WAEC SSCE - Biology - 1990 (Essay)

Bayanin Amsa

(a) Process of succession on an abandoned farm land

Succession is the gradual and orderly replacement of one community of organisms by another over a period of time until a stable community is reached. On an abandoned farm land it proceeds as follows:

1. Colonisation by pioneer plants: the bare, exposed soil is first invaded by fast-growing weeds and grasses whose light seeds are easily dispersed. These are the pioneer species.
2. Improvement of the soil: as these plants grow, die and decay, they add humus to the soil, improving its fertility, water-holding capacity and structure.
3. Replacement by larger plants: the enriched soil now supports taller herbs and shrubs, which grow up and shade out and replace the grasses (they compete for light, water and nutrients).
4. Development of woody plants: shrubs are in turn replaced by small trees and then larger trees as conditions continue to improve.
5. Climax community: eventually a stable, self-maintaining community of tall trees (a forest, in a rainforest zone) develops. This final stage, which remains in balance with the environment, is the climax community. Each stage is called a sere.

Alongside the plants, the animal community also changes, so that the animals present at each stage match the vegetation.

(b) Seven ways by which micro-organisms can be controlled to maintain good health

1. Sterilization of instruments and materials by heat (e.g. autoclaving, boiling).
2. Use of antiseptics on the skin and wounds (e.g. iodine, Dettol).
3. Use of disinfectants on floors, surfaces and toilets (e.g. bleach, phenol).
4. Pasteurization of milk and other drinks to kill harmful microbes.
5. Proper cooking and refrigeration of food to kill or slow the growth of microbes.
6. Personal and environmental hygiene such as regular washing of hands, bathing and proper disposal of refuse and sewage.
7. Immunization (vaccination) and the use of drugs such as antibiotics to fight micro-organisms in the body.

Bayanin Amsa

(a) How meiotic division contributes to variation in a population

Meiosis introduces variation in two main ways:

• Crossing over: during the first meiotic division, homologous chromosomes pair and exchange segments (genes) at the chiasmata. This produces new combinations of alleles in the resulting gametes.
• Independent (random) assortment: the homologous chromosome pairs line up and separate independently of one another, so the chromosomes are shuffled and each gamete receives a different mixture of maternal and paternal chromosomes.

Because of these events, meiosis produces gametes that are genetically different from one another and from the parent cell. When these varied gametes fuse at random during fertilization, offspring with new gene combinations are formed, giving rise to variation within the population.

(b) Mendel's first and second laws

First law (Law of Segregation): the two members of a pair of alleles controlling a character separate (segregate) during the formation of gametes, so that each gamete carries only one allele of the pair.

Second law (Law of Independent Assortment): when two or more pairs of contrasting characters are considered together, each pair of alleles segregates and is distributed into the gametes independently of the other pairs.

(c) Cross of brown rat (BB) x white rat (bb) to the second filial generation

Let B = brown (dominant), b = white (recessive).

Parents: \(BB\) (brown) \(\times\) \(bb\) (white). Gametes: \(B\) and \(b\).

F1: all \(Bb\) - brown rats.

F1 selfed: \(Bb \times Bb\). Gametes: \(B\) and \(b\) from each.

F2 genotypes: \(1\ BB : 2\ Bb : 1\ bb\).

F2 phenotypes: 3 brown : 1 white, i.e. \(3:1\).

Bayanin Amsa

(a)(i) Two reasons why termites are described as social insects

1. They live together in large organised colonies in which members depend on one another.
2. There is division of labour, with different castes performing different specialised duties for the good of the whole colony.

(a)(ii) Four castes in a termite nest, with the function of each

(b) Five behavioural adaptations of termites for survival

1. Division of labour among castes: each caste performs a special task (reproduction, feeding, defence), which makes the colony efficient and better able to survive.
2. Building of termitaria (mounds): they construct strong, ventilated mounds that protect the colony from enemies, heat and desiccation and keep conditions inside cool and moist.
3. Cooperative feeding and care of young: workers feed the queen, king, soldiers and larvae, so members that cannot feed themselves survive.
4. Cultivation of fungus gardens: workers grow fungus on chewed plant material as a reliable food supply.
5. Communication by chemicals (pheromones) and grooming: termites signal danger and coordinate activities, and constant grooming keeps the colony free from disease.

Also acceptable: swarming/nuptial flight for dispersal and starting new colonies, and remaining hidden inside tunnels away from light and predators.

Bayanin Amsa

(a) Breakdown of glucose in the cell to produce energy (ATP)

The release of energy from glucose is called cellular respiration. It takes place in two main stages:

• Glycolysis (in the cytoplasm): a molecule of glucose is broken down through a series of enzyme-controlled steps into two molecules of pyruvic acid, producing a small amount of ATP. This stage does not require oxygen.
• Aerobic breakdown (in the mitochondria): in the presence of oxygen, pyruvic acid is completely broken down through the Krebs cycle and the electron transport chain into carbon dioxide and water, releasing a large amount of ATP.

The overall aerobic reaction is:

\[ C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)} \]

In the absence of oxygen (anaerobic respiration), the pyruvic acid is only partly broken down, giving lactic acid (in animals) or ethanol and carbon dioxide (in plants and yeast), with the release of only a little energy.

(b) Two conditions under which glycogen can be converted to glucose

1. When the blood glucose level falls (e.g. during hunger/fasting or strenuous exercise), the hormone glucagon (or adrenaline) stimulates the liver to convert glycogen to glucose.
2. When the body needs extra energy quickly, such as during vigorous muscular activity.

(c) Experiment to demonstrate that oxygen is released by green plants during photosynthesis

• Place some water plants (e.g. Elodea or Hydrilla) in a beaker of pond water and cover them with a short-stemmed glass funnel, mouth downwards.
• Fill a test tube completely with water and invert it over the funnel stem so that no air enters.
• Set up a second identical apparatus in the dark as a control, and place the first in bright sunlight.

Observation: in the apparatus kept in sunlight, bubbles of gas rise from the plant and collect in the test tube, gradually displacing the water. When enough gas has collected, a glowing splint inserted into it is rekindled (relights), showing that the gas is oxygen. No gas collects in the apparatus kept in the dark.

Conclusion: green plants release oxygen during photosynthesis, and only in the presence of light.