WAEC SSCE - Biology - 1993 (Essay)

(a) Characteristics of the Northern Guinea Savanna

1. The vegetation is made up of tall grasses with scattered, short, fire- and drought-resistant trees.
2. The trees are shorter and more widely spaced than in the southern (Guinea) savanna, and many have thick, fire-resistant bark and small or thorny leaves.
3. It has a longer dry season and lower total rainfall (about 1000 to 1200 mm a year) than the forest and southern savanna.
4. The grasses dry up and are often burnt during the dry season, and shed leaves reduce water loss.
5. Common plants include shea butter, locust bean and acacia trees, and common animals include antelopes, grasscutters and grasshoppers.

(Any well-explained points describing the tall-grass-with-scattered-trees structure, seasonal drought, bush fires and drought adaptations are credited.)

(b) Estimating the density of a plant species using a quadrat

1. Select the area to be studied and lay out a quadrat (a square frame of known area, e.g. 1 m by 1 m = 1 m²).
2. Throw or place the quadrat at random within the study area to avoid bias.
3. Count the number of individuals of the chosen plant species that fall inside the quadrat.
4. Repeat the sampling several times at different random positions and record each count.
5. Find the average (mean) number of the plant per quadrat.
6. Calculate the density using \[ \text{Density} = \dfrac{\text{total number of the plant counted}}{\text{total area of all the quadrats}} \] which gives the number of individuals per unit area (e.g. per m²). Multiplying by the whole area estimates the total population.

(a)(i) Two diseases each caused by bacteria

Plants: bacterial wilt of tomato; bacterial soft rot of yam/cassava (bacterial leaf blight of rice is also acceptable).

Animals (including man): tuberculosis; cholera (others: tetanus, gonorrhoea, anthrax, typhoid).

(a)(ii) Three ways in which bacteria are useful

1. Decay bacteria (decomposers) break down dead organisms and recycle nutrients in the soil.
2. Nitrogen-fixing and nitrifying bacteria enrich the soil with nitrogenous compounds for plant growth.
3. Bacteria are used in industry to make food and drugs, e.g. yoghurt and cheese production, and antibiotics; some also aid digestion in the gut.

(b) Three methods of controlling mosquitoes and the reason for each

(c) Experiment to demonstrate the presence of bacteria under the finger nails

1. Prepare two sterile Petri dishes containing sterile nutrient agar (A and B).
2. Dish A: with an unwashed finger, scrape the material under the finger nails and gently streak it across the agar surface, then cover.
3. Dish B: leave untouched (or streak with a finger washed thoroughly with soap and disinfectant) to act as the control.
4. Incubate both dishes at about 37 °C for 2 to 3 days.
5. Observation: numerous colonies of bacteria grow on dish A, while the control B remains clear or nearly clear.
6. Conclusion: the growth of colonies on A shows that bacteria were present under the finger nails.

(a) Process of inhalation (breathing in) in man

1. The external intercostal muscles contract, pulling the ribs upwards and outwards.
2. The diaphragm muscles contract, so the diaphragm flattens and moves downwards.
3. These two movements increase the volume of the thoracic cavity (chest).
4. The increased volume lowers the air pressure inside the lungs below atmospheric pressure.
5. Because outside air is now at higher pressure, air rushes in through the nose and trachea into the lungs, and the lungs inflate.

(b) Gaseous exchange in the shoot system of flowering plants

In the shoot, gaseous exchange takes place mainly through the stomata of the leaves (and through lenticels in woody stems). Gases diffuse in and out through these openings and through the air spaces of the spongy mesophyll. By day, when photosynthesis is faster than respiration, carbon dioxide diffuses in and oxygen diffuses out. At night, when only respiration occurs, oxygen diffuses in and carbon dioxide diffuses out. The exchange is by simple diffusion along concentration gradients across the moist cell surfaces.

(c) Experiment to demonstrate the effect of distilled water on mammalian red blood cells

1. Obtain a fresh sample of blood and place a drop in a test tube.
2. Add a quantity of distilled water to the blood and mix gently.
3. Place a drop on a slide and examine under a microscope; also observe the colour of the mixture.
4. Observation: the red cells swell, become spherical and burst; the mixture turns clear red because the haemoglobin is released.
5. Explanation / conclusion: distilled water is hypotonic to the cell contents, so water enters the cells by osmosis until they burst. This bursting is called haemolysis.

(a) Four characters transmitted from parent to offspring in man

1. Blood group
2. Eye colour
3. Skin colour
4. Height / stature

(Others: hair type/colour, ability to roll the tongue, sex.)

(b) Two differences between mitosis and meiosis

(c) Importance of meiosis and fertilization

Meiosis halves the chromosome number to produce haploid gametes and, through crossing over and independent assortment, introduces variation among the offspring. Fertilization then restores the full diploid chromosome number when two gametes fuse, and combines genes from two parents, giving new gene combinations. Together they keep the chromosome number constant from generation to generation while ensuring genetic variation.

(d) Monohybrid cross

Let B = allele for blue flowers (dominant) and b = allele for white flowers (recessive). Parents: BB (blue) × bb (white). All F1 = Bb (blue).

(i) F1 self-pollinated: Bb × Bb

Offspring: 3 blue (BB, Bb, Bb) : 1 white (bb), a ratio of 3 blue : 1 white. Reason: the heterozygotes each carry a hidden recessive b, so a quarter of the offspring are homozygous recessive and show white.

(ii) F1 cross-pollinated with pure white (Bb × bb)

Offspring: 2 blue (Bb) : 2 white (bb), a ratio of 1 blue : 1 white (a test cross). Reason: half the gametes of the heterozygote carry b, so half the offspring are homozygous recessive white.