CIE A Level Biology Topic 15: Control and Coordination Exam Questions

About CIE A Level Biology Topic 15: Control and Coordination

Control and coordination in the Cambridge 9700 syllabus compares nervous and endocrine communication in mammals and then looks at coordination in plants. You need to describe neurones and reflex arcs, the resting and action potential, saltatory conduction, and the transmission of impulses across cholinergic synapses. Muscle contraction is explained by the sliding filament model at the neuromuscular junction. You also need the human menstrual cycle hormones and contraceptive pills as an example of hormonal control. In plants, you need the rapid response of the Venus fly trap and the roles of auxin in cell elongation and gibberellin in seed germination. Data on membrane potentials is commonly tested.

What you need to know

• Neurones and reflexes Sensory, relay and motor neurones form reflex arcs. Myelin sheaths made by Schwann cells insulate axons; impulses jump between nodes of Ranvier (saltatory conduction), which is faster.
• Resting and action potentials The resting potential (about -70 mV) is maintained by the sodium-potassium pump and K+ leakage. At threshold, voltage-gated Na+ channels open (depolarisation), then K+ channels open (repolarisation) and hyperpolarisation follows. The refractory period ensures one-way transmission.
• Synapses An action potential opens voltage-gated Ca2+ channels; vesicles release acetylcholine by exocytosis, which binds receptors on the postsynaptic membrane and opens Na+ channels. Acetylcholinesterase breaks it down.
• Muscle contraction At the neuromuscular junction, depolarisation spreads along T-tubules and releases Ca2+ from the sarcoplasmic reticulum. Ca2+ binds troponin, moving tropomyosin; myosin heads bind actin, perform the power stroke and use ATP to detach. Sarcomeres shorten.
• Hormonal control Endocrine glands secrete hormones into the blood. The menstrual cycle is controlled by FSH, LH, oestrogen and progesterone; contraceptive pills use synthetic hormones to inhibit FSH and LH secretion.
• Plant responses In the Venus fly trap, stimulation of sensory hairs generates action potentials that cause rapid closure. Auxin promotes cell elongation by acidifying cell walls; gibberellin stimulates amylase production in germinating seeds.

Common mistakes in Topic 15: Control and Coordination

• Saying the action potential is caused by sodium being pumped in; Na+ diffuses in through voltage-gated channels.
• Writing that myelin speeds up the impulse by itself; it allows saltatory conduction between nodes of Ranvier.
• Stating that actin and myosin filaments shorten; the filaments slide past each other and the sarcomere shortens.
• Forgetting the role of ATP in muscle contraction; ATP is needed to detach the myosin head from actin and recock it.
• Confusing nervous and hormonal communication speed and duration; nervous responses are fast and short-lived, hormonal responses slower and longer-lasting.

Try three Topic 15: Control and Coordination exam-style questions