(a) Describe an experiment to demonstrate the effect of auxin on growth of a plant shoot. (b) List three uses of auxin in agriculture. (c) Describe the mech...

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(a) Experiment to demonstrate the effect of auxin on the growth of a plant shoot

• Take several similar germinating oat (or maize) seedlings (coleoptiles) grown in the dark.
• From one seedling, cut off the tip of the coleoptile and place it on a small block of agar jelly for a few hours so that the auxin from the tip diffuses into the agar.
• Cut off the tip of a second (decapitated) seedling and place the auxin-containing agar block on one side of the cut stump. Set up a control with a plain agar block (no auxin).
• Keep all the seedlings in the dark (so light is not a variable) and observe over some hours.

Observation: the seedling with the auxin agar on one side bends away from that side (grows/curves), because the side receiving auxin grows faster and elongates more than the other side. The seedling with the plain agar block does not bend or grow.

Conclusion: auxin promotes the elongation (growth) of the cells of the shoot; unequal distribution of auxin causes unequal growth and hence bending. This confirms that auxin controls growth in the shoot.

(b) Three uses of auxin in agriculture

1. It is used as a rooting hormone to promote the growth of roots in stem cuttings.
2. It is used as a selective weed killer (in high concentration it kills broad-leaved weeds).
3. It is used to prevent premature dropping of fruits and to promote fruit development (including seedless fruits).

Also acceptable: to control ripening and to promote flowering.

(c) Mechanism of transmission of an impulse through a nerve fibre

A nerve impulse is transmitted as an electrochemical wave (action potential) along the fibre:

• In a resting nerve fibre, the outside of the membrane is positively charged and the inside negatively charged (the fibre is polarised), because sodium ions are kept outside and potassium ions inside.
• When the fibre is stimulated strongly enough (above threshold), its membrane at that point suddenly becomes permeable to sodium ions, which rush in and reverse the charge (the membrane becomes depolarised). This is the action potential.
• This local change stimulates the next region of the membrane, so the depolarisation, and with it the impulse, passes along the fibre from point to point.
• Behind the impulse the membrane quickly repolarises (potassium ions move out and the ions are pumped back to their resting positions), so the fibre is ready to carry another impulse.
• At the end of the fibre the impulse reaches a synapse, where a chemical neurotransmitter is released to carry the message across the gap to the next neurone.