Cambridge International AS & A Level Biology 9700: Control and coordination – Study Guide

This study guide teaches topic 15, Control and coordination, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers section 15.1 Control and coordination in mammals (learning outcomes 1–12) and section 15.2 Control and coordination in plants (outcomes 1–3). Topic 15 is A Level content, examined on Paper 4 (A Level Structured Questions) alongside AS knowledge, and it can supply the context for Paper 5.

Also for this topic: revision notes and practice questions. For the whole course: the Cambridge A Level Biology hub, the printable 9700 checklist and the free A Level diagnostic.

What this topic covers

Syllabus ref	What you must be able to do
15.1.1–15.1.2	Describe the endocrine system (ADH, glucagon, insulin); compare nervous and endocrine systems
15.1.3–15.1.5	Describe sensory and motor neurones; outline receptor cells; explain a taste-bud chemoreceptor
15.1.6–15.1.8	Explain resting potential, action potential, refractory period, saltatory conduction
15.1.9	Describe a cholinergic synapse and the role of calcium ions
15.1.10–15.1.12	Describe neuromuscular junctions, T-tubules, sarcoplasmic reticulum, sarcomeres and the sliding filament model
15.2.1–15.2.3	Explain the Venus fly trap, auxin and elongation, and gibberellin in barley germination

15.1 Control and coordination in mammals

The endocrine system and the nervous system

Endocrine glands are ductless: they secrete hormones straight into the blood. Hormones reach every tissue, but only target cells with the matching receptors respond. You need three examples, all met in topic 14 (see the homeostasis study guide):

Hormone	Made by	Target	Effect
ADH	Hypothalamus; released by posterior pituitary	Collecting duct cells	Aquaporins inserted, more water reabsorbed
Glucagon	α cells, islets of Langerhans	Liver cells	Glycogen broken down; glucose released
Insulin	β cells, islets of Langerhans	Liver and muscle cells	Glucose uptake and glycogen synthesis increase

Feature	Nervous system	Endocrine system
Signal	Electrical impulses (action potentials), plus chemical at synapses	Chemical (hormones)
Pathway	Neurones	Blood
Speed	Very fast	Slower
Duration	Usually short-lived	Often longer-lasting
Target	Precise: specific muscle or gland cells	Any cell with the right receptor, often widespread

Neurones

	Sensory neurone	Motor neurone
Carries impulses	From receptor to the CNS	From the CNS to an effector
Cell body	Outside the CNS, part-way along (in a ganglion)	Inside the CNS, at one end
Processes	Long dendron to cell body, then axon into CNS	Many short dendrites; one long axon to the effector

Intermediate neurones (relay neurones) connect sensory neurones to motor neurones inside the CNS. Many axons are wrapped in a myelin sheath formed by Schwann cells, with gaps called nodes of Ranvier.

Receptor cells and the taste bud

A sensory receptor cell converts the energy of a stimulus into a change in membrane potential, the receptor potential. If it is large enough to reach the threshold, an action potential is set up in the sensory neurone. A bigger stimulus gives more frequent action potentials, not bigger ones.

Chemoreceptor in a taste bud, detecting salt:

1. Na⁺ from the food diffuses through sodium ion channels into the chemoreceptor cell.
2. The inside becomes less negative: the cell is depolarised (receptor potential).
3. The depolarisation opens voltage-gated calcium ion channels; Ca²⁺ diffuses in.
4. Ca²⁺ causes vesicles to release neurotransmitter onto the sensory neurone.
5. The neurone is depolarised; if threshold is reached, an action potential passes to the brain.

Resting potential

At rest the inside of the axon is about −70 mV relative to the outside. It is maintained by:

• Na⁺–K⁺ pumps using ATP: 3 Na⁺ out for every 2 K⁺ in;
• the membrane being more permeable to K⁺ than Na⁺ (more open K⁺ channels), so K⁺ diffuses out faster than Na⁺ leaks in;
• large negatively charged molecules, such as proteins, staying inside.

The action potential

1. A stimulus opens some voltage-gated Na⁺ channels; Na⁺ diffuses in.
2. If depolarisation reaches threshold, many more Na⁺ channels open (positive feedback). This is all-or-nothing.
3. The potential rises to about +40 mV; the inside is now positive.
4. Na⁺ channels close; voltage-gated K⁺ channels open. K⁺ diffuses out: repolarisation.
5. So many K⁺ leave that the potential briefly dips below −70 mV: hyperpolarisation. The K⁺ channels then close.
6. The Na⁺–K⁺ pump and K⁺ leakage restore the resting potential. During this refractory period another action potential cannot be generated (or needs a much bigger stimulus).

Worked example 1. From −70 mV to +40 mV the membrane potential changes by 40 − (−70) = 110 mV. Write the sign: “rises from −70 mV to +40 mV”, not “from 70 to 40”.

Why the refractory period matters

• Action potentials are separate (discrete) events; they cannot merge.
• An action potential travels one way only, because the membrane behind it is refractory.
• It sets a maximum frequency of impulses. Stimulus strength is coded by frequency, so this limits how strong a signal can be.

Worked example 2. A neurone’s refractory period lasts 2.5 ms. What is the maximum frequency of impulses?

2.5 ms = 0.0025 s
maximum frequency = 1 ÷ 0.0025 = 400 impulses per second (400 Hz)

Saltatory conduction

Myelin insulates the axon, so ions cross the membrane only at the nodes of Ranvier, where the voltage-gated channels are concentrated. Local circuits of current run from one node to the next, and the action potential jumps from node to node. This is saltatory conduction, and it is much faster than conduction along an unmyelinated axon of the same diameter. The practice set works through a speed calculation.

The cholinergic synapse

Structure: a presynaptic knob with many mitochondria and vesicles of acetylcholine (ACh), voltage-gated Ca²⁺ channels in the presynaptic membrane, a synaptic cleft about 20 nm wide, and a postsynaptic membrane with ACh receptors (ligand-gated Na⁺ channels). Acetylcholinesterase sits in the cleft.

Function:

1. An action potential arrives and depolarises the presynaptic membrane.
2. Voltage-gated Ca²⁺ channels open; Ca²⁺ diffuses into the knob.
3. Ca²⁺ causes vesicles to move to and fuse with the presynaptic membrane, releasing ACh by exocytosis.
4. ACh diffuses across the cleft and binds to receptors; Na⁺ channels open and Na⁺ diffuses in.
5. The postsynaptic membrane depolarises; if threshold is reached, an action potential starts.
6. Acetylcholinesterase hydrolyses ACh to choline and ethanoic acid (acetate). Choline is taken back into the knob and recycled with ATP from the mitochondria.

Synapses make impulses travel one way (receptors are only on the postsynaptic side) and let several inputs be combined.

Neuromuscular junction and muscle contraction

A neuromuscular junction is a cholinergic synapse between a motor neurone and a muscle fibre.

1. ACh depolarises the sarcolemma (muscle cell membrane).
2. The depolarisation spreads down the T-tubules (infoldings of the sarcolemma) deep into the fibre.
3. This causes the sarcoplasmic reticulum to release Ca²⁺ into the sarcoplasm.

Ultrastructure of striated muscle

A sarcomere runs from one Z line to the next.

Band	Contains	Appearance	During contraction
I band	Thin (actin) filaments only	Light	Shortens
A band	Full length of thick (myosin) filaments, overlapping actin	Dark	Stays the same
H zone	Myosin only (centre of A band)	Paler	Shortens
M line	Holds myosin filaments together	Line at centre	—

Worked example 3. On an electron micrograph at ×20 000, one sarcomere measures 48 mm.

48 mm = 48 000 µm
actual length = 48 000 ÷ 20 000 = 2.4 µm

After contraction the sarcomere is 1.9 µm. Percentage decrease = (2.4 − 1.9) ÷ 2.4 × 100 = 20.8%. The filaments themselves do not shorten; they slide past each other.

The sliding filament model

1. At rest, tropomyosin covers the myosin-binding sites on actin, held by troponin.
2. Ca²⁺ binds to troponin, which changes shape and moves tropomyosin aside, exposing the binding sites.
3. Myosin heads, already carrying ADP and Pi, bind to actin: cross-bridges form.
4. ADP and Pi are released; the heads tilt (power stroke), pulling actin towards the M line.
5. ATP binds to each myosin head, which detaches from actin.
6. The head hydrolyses ATP (it has ATPase activity) and returns to its upright position, ready to bind again.
7. When stimulation stops, Ca²⁺ is actively transported back into the sarcoplasmic reticulum; tropomyosin blocks the sites again and the muscle relaxes.

15.2 Control and coordination in plants

The Venus fly trap

Each lobe of the modified leaf has sensitive hairs. When an insect bends a hair, ion channels in the hair’s cells open and a receptor potential forms. If a second stimulation follows within a short time, an action potential spreads across the lobes. Closure then follows: it is thought that hydrogen ions are pumped into the cell walls of certain cells, loosening them; water moves in by osmosis, the cells expand, and the lobes flip rapidly from convex to concave, snapping the trap shut. This takes a fraction of a second, making it one of the fastest plant movements.

Auxin and elongation growth

1. Auxin, made in growing tips, binds to receptors in cells of the elongation zone.
2. This stimulates proton pumps in the cell surface membrane to pump H⁺ into the cell wall.
3. The wall is acidified (lower pH). Proteins called expansins, activated at low pH, loosen the links between cellulose microfibrils.
4. The cell takes in water by osmosis; its turgor pressure stretches the loosened wall, so the cell elongates.

Gibberellin and barley germination

1. The seed absorbs water; the embryo releases gibberellin.
2. Gibberellin reaches the aleurone layer.
3. It causes the breakdown of DELLA proteins, which normally inhibit a factor that promotes transcription (see 16.3.4).
4. The amylase gene is transcribed; amylase is made and secreted into the endosperm.
5. Amylase hydrolyses starch to maltose; sugars are used by the embryo for respiration and growth.

Common errors

• “The inside becomes positive” at rest. At rest it is negative; it becomes positive only at the peak of the action potential.
• Saying K⁺ leaves through the Na⁺–K⁺ pump during repolarisation. It leaves through voltage-gated K⁺ channels.
• Forgetting Ca²⁺ at the synapse. Name voltage-gated calcium channels and exocytosis.
• “Acetylcholinesterase breaks down the receptor.” It hydrolyses acetylcholine.
• “The A band shortens.” The A band stays the same; the I band and H zone shorten.
• Leaving ATP out of detachment. ATP binding detaches the myosin head from actin; without ATP the cross-bridges stay locked. Hydrolysis of that ATP returns the head to its upright position.
• “Auxin makes cells divide” for 15.2.2. The outcome is about elongation by acid loosening of walls.

Where to go next

• Condense this page with the control and coordination revision notes, then try the practice set.
• Revise hormones in context with the homeostasis revision notes.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027 (Version 1), published by Cambridge University Press & Assessment (Cambridge International Education). Topic 15 Control and coordination: sections 15.1 Control and coordination in mammals and 15.2 Control and coordination in plants.