Theory question 102
Plant Responses and Hormones(a) Describe an experiment to demonstrate the influence of auxins on the growth of plant shoot
(b) List three uses of auxins in agriculture
(c) Describe briefly the mechanism of transmission of impulses through a nerve fibre.
Show the model answer
(a) Use young coleoptiles/seedlings; cut off the tips; place one tip on a block of agar (or insert a gelatin/jelly strip) and another on mica (impermeable); replace on the cut stumps; a control left intact; keep in the dark/uniform light for a few days
Result: shoot with agar/jelly grows; shoot with mica (auxin blocked) does not grow; conclusion – auxin made in the tip moves down and causes growth
(b) Any three: rooting of cuttings; selective weedkillers (e.g. 2,4-D); seedless (parthenocarpic) fruits; prevents premature fruit/leaf fall; promotes fruit setting and ripening control
(c) Resting nerve: outside positive, inside negative (Na⁺ pumped out, K⁺ in) – resting potential
Stimulus makes the membrane permeable to Na⁺; Na⁺ rushes in, inside becomes positive – action potential (depolarisation)
This local change stimulates the next part of the membrane, so the impulse travels along the fibre
Repolarisation: K⁺ moves out and the sodium pump restores the resting potential
Always include a control in an experiment and say what it shows.