WAEC SSCE - Biology - 1994 (Essay)

Maelezo ya Majibu

(a) General arrangement of the viscera of a named mammal (the rabbit)

The viscera are the internal organs contained in the body cavity (coelom). In a mammal such as the rabbit the coelom is divided by a muscular sheet, the diaphragm, into a front thoracic cavity and a hind abdominal cavity.

Thoracic cavity (chest):

• The heart lies centrally, enclosed in the pericardium.
• A pair of lungs lies on either side of the heart.
• The trachea runs down and divides into two bronchi entering the lungs, and the oesophagus passes behind them through the diaphragm.

Abdominal cavity (belly):

• The stomach lies just behind the diaphragm on the left.
• The large liver lies behind the diaphragm, with the gall bladder associated with it.
• The coiled small intestine and the large intestine (with a large caecum in the rabbit) fill much of the cavity, held by the mesentery.
• The pancreas lies in the loop of the duodenum and the spleen near the stomach.
• A pair of kidneys lies against the dorsal body wall, leading by ureters to the bladder.
• The reproductive organs (ovaries and uterus in the female; testes in the male) lie in the lower part of the abdomen.

(b) Process of feeding

(i) Hydra: Hydra is carnivorous. When a small animal (e.g. a water flea) touches the tentacles surrounding its mouth, the stinging cells (cnidoblasts/nematocysts) discharge threads that paralyse and hold the prey. The tentacles then push the prey through the mouth into the enteron (gastrovascular cavity), where enzymes secreted by gland cells digest it (extracellular digestion); the products are then taken up by the endodermal cells and digested further inside them (intracellular digestion). Undigested remains are egested through the mouth.

(ii) Amoeba: Amoeba feeds by phagocytosis (holozoic nutrition). When it meets a food particle such as a smaller organism, it flows around it by pushing out pseudopodia which surround and enclose the food together with a little water, forming a food vacuole. Enzymes are secreted into the vacuole to digest the food; the soluble products are absorbed into the cytoplasm, and the undigested matter is left behind (egested) as the Amoeba moves on.

Maelezo ya Majibu

(a) Three major zones in the marine habitat with one characteristic feature each

The oceanic/pelagic zone (open surface waters) is also acceptable.

(b)(i) One plant and one animal example in the marine habitat

Plant: a brown seaweed (algae) such as Sargassum. Animal: a bony fish (e.g. herring).

(b)(ii) How each is adapted to the habitat

• Seaweed (plant): it has a holdfast that anchors it firmly to rocks against the pull of waves and currents; its body (thallus) is flexible/leathery so that it bends with the waves without breaking; and it has no true roots or rigid stem since water supports it.
• Fish (animal): it has a streamlined body that reduces resistance while swimming; gills for extracting dissolved oxygen from the water; fins for swimming, steering and balance; and its body fluids/kidneys are adapted to cope with the salinity of sea water.

(c) Three behavioural adaptations of animals to seasonal changes in a terrestrial habitat

1. Migration - moving to more favourable areas during unfavourable seasons.
2. Aestivation - becoming dormant during the hot, dry season.
3. Hibernation - becoming dormant/inactive during the cold season.

(d) Three characteristics each of the plant community of the tropical rain forest and the savanna

Tropical rain forest:

1. Tall trees forming several layers (storeys) with a closed canopy.
2. Trees are evergreen with broad leaves.
3. Presence of many climbers (lianas) and epiphytes; great variety (diversity) of species.

Savanna:

1. Dominated by tall grasses with scattered, short trees.
2. Trees are widely spaced, deciduous, with small leaves and often thorny.
3. Trees show drought adaptations (thick bark, deep roots); fewer species than the forest.

Maelezo ya Majibu

(a)(i) Courtship behaviour

Courtship behaviour is a series of special activities or displays carried out by an animal (usually the male) to attract a mate of the same species, and to prepare both partners for mating. It includes displays such as special calls, songs, dances, release of scents (pheromones), colour displays, nest building and offering of food.

(a)(ii) How courtship aids reproduction

• It enables members of the same species to recognise and attract one another, preventing wasteful mating between different species.
• It helps each partner to identify the sex of the other, so that a male attracts a female and not another male.
• It ensures that the female is sexually receptive and ready to mate, so that mating and fertilization succeed.
• It synchronises the release of sperms and eggs, increasing the chance of fertilization.
• It reduces aggression between the sexes, so that mating can take place peacefully.
• It allows the female to select a strong, healthy mate, so that fit offspring are produced.

(b) Fertilization in flowering plants

Fertilization is the fusion of the male gamete with the female gamete to form a zygote. In flowering plants it follows pollination and occurs as follows:

1. After pollination, a pollen grain lands on the sticky stigma and absorbs sugary fluid.
2. The pollen grain germinates and grows a pollen tube which is guided by the tube nucleus down through the style towards the ovary.
3. The pollen tube secretes enzymes that digest the tissues of the style, making a path for its growth.
4. Inside the pollen tube the generative nucleus divides to form two male gametes (male nuclei).
5. The pollen tube enters the ovule through a small pore called the micropyle.
6. The tip of the tube bursts and releases the two male gametes into the embryo sac.
7. Double fertilization then occurs: one male gamete fuses with the egg cell (ovum) to form a diploid zygote, which develops into the embryo; the second male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus, which develops into the endosperm (food store).
8. After fertilization the ovule develops into a seed and the ovary develops into a fruit.

Maelezo ya Majibu

(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.