Edexcel International GCSE Biology 4BI1: Co-ordination and response – Revision Notes

These notes condense sub-topic 2(j), Co-ordination and response (points 2.80–2.95B), of the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3, for the June and November series examined under Issue 3. The course is untiered; point 2.95B (ADH, FSH and LH) is a bold B statement and is examined on Paper 2 only. For full explanations and worked examples, read the study guide first.

Then test yourself with the practice questions. The rest of the course is on the Edexcel IGCSE Biology hub, and the printable checklist lists every point. Earlier parts of Topic 2 are in the cell structure revision notes. The free diagnostics show which topics need most work.

Key definitions (2.80–2.82)

Term	Definition
Stimulus	A change in the environment (internal or external) that can be detected
Receptor	Detects a stimulus (e.g. light receptors in the retina)
Effector	Carries out the response: a muscle (contracts) or a gland (secretes)
Homeostasis	Maintenance of a constant internal environment
Tropism	Growth response of a plant to a directional stimulus
Reflex	Rapid, automatic response that does not need conscious thought
Hormone	Chemical secreted by a gland into the blood, acting on target organs

The two homeostasis examples the specification names: body water content and body temperature.

The co-ordinated response chain:

stimulus → receptor → co-ordinator (CNS / hormones) → effector → response

Plants: tropisms and auxin (2.83–2.85)

	Stem	Root
Phototropism (light)	Positive	Negative
Geotropism (gravity)	Negative	Positive

Method in steps – explaining a stem bending to light:

1. Auxin is made in the shoot tip.
2. Light from one side → auxin moves to the shaded side.
3. More auxin → more cell elongation on the shaded side.
4. Shaded side grows longer → stem bends towards the light.
5. Benefit: leaves absorb more light for photosynthesis.

Even light or darkness → auxin spread evenly → stem grows straight.

Plant vs animal responses: plants respond by growth, which is slow and usually permanent; animals respond with muscles and glands, which is fast and reversible. Both still follow stimulus → receptor → effector. In a stem, the tip detects the light and the elongating cells further down carry out the response.

Nervous and hormonal systems (2.86–2.89)

	Nervous	Hormonal
Signal	Electrical impulse	Chemical
Route	Neurones	Blood
Speed	Fast	Slow
Effect lasts	Short time	Longer
Target	Precise	Widespread

• CNS = brain + spinal cord, linked to sense organs by nerves.
• Receptors stimulated → impulses along nerves into the CNS → impulses out to effectors → rapid response.
• Neurones: sensory (in), relay (within CNS), motor (out).

Synapse in four steps: impulse arrives → neurotransmitter released → diffuses across gap → binds to receptors on next neurone, starting a new impulse. One-way only.

Reflex arc (2.90)

heat → receptor in skin → sensory neurone → relay neurone (spinal cord)
     → motor neurone → muscle (biceps) contracts → hand withdrawn

Fast because the route is short, has few synapses and bypasses conscious thought. Protects the body from harm.

The eye (2.91–2.92)

Part	Job
Cornea	Refracts light
Iris	Controls pupil size
Lens	Fine focusing
Ciliary muscle + suspensory ligaments	Change lens shape
Retina	Receptors: rods (dim light), cones (colour)
Fovea	Most cones; sharpest vision
Optic nerve	Impulses to brain
Blind spot	No receptors

Accommodation:

Looking at	Ciliary muscle	Ligaments	Lens
Near object	Contracts	Slack	Fat, refracts more
Distant object	Relaxes	Tight	Thin, refracts less

Pupil size:

Light	Circular muscle	Radial muscle	Pupil
Bright	Contracts	Relaxes	Constricts
Dim	Relaxes	Contracts	Dilates

Skin and temperature (2.93)

Too hot	Too cold
More sweat; evaporation removes heat	Less sweat
Vasodilation: arterioles to skin widen	Vasoconstriction: arterioles to skin narrow
More blood near surface, more heat radiated	Less blood near surface, less heat lost

Hormones (2.94, 2.95B)

Hormone	Gland	Key effect
Adrenaline	Adrenal	Heart and breathing rate up; liver glycogen → glucose; pupils widen
Insulin	Pancreas	Blood glucose down; glucose → glycogen in liver and muscles
Testosterone	Testes	Male secondary sexual characteristics; sperm production
Oestrogen	Ovaries	Female secondary sexual characteristics; builds uterus lining
Progesterone	Ovaries	Maintains uterus lining
ADH (Paper 2 only)	Pituitary	Collecting duct more permeable → more water reabsorbed → less, more concentrated urine
FSH (Paper 2 only)	Pituitary	Egg matures; ovaries make oestrogen
LH (Paper 2 only)	Pituitary	Triggers ovulation

The menstrual cycle itself is in Topic 3: see the reproduction and inheritance revision notes.

Worked reminders

Percentage change. Subtract the starting value from the final value, divide by the starting value, multiply by 100. A pupil that narrows from 5.0 mm to 2.0 mm changes by (2.0 − 5.0) ÷ 5.0 × 100 = −60%, a 60% decrease. Dividing by 2.0 instead gives a wrong answer of −150%.

Rate from a graph or table. Rate = change in the quantity ÷ time taken. Always give the unit, for example “degrees per hour” for a bending shoot or “mmol/dm³ per minute” for blood glucose.

“Use the data” questions. Quote at least one pair of numbers from the table or graph, then link them to the biology. “More auxin on the shaded side (48 vs 27 units), so more cell elongation there” is the shape that scores.

Method in steps – extended answers

Body too hot (6-mark style):

1. Temperature rise detected (receptors and brain).
2. Sweat glands release more sweat.
3. Water in sweat evaporates, using heat energy from the skin.
4. Arterioles supplying skin capillaries dilate (vasodilation).
5. More blood flows near the skin surface.
6. More heat lost to the surroundings by radiation.

Body too cold: reverse steps 2–6: less sweat; arterioles constrict (vasoconstriction); less blood near the surface; less heat lost.

Water balance (Paper 2 only, links 2.81 to 2.95B):

1. Sweating or little drinking → blood more concentrated.
2. Pituitary gland releases more ADH.
3. Collecting ducts become more permeable to water.
4. More water reabsorbed into the blood.
5. Small volume of concentrated urine.

Drinking a lot gives the opposite: less ADH, less water reabsorbed, large volume of dilute urine.

Command words for this sub-topic

Command word	What it needs here
State / Name	A single term: “relay neurone”, “pancreas”
Describe	The sequence of events, in order, no reasons needed: the reflex pathway, accommodation steps
Explain	Each step plus why: “less blood near the surface, so less heat lost”
Suggest	Apply the ideas to an unfamiliar organism or set-up
Calculate	Show the working and give a unit

Must-know distinctions

• Glycogen (storage carbohydrate) vs glucagon (a different hormone, not on this specification list). Insulin turns glucose into glycogen.
• Vasodilation (widening, when hot) vs vasoconstriction (narrowing, when cold).
• Positive tropism (towards) vs negative tropism (away).
• Receptor (detects) vs effector (responds).
• Contracting ciliary muscle = near focus; relaxing = far focus.
• Neurone (one cell) vs nerve (a bundle of neurones).

Quick self-test

1. Name the two examples of homeostasis given in the specification.
2. A root is placed horizontally. In which direction does it grow, and what is the response called?
3. To which side of a stem lit from the right does auxin move?
4. State what auxin does to cells in a stem.
5. Name the parts of the central nervous system.
6. What crosses the gap at a synapse, and by what process?
7. Name the neurone found in the spinal cord in a reflex arc.
8. A person looks up from a book to a distant hill. What happens to the ciliary muscle and the lens?
9. A pupil widens from 3.0 mm to 6.6 mm in a dark room. Calculate the percentage increase.
10. A runner loses sweat at 0.6 dm³ per hour for 2.5 hours. Calculate the volume of sweat lost.
11. Name the gland that secretes insulin and state its effect on blood glucose.
12. (Paper 2 only) Name the hormone that triggers ovulation and the gland that releases it.

Answers

1. Body water content and body temperature.
2. Downwards (towards gravity): positive geotropism.
3. The left (shaded) side.
4. Stimulates cell elongation.
5. Brain and spinal cord.
6. Neurotransmitter; diffusion.
7. Relay neurone.
8. Ciliary muscle relaxes; suspensory ligaments tighten; lens becomes thinner and refracts light less.
9. (6.6 − 3.0) ÷ 3.0 × 100 = 120%.
10. 0.6 × 2.5 = 1.5 dm³.
11. Pancreas; lowers blood glucose.
12. LH; pituitary gland.

Where marks are usually lost

• Writing “auxin moves away from light” without saying which side of the stem it ends up on.
• Saying auxin makes cells divide rather than elongate.
• Mixing up which organ (root or stem) is positive for each tropism.
• Describing a reflex arc without the relay neurone or without naming the spinal cord.
• Forgetting that the neurotransmitter diffuses and binds to receptors on the next neurone.
• Reversing accommodation: the ciliary muscle contracts for near objects.
• Mixing up the circular and radial muscles of the iris.
• Saying capillaries “move closer to the skin” instead of arterioles widening.
• Writing “glucagon” when you mean glycogen.
• Giving a hormone’s effect without its source gland when the question asks for both.

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.