WAEC SSCE - Biology - 1996 (Essay)

(a) Osmoregulation is the control of the water and solute (salt) balance of an organism so that its internal osmotic pressure is kept steady.

(i) In a named unicellular organism (Amoeba / Paramecium)

Amoeba lives in fresh water. The surrounding water is more dilute (hypotonic) than the cell contents, so water continuously enters the cell by osmosis across the cell membrane. To avoid bursting, the excess water collects in a contractile vacuole. The vacuole gradually fills, moves to the surface and discharges the water to the outside (osmoregulation by active removal of water). This is repeated continually, using energy (ATP).

(ii) In man

Osmoregulation is carried out mainly by the kidneys under the control of antidiuretic hormone (ADH) from the pituitary gland.

• When the blood is too concentrated (little water), the pituitary releases more ADH, which makes the kidney tubules (nephrons) reabsorb more water back into the blood, so a small volume of concentrated urine is produced and water is conserved.
• When the blood is too dilute (excess water), less ADH is released, less water is reabsorbed, and a large volume of dilute urine is produced.
• Water is also lost through sweating and in expired air, and gained by drinking and from food; the kidney adjusts urine output to keep the balance.

(b) How mammalian skin regulates body temperature in a hot room

In a hot environment the skin acts to lose heat and cool the body:

• Vasodilation: the blood vessels (arterioles) supplying the skin surface widen, so more warm blood flows near the surface and heat is lost by radiation and convection, giving the skin a flushed appearance.
• Sweating: the sweat glands become active and pour out sweat onto the skin surface; as the sweat evaporates it takes latent heat from the body, cooling it.
• Hair lies flat: the erector (hair) muscles relax so the hairs lie flat, trapping less air, allowing heat to escape.
• Reduced metabolic heat: the rate of metabolism (heat production) is lowered.

(a) Transmittable (heritable) characters

(i) In human beings:

• Colour of the eyes (eye colour)
• Colour of the skin
• Blood group (A, B, AB, O)

(ii) In plants:

• Height of the plant (tallness or shortness)
• Colour of the flower
• Shape or colour of the seed

(b) Three ways genetics has contributed to the improvement of agriculture

• Production of improved (high-yielding) varieties: selective breeding and hybridisation have produced crops and animals that give higher yields of grain, milk, meat or eggs.
• Development of disease- and pest-resistant varieties: genetics is used to breed crops and livestock that resist diseases and pests, reducing losses.
• Breeding for desirable qualities and adaptation: production of varieties that are early maturing, drought resistant, or tolerant of poor soils, and animals with better feed conversion (also improved food quality).

(c) Cross between a homozygous tall parent and a homozygous short parent

Let the allele for tallness be T (dominant) and for shortness be t (recessive).

Parental genotypes: tall = TT; short = tt.

Gametes: T from the tall parent; t from the short parent.

First filial (F₁) generation: all offspring are Tt (heterozygous) and are therefore all tall, since T is dominant. Ratio: 100% tall.

(a) Competition is the struggle or interaction between organisms for the same limited resources (such as food, water, light, space, mates or shelter) when the supply of the resource is not enough for all of them.

(b)(i) Interspecific (interspecies) competition is competition that takes place between organisms of different species for a common resource, for example a lion and a hyena competing for the same prey, or maize and weeds competing for soil nutrients and light.

(b)(ii) Intraspecific (intraspecies) competition is competition that takes place between organisms of the same species for the same resource, for example two cocks competing for the same hen, or maize plants in a crowded field competing for light and nutrients. It is usually more intense because the needs are identical.

(c) Structural adaptations of the toad/frog

(i) For obtaining food

• A long, sticky, protrusible tongue attached at the front of the mouth which can be shot out rapidly to trap insects.
• A wide gape (large mouth) and backwardly directed teeth (in the frog) that hold the prey and prevent its escape; large bulging eyes that help detect moving prey.

(ii) For protection

• Camouflage (cryptic) colouration of the skin (greenish/brown) that blends with the surroundings and hides it from enemies.
• Poison (mucous) glands in the skin (especially the parotid glands of the toad) that secrete an irritating/toxic fluid to discourage predators.

(iii) For movement

• Long, muscular hind limbs that are folded and act like springs to leap on land.
• Webbed hind feet that increase surface area and push against water for efficient swimming.