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Notes/Biology/Paper 4/Control and Coordination
CAIEA Level9700§15

Control and Coordination

The endocrine system (ADH, glucagon, insulin) versus the nervous system; structure of sensory and motor neurones; sensory receptor cells; the action potential in a chemoreceptor cell; the resting potential and how it is maintained; depolarisation, repolarisation, hyperpolarisation and the refractory period; saltatory conduction in myelinated axons; cholinergic synapses and the role of Ca²⁺; neuromuscular junctions, T-tubules and the sarcoplasmic reticulum; striated-muscle ultrastructure and the sarcomere; the sliding-filament model (troponin, tropomyosin, Ca²⁺, ATP); the Venus fly-trap; auxin in elongation growth; and gibberellin in barley germination and the DELLA-repressor mechanism.

200 min read 9 sub-topics
170
question parts
2021–2025 · 37 papers
14 marks
per paper
≈ 14% of the paper
2.4/3
avg difficulty
demanding
#3
most examined
of 10 topics by marks

Every multicellular organism has to coordinate the activity of its cells — sometimes in milliseconds (a hand on a hot surface, a predator lunging), sometimes over hours or days (growth, germination, the menstrual cycle). Mammals do this with two systems that work in parallel: the nervous system (fast, electrical, point-to-point) and the endocrine system (slower, hormonal, body-wide). The first part of this note (§§01–07) is the nervous system and the muscle it drives; the second part (§08) is the plant equivalent — touch responses, growth hormones, and germination.

The route through the note is: §01 the two coordination systems side by side; §02 the three types of neurone (sensory, motor, intermediate) and what each looks like under the microscope; §03 the action potential — what the resting potential is, how a stimulus triggers depolarisation, and how the cell returns to rest (the chemoreceptor-cell example from the syllabus is used throughout); §04 why myelinated axons transmit impulses faster, with the nodes of Ranvier and saltatory conduction; §05 the cholinergic synapse — the structure of the presynaptic terminal, the role of Ca²⁺, and how neurotransmitters carry the signal across the synaptic cleft; §06 the neuromuscular junction and the T-tubule / sarcoplasmic-reticulum system that delivers Ca²⁺ deep into the muscle fibre; §07 striated-muscle ultrastructure (sarcomere, A-band, I-band, H-zone, Z-line) and the sliding-filament model (troponin, tropomyosin, Ca²⁺, ATP, cross-bridge cycling); §08 the plant half — the Venus fly-trap's electrical response, auxin's proton-pump mechanism for elongation, and gibberellin's release of DELLA-repressor proteins to allow α-amylase synthesis in germinating barley.

Across 2021–2025 this is one of the heavier A2 topics on Paper 4 — 170 leaf parts, 522 marks, mean difficulty 2.42, across 37 papers. The mark schemes test it in many guises: action-potential traces to read and interpret, synapses to label, sliding-filament diagrams with proteins to identify, T-tubule / sarcoplasmic-reticulum flow charts, and auxin-and-gibberellin graphs from the germination experiments. Several of these are MCQ-style on Paper 1 too (sensory-receptor type, hormone identity, sarcomere band) — so the same diagram is reused twice. Almost every question has a real exam image on the front, so the note uses the question-bank 'image_path' wherever the original paper prints one.

Before you start you should be able to

  • The cell-signalling ideas introduced in §6 (nucleic acids and protein synthesis) — that cell-surface receptors bind signalling molecules and trigger intracellular cascades

  • The membrane-transport work in §4 — that diffusion moves solutes down their concentration gradient, that facilitated diffusion uses protein channels, and that active transport requires ATP and carrier proteins

  • The enzyme work in §3 — that enzymes are specific to their substrate (essential for the gibberellin → α-amylase link in §08) and that temperature affects enzyme rate

  • The transport-in-mammals work in §8 — that the blood carries hormones from their gland of origin to their target cells, and that tissue fluid is the immediate environment of every cell

  • The homeostasis work in §14 — that insulin and glucagon control blood glucose, that ADH controls water reabsorption, and the cAMP second-messenger cascade (which §05 reuses, but with Ca²⁺ as the second messenger instead of cAMP)

By the end of this page you can

  • Describe the features of the endocrine system with reference to the hormones ADH, glucagon and insulin (see §14.1.8, §14.1.9 and §14.1.10)

  • Compare the features of the nervous system and the endocrine system

  • Describe the structure and function of a sensory neurone and a motor neurone and state that intermediate neurones connect sensory neurones and motor neurones

  • Outline the role of sensory receptor cells in detecting stimuli and stimulating the transmission of impulses in sensory neurones

  • Describe the sequence of events that results in an action potential in a sensory neurone, using a chemoreceptor cell in a human taste bud as an example

  • Describe and explain changes to the membrane potential of neurones, including: how the resting potential is maintained; the events that occur during an action potential; how the resting potential is restored during the refractory period

  • Describe and explain the rapid transmission of an impulse in a myelinated neurone with reference to saltatory conduction

  • Explain the importance of the refractory period in determining the frequency of impulses

  • Describe the structure of a cholinergic synapse and explain how it functions, including the role of calcium ions

  • Describe the roles of neuromuscular junctions, the T-tubule system and sarcoplasmic reticulum in stimulating contraction in striated muscle

  • Describe the ultrastructure of striated muscle with reference to sarcomere structure using electron micrographs and diagrams

  • Explain the sliding filament model of muscular contraction including the roles of troponin, tropomyosin, calcium ions and ATP

  • Describe the rapid response of the Venus fly trap to stimulation of hairs on the lobes of modified leaves and explain how the closure of the trap is achieved

  • Explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls

  • Describe the role of gibberellin in the germination of barley (see §16.3.4)

  • Explain that gibberellin stimulates the breakdown of DELLA repressor proteins

01

Two coordination systems — nervous and endocrine

Syllabus requirement · §15.1

“

describe the features of the endocrine system with reference to the hormones ADH, glucagon and insulin (see §14.1.8, §14.1.9 and §14.1.10); compare the features of the nervous system and the endocrine system.

”

Why two systems?

Every multicellular animal has to coordinate the activity of millions of cells. Some responses have to be fast (a finger on a hot kettle, a sudden sound, the moment a gazelle spots a lion) — the reaction is over in milliseconds. Other responses are slower but longer-lasting (digestion, growth, water balance, blood glucose, reproduction) — they take seconds to days. Mammals solve both problems with two parallel coordination systems, each tuned to a different timescale:

  • The nervous system — fast (ms), electrical, point-to-point.
  • The endocrine system — slow (s to days), hormonal, body-wide.

The two systems overlap (the hypothalamus is part of both; an impulse travelling down a motor neurone causes the release of a neurotransmitter, which is a chemical signal, and is therefore a tiny "endocrine" event in a localised area), but the comparison below is what the mark scheme tests.

The endocrine system, in one sentence

The endocrine system is a set of ductless glands that secrete hormones directly into the blood. The hormones travel in the blood to target cells that have the right receptor, and they trigger a slower but longer-lasting response than a nerve impulse. The three hormones the syllabus names in this topic are the same three that §14 already taught in the context of homeostasis: ADH (made in the hypothalamus, released from the posterior pituitary — see §14.1.8), glucagon (made in the α cells of the pancreatic islets — see §14.1.9), and insulin (made in the β cells of the pancreatic islets — see §14.1.10). The endocrine features the MS tests here are:

  • Ductless glands — hormones are secreted into the blood, not into a duct.
  • Hormones travel in the blood — they reach every cell in the body, but only the target cells (with the right receptor) respond.
  • Slow to start, slow to stop — the response takes seconds to set up, but persists for seconds to days.
  • Amplified cascade — many hormone molecules can activate many intracellular cascades, so a small hormonal signal can produce a large physiological response.

The nervous system, in one sentence

The nervous system is a network of neurones that transmit electrical impulses along their cell-surface membranes. An impulse travels along a neurone at up to 100 m s⁻¹, and at a synapse the impulse triggers the release of a neurotransmitter (a chemical signal) that carries the message across the synaptic cleft to the next neurone (or to an effector such as a muscle or a gland). The features the MS tests here are:

  • Neurones — the cellular units of the system, with a cell body, dendrites and an axon.
  • Electrical impulses — the message travels as a wave of depolarisation (an action potential, see §03) along the axon.
  • Fast and point-to-point — the response is over in milliseconds and is targeted to one specific effector.
  • Neurotransmitter at the synapse — the signal is converted from electrical to chemical and back to electrical.

Feature

Nervous system

Endocrine system

Signal type

Electrical impulse (action potential)

Chemical hormone in the blood

Pathway

Along a neurone; across a synapse by neurotransmitter

In the blood, throughout the body

Speed of response

Very fast (milliseconds)

Slow (seconds to days)

Duration of response

Short (milliseconds to seconds)

Long (seconds to days)

Target

Specific effector (one muscle, one gland)

Any cell with the right receptor — potentially body-wide

Cellular units

Neurones

Endocrine (ductless) glands

Example of stimulus

Touching a hot kettle

Low blood glucose

Example of response

Withdraw the hand

Glucagon → liver releases glucose

Example of messenger

Acetylcholine (at a cholinergic synapse, §05)

ADH, glucagon, insulin (§14)

Comparing the nervous and endocrine systems. The MS credits both halves of any comparison — the two columns are tested by 'compare and contrast' questions, and the MS gives one mark per row of meaningful difference.

The MS only credits real differences

A common mark-losing answer is to write a long sentence that says "the nervous system is fast and the endocrine system is slow, and the nervous system uses neurones and the endocrine system uses hormones, and the nervous system is short and the endocrine system is long". The MS gives one mark per row of difference, not one mark per word. A better answer uses a table (like the one above) — each row is one mark, and the comparison is visible to the examiner at a glance. A non-table answer should still aim for 4–5 distinct rows of comparison.

Comparing nervous and endocrine systems

9700/42 O/N 2023 Q9(b)4 marks

Outline the differences between the endocrine system and the nervous system.

Show full working
  1. 1

    Speed: the nervous system is fast (milliseconds); the endocrine system is slow (seconds to days). The nervous system transmits electrical impulses along neurones; the endocrine system secretes hormones into the blood.

    Mark 1. The MS credits 'fast vs slow'.

  2. 2

    Duration: the nervous system response is short-lived; the endocrine system response is long-lasting. A nerve impulse is over in milliseconds; a hormone may stay in the blood for minutes to hours.

    Mark 2. The MS credits 'short vs long (duration)'.

  3. 3

    Target: the nervous system targets a specific effector (one muscle, one gland); the endocrine system targets any cell with the right receptor. Nerve impulses travel point-to-point; hormones travel in the blood and reach every cell, but only target cells respond.

    Mark 3. The MS credits 'specific / point-to-point vs body-wide / any cell with receptor'.

  4. 4

    Mechanism: the nervous system uses neurones and electrical impulses (with neurotransmitter at the synapse); the endocrine system uses ductless glands and hormones in the blood. This is a fourth distinct row of difference.

    Mark 4. The MS credits 'different mechanism / cellular units'.

Answer

The nervous system is fast and short-lived, and targets a specific effector via electrical impulses along neurones. The endocrine system is slower, longer-lasting, and targets any cell with the right receptor via hormones in the blood.

On a 4-mark 'outline the differences' question, the answer is four distinct rows of difference. An 'outline the differences' question is not asking for a description of either system on its own — it is asking for the differences, and the MS gives one mark per difference. Do not list features of the nervous system in one paragraph and features of the endocrine system in another; the comparison must be visible in the answer.

Your turn — coordination systems

  1. 14 marks

    A student writes: "The endocrine system and the nervous system are different because one is in the brain and the other is in the body."

    (a) State two errors in this statement.
    (b) Rewrite the statement correctly.

    Stuck? Show hint

    (a) Two errors: the location claim, and the verb. (b) The location is wrong for both systems, and the difference is not where they are but how they signal.

    Show solution
    1. 1

      Error 1: the nervous system is not just in the brain. The nervous system includes the brain, the spinal cord, and the peripheral nerves that run throughout the body. Endocrine glands (e.g. pancreas, adrenal, thyroid) are also throughout the body, not just in one place.

      Mark 1. The MS credits the location error.

    2. 2

      Error 2: the difference between the two systems is not location, but the type of signal (electrical impulse along neurones vs hormone in the blood) and the speed/duration of the response.

      Mark 2. The MS credits the framing error.

    3. 3

      (b) Correct statement: the endocrine system and the nervous system differ in the type of signal (hormones in the blood vs electrical impulses along neurones), the speed of the response (slow vs fast), the duration of the response (long vs short), and the target (body-wide via the blood vs a specific effector). Both systems are distributed throughout the body — the nervous system via the brain, spinal cord and peripheral nerves, and the endocrine system via ductless glands such as the hypothalamus, pituitary, pancreas, adrenal, thyroid and gonads.

      Marks 3 and 4. The MS credits the corrected framing.

    Answer

    (a) Two errors: the nervous system is not just in the brain (it includes the spinal cord and peripheral nerves), and the difference is not location but the type of signal (electrical vs hormonal) and the speed / duration / target of the response. (b) The two systems differ in the type of signal (hormones in the blood vs electrical impulses along neurones), the speed of the response (slow vs fast), the duration (long vs short), and the target (body-wide vs specific effector). Both systems are distributed throughout the body.

Practise the two coordination systemsReal past-paper questions · Comparing nervous and endocrine systems

The rest of this note

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Can you do all of these?

  • Compare the nervous system (fast, electrical, point-to-point, short-lived) with the endocrine system (slow, hormonal, body-wide, long-lived); give one example of each

  • Describe the three types of neurone (sensory, intermediate, motor) and where their cell bodies and axons sit

  • Outline the role of a chemoreceptor cell in a taste bud: tastant binds microvillar receptor → generator potential → if threshold reached, action potential in the sensory neurone

  • Explain how the resting potential is maintained: more permeable to K⁺ than Na⁺ at rest → K⁺ diffuses out → fixed anions make the inside negative → Na⁺/K⁺ pump maintains the gradients (3 Na⁺ out, 2 K⁺ in per ATP)

  • Describe the action potential: stimulus → threshold → voltage-gated Na⁺ channels open → depolarisation to +30 mV → Na⁺ channels inactivate, K⁺ channels open → repolarisation → hyperpolarisation → Na⁺/K⁺ pump restores the resting potential

  • Explain why the action potential is all-or-nothing (above threshold, the size is fixed; a stronger stimulus gives a higher frequency of action potentials, not a larger one)

  • Explain the refractory period: absolute (Na⁺ channels inactivated — no stimulus can trigger another AP) and relative (membrane hyperpolarised — only a stronger stimulus can trigger another AP); the refractory period limits the frequency of impulses and enforces one-way propagation

  • Explain saltatory conduction: myelin insulates the axon between nodes of Ranvier; the action potential is regenerated only at the nodes; the depolarisation 'jumps' from node to node; this is faster than continuous conduction

  • Describe the structure of a cholinergic synapse: presynaptic terminal with vesicles and mitochondria; synaptic cleft ~20 nm; postsynaptic membrane with receptors

  • Explain the role of Ca²⁺ at a cholinergic synapse: action potential opens voltage-gated Ca²⁺ channels → Ca²⁺ flows in → Ca²⁺ triggers vesicles to fuse with the presynaptic membrane → ACh is released by exocytosis → ACh diffuses across the cleft → ACh binds postsynaptic receptors → Na⁺ channels open → new action potential if threshold reached

  • Explain why a synapse is one-way: neurotransmitter is only in the presynaptic terminal; receptors are only on the postsynaptic membrane; Ca²⁺ channels are only on the presynaptic membrane

  • Explain the role of the neuromuscular junction, T-tubules and sarcoplasmic reticulum: NMJ releases ACh → sarcolemma depolarises → depolarisation spreads down the T-tubules → SR releases Ca²⁺ into the cytoplasm → contraction

  • Identify the parts of the sarcomere: Z-line, I-band (thin filaments only), A-band (thick filaments only, full length of myosin), H-zone (centre of A-band, thick filaments only at rest), M-line (centre of H-zone)

  • Explain the sliding-filament model: thin filaments slide over the thick filaments toward the M-line; the filaments do not change length; the sarcomere shortens; the I-band and H-zone shorten, but the A-band does not

  • Describe the cross-bridge cycle: ATP hydrolyses → myosin head cocks → myosin binds actin (cross-bridge) → power stroke (ADP + Pi released) → new ATP binds → myosin detaches → cycle repeats

  • Explain the role of troponin, tropomyosin, Ca²⁺ and ATP: in a relaxed muscle, tropomyosin blocks the myosin-binding sites on actin; Ca²⁺ (from the SR) binds troponin; troponin pulls tropomyosin away, exposing the binding sites; myosin can now bind and the cycle runs; ATP is needed to cock the myosin head and to break the cross-bridge

  • Describe the Venus fly-trap: trigger hair touched twice → action potential → Ca²⁺ influx → K⁺ and Cl⁻ efflux from the inner layer of the lobe → water follows by osmosis → inner layer flaccid, outer layer turgid → trap snaps shut

  • Explain the role of auxin in elongation growth: auxin binds membrane receptor → activates proton pump → H⁺ pumped into cell wall → acidic pH activates expansins → cell wall loosens → water enters by osmosis → cell elongates

  • Explain the role of gibberellin in barley germination: embryo releases gibberellin → gibberellin binds GID1 receptor in aleurone cells → complex binds DELLA repressor → DELLA is broken down → α-amylase gene de-repressed → α-amylase synthesised and secreted → α-amylase hydrolyses starch to maltose (then to glucose) → glucose absorbed by the embryo and respired

Now do the questions
170 real Paper 4 parts from 2021–2025, sorted by difficulty, with mark schemes