. Auxin makes existing cells longer in this response; it is not “the cells on the shaded side divide faster”.

Worked example 1

A seedling is lit from the left. After 8 hours the stem tip has bent 28° to the left. Calculate the mean rate of bending, then explain the result.

• Rate = angle ÷ time = 28 ÷ 8 = 3.5° per hour.
• Explanation: auxin made in the tip moves to the shaded (right-hand) side [point 1]; it causes greater cell elongation on the right [point 2]; the right side grows longer, so the stem curves to the left, towards the light [point 3].

A seedling grown in the dark, or lit evenly from above, grows straight up because auxin is spread evenly.

3. Nervous and hormonal communication (2.86–2.89)

Humans co-ordinate responses in two ways.

Feature	Nervous	Hormonal
Signal	Electrical impulses	Chemicals (hormones)
Carried by	Neurones (nerve cells)	Blood plasma
Speed	Very fast	Slower
Duration of effect	Short-lived	Usually longer-lasting
Area affected	Precise, one place	Can affect several organs

The central nervous system (CNS) is the brain and spinal cord. It is linked to the sense organs and to effectors by nerves. When receptors in a sense organ are stimulated, they send electrical impulses along nerves into the CNS. The CNS sends impulses out along other nerves to effectors. This route gives rapid responses.

Three types of neurone carry these impulses: sensory neurones (receptor to CNS), relay neurones (inside the CNS) and motor neurones (CNS to effector).

Synapses

Neurones do not touch. The small gap where one meets the next is a synapse.

1. An impulse reaches the end of the first neurone.
2. A chemical called a neurotransmitter is released into the gap.
3. The neurotransmitter diffuses across the gap.
4. It binds to receptors on the next neurone and starts a new impulse.

Because neurotransmitter is released from one side only, impulses cross a synapse in one direction.

4. The reflex arc (2.90)

A reflex is a rapid, automatic response to a stimulus. It does not need conscious thought, so it protects the body quickly. The specification uses withdrawing a finger from a hot object.

Stage	In the example
Stimulus	Heat from the hot object
Receptor	Temperature (and pain) receptors in the skin of the finger
Sensory neurone	Carries impulses to the spinal cord
Relay neurone	In the spinal cord; passes impulses on, across synapses
Motor neurone	Carries impulses out to the arm
Effector	Muscle in the arm (the biceps) contracts
Response	Hand pulled away from the hot object

Worked example 2

Explain why the hand is pulled away before the person feels pain.

• The pathway passes through the spinal cord, not through the conscious parts of the brain [1].
• Only a few neurones and synapses are involved, so the impulse travels a short route quickly [1].
• Impulses do also travel up the spinal cord to the brain, but this takes longer, so pain is felt after the hand has already moved [1].

5. The eye (2.91–2.92)

The eye is a sense organ containing light receptors.

Structure	Function
Cornea	Transparent front; refracts (bends) light into the eye
Iris	Muscles that control the size of the pupil
Pupil	Hole that lets light in
Lens	Changes shape to focus light on the retina
Ciliary muscle	Contracts or relaxes to change the shape of the lens
Suspensory ligaments	Attach the lens to the ciliary muscle
Retina	Contains light receptors: rods (dim light) and cones (colour)
Fovea	Part of the retina with most cones; gives the sharpest image
Optic nerve	Carries impulses from the retina to the brain
Blind spot	Where the optic nerve leaves; no receptors
Sclera	Tough white outer layer; protects the eye

Focusing (accommodation)

	Near object	Distant object
Ciliary muscle	Contracts	Relaxes
Suspensory ligaments	Slacken	Pulled tight
Lens shape	Fatter (more convex)	Thinner (pulled flatter)
Refraction	More	Less

Responding to light intensity

The iris has circular and radial muscles.

• Bright light: circular muscles contract, radial muscles relax, pupil gets smaller. Less light enters, so the retina is not damaged.
• Dim light: radial muscles contract, circular muscles relax, pupil gets wider. More light reaches the retina.

Worked example 3

A pupil narrows from 7.2 mm in dim light to 2.7 mm in bright light. Calculate the percentage change.

• Change = 2.7 − 7.2 = −4.5 mm.
• Percentage change = (−4.5 ÷ 7.2) × 100 = −62.5% (a 62.5% decrease).
• Always divide by the starting value.

6. Temperature control by the skin (2.93)

Temperature receptors and the brain detect a change in body temperature. The skin then acts as an effector.

When the body is too hot:

• Sweating increases. Sweat glands release sweat onto the skin. Evaporation of the water in sweat uses heat energy from the skin, cooling it.
• Vasodilation. Arterioles (small arteries) supplying the skin capillaries widen. More blood flows near the skin surface, so more heat is lost by radiation.

When the body is too cold:

• Sweating decreases, so less heat is lost by evaporation.
• Vasoconstriction. Arterioles supplying the skin capillaries narrow. Less blood flows near the surface, so less heat is lost.

The blood vessels do not move up or down in the skin. The arterioles change their width, which changes how much blood reaches the surface capillaries.

7. Hormones (2.94 and 2.95B)

A hormone is a chemical secreted by a gland into the blood, which carries it to target organs.

Hormone	Source	Role and effects
Adrenaline	Adrenal glands	Prepares the body for action (“fight or flight”): raises heart rate and breathing rate, raises blood glucose by causing the liver to break down glycogen, widens pupils
Insulin	Pancreas	Lowers blood glucose: causes the liver and muscles to convert glucose to glycogen and cells to take up glucose
Testosterone	Testes	Male secondary sexual characteristics; sperm production
Oestrogen	Ovaries	Female secondary sexual characteristics; repairs and thickens the uterus lining in the menstrual cycle
Progesterone	Ovaries (and placenta in pregnancy)	Maintains the uterus lining

Paper 2 only (2.95B):

Hormone	Source	Role and effects
ADH	Pituitary gland	Makes the collecting ducts of the kidney more permeable to water, so more water is reabsorbed into the blood; urine becomes smaller in volume and more concentrated
FSH	Pituitary gland	Causes an egg to mature in the ovary; stimulates the ovaries to make oestrogen
LH	Pituitary gland	Triggers ovulation (release of the egg)

ADH links back to homeostasis of body water content. If the blood becomes too concentrated (after sweating heavily, for example), more ADH is released and less water is lost in urine. If you drink a lot, less ADH is released and more dilute urine is produced. The full menstrual cycle sequence belongs to Topic 3; see the reproduction and inheritance revision notes.

Common errors

• Saying auxin is “destroyed by light” or that it makes the shaded side cells “divide”. Use “moves to the shaded side” and “cell elongation”.
• Swapping signs: roots are positively geotropic, stems negatively geotropic.
• Writing “the nerve sends a message”. Write “electrical impulses travel along neurones”.
• Missing the relay neurone, or putting the reflex in the brain instead of the spinal cord.
• Saying the ciliary muscles “pull the lens fat”. Contracting ciliary muscles let the ligaments slacken, so the lens becomes fatter by itself.
• “Capillaries move nearer the skin” – they do not. Arterioles widen or narrow.
• “Sweat cools you.” It is the evaporation of sweat that removes heat.
• Calling glycogen “glucagon” when describing insulin’s effect.

Next steps

Condense this into the revision notes, then test yourself with the practice questions. The free diagnostics show which topics need most work.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1) specification, Issue 3, first examination June 2019, Pearson Education Limited – Topic 2, Structures and functions in living organisms, sub-topic (j) Co-ordination and response.