WAEC SSCE - Biology - 2002 (Essay)

Bayanin Amsa

(a) Explanation of terms

• (i) Parasitism: An association between two organisms of different species in which one (the parasite) lives on or inside the other (the host), obtaining food and shelter from it and harming the host in the process. For example a tapeworm in the human intestine.
• (ii) Commensalism: An association between two organisms of different species in which one (the commensal) benefits (usually for food or shelter) while the other (the host) is neither harmed nor benefited. For example the remora fish attached to a shark, or epiphytes growing on a tree.

(b) Named parasites and hosts

• (i) A fungal parasite of a plant and its host: Puccinia (rust fungus) on wheat/cereal (also acceptable: Phytophthora on Irish potato).
• (ii) A bacterial parasite of humans: Vibrio cholerae (causing cholera) (also acceptable: Mycobacterium tuberculosis, Salmonella typhi).

(c)(i) Four symptoms of malaria

1. High fever with intermittent chills and shivering.
2. Headache.
3. Body/joint pains and general weakness.
4. Sweating, loss of appetite and sometimes vomiting.

(c)(ii) Six methods of controlling malaria in West Africa

1. Clearing bushes and draining/filling stagnant water to destroy mosquito breeding sites.
2. Spraying breeding waters with oil or larvicides to kill mosquito larvae.
3. Sleeping under insecticide-treated mosquito nets.
4. Using insecticides and mosquito repellents, and screening windows and doors with wire gauze.
5. Introducing larva-eating fish (for example Gambusia/Tilapia) into ponds (biological control).
6. Taking anti-malarial drugs and seeking prompt treatment; and public health education.

Bayanin Amsa

(a) Explanation of terms

• (i) Producer: an autotrophic organism, usually a green plant, that manufactures its own food from simple inorganic substances by photosynthesis, forming the first trophic level of a food chain.
• (ii) Consumer: a heterotrophic organism that cannot make its own food and feeds on other organisms (plants or animals) for its energy and nutrients.
• (iii) Community: all the populations of different species (plants, animals and microbes) living and interacting together in a particular habitat at the same time.
• (iv) Decomposer: an organism, such as bacteria and fungi, that feeds on dead organic matter, breaking it down and releasing nutrients back into the environment.

(b)(i) Four characteristics of trees of the Tropical Rain Forest

• Very tall trees with straight trunks and buttress roots for support.
• Broad, evergreen leaves with drip tips.
• Form distinct layers (canopy, under-canopy) with a closed canopy.
• Have many climbers (lianes) and epiphytes growing on them.

(ii) Four characteristics of trees/shrubs of the Savanna

• Trees are short, scattered and widely spaced.
• Have small, narrow or reduced leaves to reduce water loss.
• Have thick, fire-resistant (corky) bark.
• Have deep or extensive root systems to reach water; many are deciduous, shedding leaves in the dry season.

Bayanin Amsa

(a)(i) Ecological succession is the gradual and orderly replacement of one community of organisms by another in a given area over a period of time, until a stable climax community is established.

(ii) Four characteristics of ecological succession of plants

• It is a gradual, orderly and progressive process.
• It begins with pioneer species and passes through several intermediate (seral) stages.
• Each stage modifies the environment, making it suitable for the next community.
• It ends in a stable, self-perpetuating climax community.

(b) Four factors which affect population size

• Natality (birth rate): a high birth rate increases population size.
• Mortality (death rate): a high death rate decreases population size.
• Immigration: movement of organisms into the area increases population size.
• Emigration: movement of organisms out of the area decreases population size. (Availability of food, predation and disease also apply.)

(c) Adaptations of tapeworm (Taenia solium) to its habitat in the human gut

• Has hooks and suckers on the scolex (head) for attachment to the gut wall so it is not washed out.
• Has a thick cuticle that resists digestion by the host's enzymes.
• Secretes anti-enzymes to protect itself from being digested.
• Has a long, flat, thin body giving a large surface area for absorbing already-digested food directly through its body wall (no gut needed).
• Is hermaphroditic and produces very many eggs, ensuring survival and spread of the species.
• Can respire anaerobically, coping with the low-oxygen conditions of the gut.

Bayanin Amsa

(a) Main constituents of mammalian blood

• Plasma (liquid matrix)
• Red blood cells (erythrocytes)
• White blood cells (leucocytes)
• Platelets (thrombocytes)

(b) Two functions of each constituent

• Plasma: transports dissolved food (glucose, amino acids), salts, hormones and excretory wastes (urea, carbon dioxide); distributes heat evenly to regulate body temperature.
• Red blood cells: carry oxygen from the lungs to the tissues as oxyhaemoglobin; help transport carbon dioxide from tissues to the lungs.
• White blood cells: engulf and destroy invading germs by phagocytosis; produce antibodies and antitoxins for defence against disease.
• Platelets: initiate blood clotting at wounds; help seal cuts to prevent excessive blood loss and entry of pathogens.

(c) Six differences between arteries and veins

(d) Main artery transporting blood to the

• (i) Lung: pulmonary artery
• (ii) Kidney: renal artery

Bayanin Amsa

(a) Receptor

A receptor is a specialised sensory cell or group of sensory cells that detects a particular stimulus and converts it into nerve impulses which are transmitted to the central nervous system.

(b) General features common to receptors

• They are made up of specialised sensory cells, often occurring in groups.
• The free ends of the sensory cells usually bear short hair-like processes or dendrites that receive the stimulus.
• Nerve fibres arise from, or are connected to, their basal regions.
• These nerve fibres lead through sensory neurones to the cranial nerves or spinal nerves and then to the central nervous system.
• Each receptor is adapted to respond most readily to a particular kind of stimulus.

(c)(i) Labelled diagram of the outer and middle ear of a human

The diagram is drawn to a length of approximately 9 cm and shows only the outer and middle ear.

(c)(ii) Ways of caring for the ear

1. Avoid inserting sharp objects, pins or matchsticks into the ear.
2. Keep the outer ear clean and dry; do not force objects deeply into the auditory canal.
3. Avoid prolonged exposure to loud noise and excessive use of earphones at high volume.
4. Report ear pain, discharge, injury or difficulty in hearing promptly to a medical doctor.
5. Seek prompt treatment for infections such as measles and meningitis, which may cause deafness.

Bayanin Amsa

(a) Sexual reproduction is the production of new offspring by the fusion (fertilisation) of two haploid sex cells (gametes), a male gamete and a female gamete, to form a diploid zygote which develops into a new individual.

(b) Five functions of the placenta

• Allows diffusion of oxygen from the mother's blood to the foetus.
• Allows dissolved food (glucose, amino acids, minerals, vitamins) to pass from mother to foetus.
• Removes excretory wastes (carbon dioxide, urea) from the foetus into the mother's blood.
• Secretes hormones (progesterone and oestrogen) that maintain pregnancy.
• Acts as a barrier that prevents the mixing of maternal and foetal blood and keeps out some harmful microbes.

Four functions of the umbilical cord

• Connects the foetus to the placenta.
• Contains the umbilical vein that carries oxygenated, food-laden blood from placenta to foetus.
• Contains umbilical arteries that carry deoxygenated, waste-laden blood from foetus to placenta.
• Provides some support and allows the foetus to move freely in the amniotic fluid.

(c) Four types of placentation in flowering plants

• Marginal placentation
• Axile placentation
• Parietal placentation
• Basal placentation (free-central is also acceptable)

(d) Four conditions necessary for seed germination

• Water (moisture)
• Suitable (warm) temperature
• Oxygen (air)
• Viable, living embryo (with adequate stored food)