CCEA A Level Biology 4.3 Co-ordination and Control Exam Questions

About CCEA A Level Biology 4.3 Co-ordination and Control

Section 4.3 of Unit A2 1 is split into plants and animals. For plants, CCEA requires phytochromes (P660 and P730), critical night length and flowering in short-day and long-day plants, plus the roles of auxins, cytokinins and gibberellins. For animals you need neurone structure, resting and action potentials, saltatory conduction, synaptic transmission with excitatory and inhibitory postsynaptic potentials, the structure and function of the eye including rods, cones, accommodation and dark adaptation, and the sliding filament model of muscle contraction. Examiners often set graphs of action potentials, sarcomere diagrams and essays on synapses or vision.

What you need to know

• Phytochromes P660 absorbs red light and converts to P730; P730 absorbs far-red light and converts back, and also slowly reverts in the dark. Short-day plants flower when the night is long enough to remove P730; long-day plants flower when P730 is not removed. Growers manipulate photoperiod to flower plants out of season.
• Plant growth substances Auxins promote cell elongation, cytokinins promote cell division and gibberellins promote internode elongation.
• Nerve impulses Resting potential is about -70 mV. If threshold is reached, sodium channels open and the membrane depolarises (all-or-nothing), then potassium channels open and it repolarises. The refractory period ensures one-way travel. Myelination (saltatory conduction between nodes of Ranvier) and wider axons increase speed.
• Synapses An impulse causes vesicles to release neurotransmitter (acetylcholine, noradrenaline or GABA) by exocytosis. It binds to receptors on the postsynaptic membrane, producing an EPSP; summed EPSPs can trigger an action potential. Inhibitory transmitters cause hyperpolarisation (IPSP). Acetylcholinesterase breaks down acetylcholine.
• The eye Iris muscles control pupil size; ciliary body and suspensory ligaments change lens shape (accommodation). Rods show convergence, so are sensitive but give low acuity, and use rhodopsin, which must be resynthesised for dark adaptation. Cones in the fovea give high acuity and colour vision. Binocular vision gives depth perception.
• Muscle contraction A sarcomere runs Z-line to Z-line. In contraction the I-band and H-zone shorten but the A-band stays the same length. Calcium ions expose binding sites on actin, myosin heads form cross bridges and pull actin, and ATP is used to detach and reset the heads.

Common mistakes in 4.3 Co-ordination and Control

• Saying the action potential gets bigger with a stronger stimulus. It is all-or-nothing; a stronger stimulus increases impulse frequency.
• Saying the A-band shortens during contraction. The A-band is the length of the myosin filaments and stays the same.
• Saying cones work in dim light. Rods are sensitive to low light intensity; cones need bright light.
• Getting the phytochrome forms the wrong way round. P660 absorbs red and becomes P730; P730 is the active form that affects flowering.
• Saying impulses jump along myelinated axons because the myelin conducts. Depolarisation only occurs at the nodes of Ranvier, so local circuits jump between nodes.

Try three 4.3 Co-ordination and Control exam-style questions