WAEC SSCE - Biology - 1991 (Essay)

Answer Details

(a) Labelled diagram of the internal structure of the mammalian ear

(b) Mechanism of hearing in mammals

1. The pinna collects sound waves and directs them through the auditory canal (auditory meatus) to the tympanic membrane or ear drum.
2. The sound waves make the ear drum vibrate.
3. These vibrations are transmitted through, and amplified by, the three ossicles: malleus, incus and stapes.
4. The stapes transmits the vibrations to the oval window. This causes the fluid in the cochlea to vibrate.
5. The vibrations stimulate the sensory hair cells in the organ of Corti in the cochlea, producing nerve impulses.
6. The auditory nerve carries the impulses to the brain, where they are interpreted as sound.

(c) Care of the ear

1. Keep the outer ear clean, but do not insert pins, sticks or other sharp objects into the ear canal.
2. Avoid excessive loud noise and consult a doctor promptly if there is ear pain, discharge or infection.

Answer Details

(a)(i) Three components of an ecosystem

• Producers (green plants/autotrophs).
• Consumers (herbivores, carnivores, omnivores).
• Decomposers (bacteria and fungi).

(The non-living abiotic component of water, air, soil and light is also part of an ecosystem.)

(a)(ii) Interaction among the components

The producers trap solar energy and manufacture food by photosynthesis. The consumers feed on the producers (and on other consumers), so energy and materials are transferred along food chains and food webs. When producers and consumers die, the decomposers break down their bodies and wastes, releasing mineral nutrients back into the soil and air, which the producers reuse. In this way energy flows through the system and nutrients are recycled among the components.

(b) Three major steps in studying plants and animals in a community

1. Sampling / collection of organisms using suitable methods (quadrat for plants and sedentary animals; pitfall traps, nets or sweep nets for mobile animals).
2. Identification and counting of the organisms collected, and estimation of their abundance/distribution.
3. Recording and analysis of the data (frequency, density, and the physical/abiotic factors measured) to describe the community.

(c) Importance of each factor and its effect on population growth

• (i) Competition: Members compete for limited resources (food, space, light, mates). Intense competition reduces the survival and reproductive rate of the weaker individuals, so it checks and limits population growth.
• (ii) Pathogens: Disease-causing organisms infect members of the population. Outbreaks of disease increase the death rate, thereby reducing the size and slowing the growth of the population.
• (iii) Water: Water is essential for all life processes. An adequate supply promotes survival, reproduction and rapid growth of the population, while a shortage of water raises the death rate and reduces population growth.

Answer Details

(a)(i) Five excretory organs found in living organisms, with (ii) an example organism for each

(The leaf/stoma of green plants and the general body surface are also acceptable examples.)

(b)(i) Two ways the excretory organ of the earthworm is similar to that of mammals

• Both use tubular structures (nephridia in earthworm, nephrons in the mammalian kidney) that filter body fluids/blood.
• Both remove nitrogenous waste and excess water and reabsorb useful substances, producing urine.

(b)(ii) Three differences between them

(c) Mechanism of opening and closing of the stomata

• Opening (in light/day): In the presence of light the guard cells photosynthesise and actively pump in potassium ions; sugar/solute concentration in the guard cells rises, lowering their water potential. Water then moves into the guard cells by osmosis, making them turgid. Because the inner wall of each guard cell is thicker and less elastic than the outer wall, the turgid guard cells curve apart, opening the stoma.
• Closing (at night/water shortage): Potassium ions and sugars leave the guard cells, raising their water potential; water passes out of the guard cells by osmosis and they become flaccid. The guard cells straighten and come together, closing the stoma. This also happens when the plant is short of water, to reduce transpiration.

Answer Details

(a)(i) The cell theory

The cell theory states that:

• All living organisms are made up of one or more cells, and of the products of cells.
• The cell is the basic structural and functional unit of life (the smallest unit that can carry out life processes).
• All cells arise from pre-existing cells by cell division.

(a)(ii) Two scientists who contributed to the cell theory

Robert Hooke and Matthias Jakob Schleiden. (Theodor Schwann and Rudolf Virchow also made important contributions.)

(b) Large labelled diagram of a typical plant cell

The diagram shows the rigid cellulose cell wall forming the outer boundary, the cell (plasma) membrane lining it, the cytoplasm in which the organelles are suspended, the nucleus with its nuclear membrane and nucleolus, the large central vacuole bounded by the tonoplast, several green chloroplasts, and the mitochondria.

(c) Three ways a typical plant cell differs from a typical animal cell

(d) Differences between Hydra and mammals