WAEC SSCE - Biology - 2001 (Essay)

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(a) What is an ecosystem?

An ecosystem is a natural unit made up of a community of living organisms (the biotic component) interacting with one another and with the non-living (abiotic) part of their environment, together with the exchange of materials and flow of energy between them. Examples include a pond, a forest and a grassland.

(b) Two abiotic factors each affecting plants and animals in a terrestrial habitat

Factors affecting plants: (i) light intensity; (ii) rainfall (water/humidity). Others: temperature, wind, soil nutrients.

Factors affecting animals: (i) temperature; (ii) humidity. Others: light, wind.

(c) Effect of three abiotic factors on a named plant and a named animal

Named plant: maize plant. Named animal: grasshopper (in a grassland habitat).

• Light: Light provides the energy for photosynthesis in the maize plant; without adequate light the maize becomes pale (etiolated) and grows poorly. In the grasshopper, light controls its daily activity, feeding and breeding rhythm.
• Temperature: A suitable temperature speeds up the enzyme reactions of photosynthesis and growth in the maize; very high or very low temperatures reduce growth and may cause wilting or death. Being cold-blooded (poikilothermic), the grasshopper's body temperature and rate of activity rise and fall with the surrounding temperature; extreme cold makes it sluggish and extreme heat can be fatal.
• Water (rainfall/humidity): Water is needed by the maize for photosynthesis, transport of materials and turgidity; shortage of water causes wilting and stunted growth. For the grasshopper, humidity affects water loss from its body; low humidity increases the risk of desiccation, so it seeks moist shelter.

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(a) Eight human activities that may pollute water bodies

1. Discharge of untreated sewage and human excreta into rivers and streams.
2. Dumping of industrial effluents and chemical wastes into water bodies.
3. Washing of clothes and bathing with detergents and soaps in streams.
4. Application of fertilizers on farms which are washed by run-off into water (causing eutrophication).
5. Spraying of pesticides and herbicides that drain into water bodies.
6. Oil spillage from tankers, refineries and pipelines.
7. Dumping of refuse and solid domestic waste into rivers and lakes.
8. Discharge of hot water (thermal pollution) from factories and power stations.

Others include mining activities that release heavy metals, and dumping of dead animals into water.

(b) Measures to prevent pollution of water bodies

• Treatment of sewage and effluents before they are discharged, so that harmful materials are removed.
• Enactment and enforcement of laws that prohibit the dumping of waste into water bodies, with penalties for offenders.
• Public enlightenment and education on the dangers of water pollution and the need for clean water.
• Proper disposal of refuse in designated dumps or by controlled incineration rather than in rivers.
• Controlled and moderate use of fertilizers and pesticides, and the use of biodegradable chemicals.
• Provision of pit latrines and toilets sited away from streams to prevent faecal contamination.
• Prompt control of oil spillage and proper maintenance of pipelines and tankers.

(c) Four methods of purifying water

1. Boiling - heating water to kill micro-organisms.
2. Filtration - passing water through sand and gravel beds to remove suspended particles.
3. Chlorination - adding chlorine or chlorine compounds to kill germs.
4. Distillation - boiling water and condensing the steam to give pure water.

Sedimentation with alum (coagulation) is also acceptable.

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(a)(i) Longitudinal section of a mammalian heart

Labels: aorta, pulmonary artery, pulmonary veins, superior vena cava, inferior vena cava, right atrium, left atrium, right ventricle, left ventricle, tricuspid valve, bicuspid (mitral) valve, semilunar valves and septum.

(a)(ii) Pulmonary circulation

Deoxygenated blood is pumped from the right ventricle through the pulmonary semilunar valve into the pulmonary artery. The pulmonary artery carries it to the capillaries of the lungs, where carbon dioxide diffuses out of the blood and oxygen diffuses into it. The oxygenated blood then returns to the left atrium through the pulmonary veins.

(b) Processes by which the mammalian body reduces its temperature

1. Increased sweating: Sweat is secreted onto the skin. Its evaporation absorbs heat from the body and cools it.
2. Vasodilation: Skin arterioles widen, allowing more warm blood to flow through surface capillaries, where heat is lost to the surroundings.
3. Relaxation of hair erector muscles: Body hairs lie flat, reducing the layer of trapped insulating air and increasing heat loss.
4. Reduced muscular activity: Less heat is produced by muscle contraction.
5. Reduced metabolic activity: The rate of heat-producing chemical reactions in the body is lowered.

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(a)(i) Labelled diagram of a human sperm cell

(ii) Sperm cells are produced in the seminiferous tubules of the testes and are stored temporarily in the epididymis.

(b) How the structure of the sperm cell enables it to function in fertilisation

1. The acrosome, situated at the tip of the head, contains enzymes which digest the egg membrane and enable the sperm to penetrate the egg.
2. The nucleus in the head contains a haploid set of chromosomes. It carries the male genetic material which fuses with the female nucleus during fertilisation.
3. The middle piece contains numerous mitochondria which release energy for movement of the sperm.
4. The long, whip-like tail or flagellum beats to propel the sperm through the female reproductive tract towards the egg.
5. Its small, streamlined shape reduces resistance as it swims towards the ovum.

(c) Four advantages of sexual reproduction

1. It produces variation among offspring.
2. The variation enables some individuals to survive changes in the environment and resist diseases.
3. It permits the combination of desirable characteristics from two parents, producing improved varieties or hybrids.
4. It maintains the chromosome number of a species from one generation to the next.

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(a)(i) Definitions

Tropic movement (tropism): a growth movement of a plant or part of a plant in response to an external stimulus that comes from a definite direction, the direction of the movement being determined by the direction of the stimulus.

Nastic movement (nasty): a non-directional movement of a plant part in response to a diffuse (non-directional) stimulus, where the direction of the response is independent of the direction of the stimulus.

(a)(ii) Four differences between tropic and nastic movements

(b) Experiment to show that auxins are produced in the apical cells (tip) of a shoot

• Take several similar germinating oat (or maize) seedlings (coleoptiles).
• Leave one seedling intact as a control.
• From a second seedling, cut off the tip of the coleoptile.
• On a third seedling, cut off the tip and then replace it on the stump.
• On a fourth, cut off the tip and place it back with a thin block of agar jelly between the tip and the stump.
• Keep all seedlings under the same conditions and illuminate them from one side.

Observation: the intact seedling and the ones with the tip replaced (directly or through agar) continue to grow and bend towards the light. The seedling whose tip was removed stops growing and does not bend.

Conclusion: because growth and curvature occur only when the tip is present, and the influence passes through the agar block, the growth substance (auxin) is produced in the apical cells of the shoot.

(c)(i) Two plant hormones other than auxins

1. Gibberellin (gibberellic acid).
2. Cytokinin (kinin).

Abscisic acid and ethylene are also acceptable.

(c)(ii) Two examples of tropism

1. Phototropism - shoot bending towards light.
2. Geotropism - root growing downwards towards gravity.