WAEC SSCE - Biology - 1989 (Essay)

(a) Experiment to demonstrate the influence of auxins on the growth of a plant shoot (phototropism)

• Materials: several germinating oat or maize seedlings (coleoptiles), sharp blade, a box with a hole cut in one side, and a dark cupboard.
• Method: Take three sets of seedlings.
  • Set A: leave the tip of the coleoptile intact (control).
  • Set B: cut off the extreme tip of the coleoptile.
  • Set C: cut off the tip, then replace it on the stump.
• Place all sets in the box so that light enters only from one side (unilateral light) and leave for a day or two.
• Observations: Set A (tip intact) bends and grows towards the light. Set B (tip removed) does not grow or bend. Set C (tip replaced) grows and bends towards the light like Set A.
• Explanation/Conclusion: the growing tip produces a hormone (auxin) which, in unilateral light, moves to the shaded side of the shoot and causes greater cell elongation there, so the shoot bends towards the light. Removing the tip removes the source of auxin, so no growth or bending occurs. This shows that auxin produced by the tip controls the growth (and phototropic curvature) of the shoot.

(b) Three uses of auxins in agriculture

• Used as a rooting hormone to promote the growth of roots on stem cuttings.
• Used as selective weed killers (herbicides) to destroy broad-leaved weeds.
• Used to prevent premature fruit drop and to promote the development of fruits (including seedless/parthenocarpic fruits).

(c) Mechanism of transmission of impulses through a nerve fibre

A resting nerve fibre is polarised: the outside of the membrane is positively charged and the inside negatively charged (resting potential), maintained by the sodium-potassium pump. When the fibre is stimulated strongly enough (above threshold), the membrane at that point becomes depolarised: sodium ions rush in, reversing the charge so the inside becomes positive. This change is the action potential (nerve impulse). The depolarised region stimulates the next region, so the impulse travels as a wave along the fibre. Behind the impulse the membrane is quickly repolarised (returns to resting state). At the end of the fibre the impulse reaches a synapse, where a chemical (neurotransmitter) is released to carry the impulse across the gap to the next neurone.

(a) Morphological features of plants and their functions

(i) Tropical rainforest plants

• Broad leaves: increase surface area for trapping the limited light and for photosynthesis.
• Drip tips on leaves: allow excess rain water to run off quickly, keeping the leaf dry.
• Buttress and prop roots: give support to the tall trees in the shallow forest soil.
• Tall, straight trunks: raise the crown up to reach sunlight above the canopy.
• Climbing and epiphytic habit (lianes, climbers): enable weaker plants to reach light without building thick stems.

(ii) Savanna plants

• Small or needle-like/reduced leaves: reduce surface area and water loss by transpiration.
• Thick, corky (fire-resistant) bark: protects the plant against bush fires and heat.
• Deep and extensive root systems: reach water deep in the soil during the dry season.
• Thick waxy cuticle / hairy leaves: reduce transpiration.
• Water-storage tissue in stems (succulence): stores water for use in the dry season.

(b) Ecological succession is the gradual and orderly replacement of one community of organisms by another in the same area over a period of time, until a stable, self-maintaining community (the climax community) is reached.

(c) The carbon cycle (diagram only)

The diagram should show a closed loop with labelled arrows illustrating:

• Photosynthesis: green plants take in carbon dioxide from the air and fix it into organic food.
• Feeding: animals obtain carbon compounds by eating plants (and other animals).
• Respiration: plants and animals release carbon dioxide back to the air.
• Decomposition: decomposers (bacteria and fungi) break down dead plants and animals, releasing carbon dioxide.
• Combustion: burning of wood and fossil fuels (coal, oil) returns carbon dioxide to the atmosphere.

Its importance: it keeps the amount of carbon dioxide in the atmosphere fairly constant, supplies the carbon needed to build all organic molecules (the basis of all life), and links producers, consumers and decomposers.

(a) How the floral parts of a named wind-pollinated flower are adapted to wind pollination (for example maize or guinea grass)

• Large, feathery (branched) stigmas that hang outside the flower and expose a large surface to catch pollen carried by the wind.
• Long filaments with anthers that hang out (are versatile and pendulous) so that the pollen is freely exposed to and easily shaken off by the wind.
• Pollen grains are small, light, smooth and dry and are produced in very large quantities so that they are easily blown far by the wind.
• Petals are small, dull or absent, with no scent and no nectar, since insects are not needed for pollination.
• The flowers are often held high above the leaves and open before the leaves expand, so nothing obstructs the movement of pollen by wind.

(b) How each behaviour affects reproduction in animals

• (i) Territoriality: an animal defends an area (territory) against others of its own species. This ensures that the holder has enough food, space and nesting sites, reduces overcrowding, and secures mates, so that mating, egg-laying and the raising of the young take place successfully with less competition.
• (ii) Display: special courtship behaviours (colours, dances, calls, movements) used to attract a mate of the opposite sex. Display helps the animals to recognise, attract and select a suitable mate of the same species, stimulates the partner to be ready for mating, and so ensures successful fertilisation and reproduction.
• (iii) Seasonal migration: the regular movement of animals from one area to another at particular seasons. Animals migrate to reach areas with favourable conditions (food, warmth, safe breeding grounds), so that breeding, egg-laying and the survival and growth of the young take place under the best possible conditions.