Control and coordination

• Every event in this chapter is an ion passing through a named protein, in a fixed order: Na+ in, K+ out, Ca2+ in to release a transmitter, Ca2+ out of the SR to start contraction, H+ out into the plant cell wall.

• 15.1.1 I can describe — 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)
• 15.1.2 I can compare — compare the features of the nervous system and the endocrine system
• 15.1.3 I can describe — describe the structure and function of a sensory neurone and a motor neurone and state that intermediate neurones connect sensory neurones and motor neurones
• 15.1.4 I can outline — outline the role of sensory receptor cells in detecting stimuli and stimulating the transmission of impulses in sensory neurones
• 15.1.5 I can describe — 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
• 15.1.6 I can describe — 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
• 15.1.7 I can describe — describe and explain the rapid transmission of an impulse in a myelinated neurone with reference to saltatory conduction
• 15.1.8 I can explain — explain the importance of the refractory period in determining the frequency of impulses
• 15.1.9 I can describe — describe the structure of a cholinergic synapse and explain how it functions, including the role of calcium ions
• 15.1.10 I can describe — describe the roles of neuromuscular junctions, the T-tubule system and sarcoplasmic reticulum in stimulating contraction in striated muscle
• 15.1.11 I can describe — describe the ultrastructure of striated muscle with reference to sarcomere structure using electron micrographs and diagrams
• 15.1.12 I can explain — explain the sliding filament model of muscular contraction including the roles of troponin, tropomyosin, calcium ions and ATP
• 15.2.1 I can describe — 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
• 15.2.2 I can explain — explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls
• 15.2.3 I can describe — describe the role of gibberellin in the germination of barley (see 16.3.4)

Precise vocabulary is part of the biology. Water moves down a water potential gradient; an active site is complementary to its substrate; enzymes are denatured (they are not alive); ATP releases energy when it is hydrolysed, and respiration never produces energy. Give a calculated answer to the same number of significant figures as the least precise data, or one more, with its unit. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as the skills those papers test — method, recording, analysis and evaluation — rather than as theory.

• “When a muscle contracts, every band of the sarcomere gets shorter.” Correct The A band does not change length: it is the length of the myosin filaments, and no filament shortens. The sarcomere, the I band and the H zone shorten because the filaments slide further over each other.
• “The Na+/K+ pump moves 3 K+ out and 2 Na+ in.” Correct 3 Na+ out, 2 K+ in for each ATP hydrolysed. Sodium is pumped out; that is why Na+ rushes in when its channels open.
• “The pump makes the resting potential.” Correct The pump builds the concentration gradients. The potential itself is mainly caused by the membrane being far more permeable to K+ than to Na+, so K+ diffuses out and leaves the inside negative.
• “Repolarisation happens because Na+ is pumped back out.” Correct Repolarisation is K+ diffusing out through voltage-gated K+ channels. The pump is far too slow to reverse an action potential.
• “A stronger stimulus makes a bigger action potential.” Correct Action potentials are all-or-nothing. A stronger stimulus gives a larger receptor potential and a higher frequency of action potentials.
• “Ca2+ binds to tropomyosin.” Correct Ca2+ binds to troponin; troponin changes shape and moves tropomyosin off the myosin-binding sites on actin.
• “Auxin makes the shoot bend towards the light.” Correct At A Level, auxin is explained by acid growth: it stimulates proton pumps, which acidify the cell wall so that it loosens and the cell elongates. Growth responses to light are not part of this syllabus.
• “Hormones travel along nerves to their target organs.” Repair Hormones travel in the blood plasma and act on target cells with specific receptors; impulses travel along neurones.
• “The cell body of a sensory neurone is in the spinal cord.” Repair It is in a ganglion just outside the CNS, on a side branch. The motor neurone’s cell body is in the CNS.
• “In the taste bud, Na+ enters the sensory neurone and causes an action potential.” Repair Na+ enters the chemoreceptor cell, giving a receptor potential; Ca2+ entry then releases a neurotransmitter onto the sensory neurone, which fires if it reaches threshold.
• “The resting potential is caused by the pump alone.” Repair The pump sets up the gradients; the potential is mainly due to the membrane being more permeable to K+, so K+ diffuses out.
• “The Na+/K+ pump moves 3 K+ out and 2 Na+ in.” Repair 3 Na+ out, 2 K+ in, per ATP.
• “During repolarisation Na+ is pumped out.” Repair Repolarisation is K+ diffusing out through voltage-gated K+ channels.
• “A stronger stimulus gives a bigger action potential.” Repair Action potentials are all-or-nothing; a stronger stimulus gives a higher frequency of them.
• “Myelin speeds up the impulse by conducting it.” Repair Myelin insulates; action potentials occur only at the nodes of Ranvier, and local circuits run between nodes.
• “The refractory period is when the neurone is resting.” Repair It is the recovery time after an action potential, when the voltage-gated Na+ channels cannot reopen; it limits the maximum frequency.
• “ACh is released when Na+ enters the presynaptic knob.” Repair Ca2+ entry through voltage-gated Ca2+ channels triggers exocytosis.
• “Acetylcholinesterase makes acetylcholine.” Repair It hydrolyses ACh in the cleft, so the receptor channels close.
• “The T-tubules store calcium.” Repair The T-tubules carry the action potential into the fibre; the sarcoplasmic reticulum stores and releases Ca2+.
• “In contraction the actin and myosin filaments shorten.” Repair The filaments slide; the sarcomere, I band and H zone shorten; the A band stays the same.
• “Ca2+ binds to tropomyosin.” Repair Ca2+ binds to troponin, which moves tropomyosin.
• “ATP is used to make the myosin head bind to actin.” Repair ATP binding detaches the head; ATP hydrolysis re-cocks it.
• “The Venus fly trap closes because its cells lose water and it collapses shut, and this is fully understood.” Repair The lobes flip from convex to concave, releasing stored elastic energy, after rapid turgor changes; the cellular account (H+ pumped into walls, water entering outer cells) is the A Level account and is still being researched.
• “Auxin makes cells divide to grow longer.” Repair Auxin makes cells elongate by stimulating proton pumping, which acidifies and loosens the wall.
• “Gibberellin digests the starch in the seed.” Repair Gibberellin switches on the amylase gene in the aleurone layer; amylase hydrolyses the starch.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. Each has the syllabus’s own meaning. State: express in clear terms. Give: produce an answer from a given source or recall/memory. Identify: name/select/recognise. Define: give a precise meaning. Outline: set out the main points. Describe: state the points of a topic / give characteristics and main features. Explain: set out purposes or reasons / make the relationships between things clear / say why and/or how and support with relevant evidence. Compare: identify/comment on similarities and/or differences; contrast: identify/comment on differences. Discuss: write about issue(s) or topic(s) in depth in a structured way; assess: make an informed judgement. Suggest: apply knowledge and understanding to situations where there are a range of valid responses in order to make proposals / put forward considerations. In this chapter the membrane potential changes (15.1.6) and saltatory conduction (15.1.7) are describe and explain outcomes, so an answer needs both the events and the reasons for them.
• Choose the right system for a new example. If a question describes a response that begins within a fraction of a second and affects one muscle, it is nervous. If it takes minutes, lasts hours and changes the metabolism of many tissues, it is endocrine. The reasoning is the same four rows of the table: signal, carrier, speed, extent.
• Interleave with the chapters that use this one. Chapter 16 (inheritance) teaches the DELLA mechanism behind gibberellin (16.3.4): when you reach it, re-answer ‘describe the role of gibberellin in barley germination’ with the gene-control step in full. Chapter 14 (homeostasis) supplies the hormones used here as endocrine examples: when you revise it, re-answer the nervous–endocrine comparison. And any later Paper 5 practice on a statistical test is a chance to redo the acid-growth t-test. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 15 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• Topic 15 is not in Paper 1, so there are no multiple-choice items on it. A Paper 4 question asks you to describe a sequence (the taste bud, the synapse, the neuromuscular junction, the cross-bridge cycle) with each ion and each channel or pump named; to explain a membrane potential change; to compare the nervous and endocrine systems row by row; and to explain how a plant response is achieved.
• An action-potential trace to read (values in mV and ms, supplied by the question); a conduction speed to calculate from a distance and a time; a maximum frequency from a refractory period; an electron micrograph of sarcomeres to label and measure, with actual size from the magnification; a table of transmitter release or growth to describe with figures quoted.
• No investigation is named in this topic. Barley half-grains on starch agar supply one: gibberellin concentration (made by serial dilution) is the independent variable, the width of the clear zone after iodine the dependent one, with grain batch, temperature and time standardised, grains with no gibberellin as the control, and the fuzzy edge of the zone as the main random error. Stem segments in acidic and neutral buffer supply a Paper 5 t-test.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. Each has the syllabus’s own meaning. State: express in clear terms. Give: produce an answer from a given source or recall/memory. Identify: name/select/recognise. Define: give a precise meaning. Outline: set out the main points. Describe: state the points of a topic / give characteristics and main features. Explain: set out purposes or reasons / make the relationships between things clear / say why and/or how and support with relevant evidence. Compare: identify/comment on similarities and/or differences; contrast: identify/comment on differences. Discuss: write about issue(s) or topic(s) in depth in a structured way; assess: make an informed judgement. Suggest: apply knowledge and understanding to situations where there are a range of valid responses in order to make proposals / put forward considerations. In this chapter the membrane potential changes (15.1.6) and saltatory conduction (15.1.7) are describe and explain outcomes, so an answer needs both the events and the reasons for them.