(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.
Explanation
(a) Experiment to demonstrate the influence of auxins:
Aim —To demonstrate the influence of auxins on the growth of plant shoot .
Apparatus: Young millet seedlings, agar block, blade, indole acetic acid (IAA), red light.
Method: Use young millet seedlings grown in the dark whose coleoptiles is up to I cm long. Label the seedlings A, B and C. Cut off the growing tips of the coleoptiles in a dark room in the presence of red light. Prepare three small agar blocks: (A) Agar block containing auxin, (B) Agar block containing indole acetic acid (IAA), (C) Agar block which is not treated as control experiment. Agar block which contains auxin is placed on cut coleoptile tip A to allow auxin in the tip to diffuse into the agar block. Agar block which contains indole acetic acid (IAA) is put on the tip of the coleoptile B.The untreated agar block is put on the tip of cut coleoptile C as control. Then leave the three coleoptiles in a unilateral source of light for few hours.
Observation: The seedlings in experiment A and B show accelerated growth curvature on the sides the agar blocks were placed. Experiment C which is the control shows no growth curvature. Conclusion: This shows that auxins promote growth in coleoptile or shoot.
(b) Uses of auxins in agriculture :
(1)Auxin spray prevents leaf fall.
(2) It enhances the formation of roots.
(3) It stimulates the formation of female flowers.
(4) It promotes the development of fruits
(5) It induces cells to differentiate into xylem to increase water and mineral transport and to provide support.
(c) Mechanism of transmission of impulses through a nerve fibre: This can be categorised into three phases:
(1) The resting potential
(2) The action potential
(3) Repolarisation phase. At the resting phase, the outer surface of a nerve fibre is positively charged and the inner surface is negatively charged. Stimulus from outside is detected by the receptor organs and this is transmitted along the nerve fibres as an electrical impulse. The change in electrical potential brings about transmission. Any part of the nerve fibre along which the impulse passes is negatively charged relative to other parts of the nerve fibre as a result of the flow of sodium and potassium ions via the membranes of the nerve fibre. Or At the resting stage of the axon, the outside and the inside are positively and negatively charged respectively. Membrane surrounding the axon is polarised, as a result of which it can maintain different electric charge on its two sides. Sodium ions are concentrated outside and potassium ions inside the fibre. Stimulation of the fibre results in change in permeability of the cell membrane. The sodium ions move in while potassium ions move out of the nerve, hence we have sudden reversal of the resting potential. The inner part of the axon becomes positive while the outer part is negative, which is the action potential. Change in the ionic charge along the fibre results in the movement of electrical current along the fibre, hence impulse is transmitted. As soon as the impulse passes on, the resting potential is restored