WAEC SSCE - Biology - 2009 (Practical)

Maelezo ya Majibu

The farmer's results are tabulated below and used to plot the graph in part (a).

(a) Graph of cytokinin concentration against number of new shoots

Cytokinin concentration (the independent variable) is plotted on the horizontal (x) axis and the number of new shoots (the dependent variable) on the vertical (y) axis. Each of the twelve readings is plotted and the points joined with a smooth curve.

(b) Effect of cytokinin concentration on shoot development

Reading directly from the curve:

• As the concentration of cytokinin is increased from \(0\) to \(4.5\ \text{mg/litre}\), the number of new shoots produced increases steadily, rising from 2 shoots to a maximum of 25 shoots.
• Shoot development reaches its peak of 25 shoots at the optimum concentration of 4.5 mg/litre.
• Above this optimum concentration, shoot development declines progressively, falling to about 5 shoots and then levelling off (remaining roughly constant) at the highest concentrations.

Thus cytokinin promotes shoot development only up to an optimum concentration; beyond that optimum, higher concentrations become inhibitory.

(c) Advice to the farmer

The farmer should apply cytokinin at a concentration of about 4.5 mg/litre, because this gives the greatest number of new shoots (25). He should avoid concentrations higher than the optimum, since these reduce shoot production, waste the chemical and could harm the plants.

(d) Substances with a similar (growth-regulating) effect to cytokinins

(i) In plants:

• Auxins
• Gibberellins

(ii) In animals:

• Growth hormone (somatotrophin)
• Thyroxine

Maelezo ya Majibu

(a) Habitat of Specimen J (toad): wet, damp places on land, such as under leaves, stones and in small holes near ponds and streams.

(b)(i) Five observable features of Specimen J (toad): webbed digits on the hind limbs; a moist, warty skin; bulging eyes on top of the head; nostrils placed on top of the head; short, stout fore-limbs and long, strong hind limbs.

(b)(ii) How the features adapt Specimen J to its habitat: the webbed hind digits are used for swimming and paddling in water; the moist skin provides a surface for gaseous exchange, while the warty skin reduces water loss; the bulging eyes on top of the head allow it to see while the body is submerged; the nostrils on top of the head let it breathe air while partly submerged; the short, stout fore-limbs act as shock absorbers on landing, and the long, strong hind limbs are used for hopping on land.

(c)(i) Two differences between Specimens H and J

(c)(ii) Three similarities between Specimen H and the skin of Specimen J: both have thin surface membranes; both have a moist surface; and both have a large surface area richly supplied with blood vessels (highly vascularised).

(c)(iii) How these features adapt Specimen H (lung) for its function: the thin membranes allow easy diffusion of oxygen and carbon dioxide; the moist surface dissolves the gases before they diffuse; the abundant blood vessels transport the respiratory gases; and the large surface area gives room for rapid exchange of gases.

(d)(i) Classification of Specimen K (maize leaf): Phylum Angiospermophyta (Magnoliophyta); Class Monocotyledoneae.

(d)(ii) Three reasons for placing it in that class: it has a long, narrow blade; parallel venation; and a leaf sheath at its base.

(d)(iii) Three observable features of Specimen K and how they adapt it to its mode of life: the broad, flat blade gives a large surface area to absorb maximum sunlight; the green colour (chlorophyll) absorbs light energy for photosynthesis; and the numerous parallel veins transport water to and manufactured food away from the leaf.

Maelezo ya Majibu

(a) Habitats of specimens A, B and C

(b)(i) Two observable features and (ii) how each adapts the specimen to its habitat

Specimen A (Water lettuce)

• Broad, spongy leaves borne in a rosette – keep the plant buoyant and expose the maximum surface to sunlight for photosynthesis.
• Numerous fine adventitious roots hanging in the water – absorb water and mineral salts and aid flotation; fine hairs on the leaves also prevent water from blocking the stomata.

Specimen B (Cactus)

• Leaves modified into spines – greatly reduce water loss by transpiration and deter herbivores.
• Succulent green stem with a thick waxy cuticle – stores water and checks water loss while still carrying out photosynthesis.

Specimen C (Waterleaf / Talinum sp.)

• Well-developed swollen taproot – anchors the plant, absorbs water and mineral salts, and stores food.
• Erect branched stem bearing numerous green leaves – holds the leaves up to receive maximum sunlight for photosynthesis.

(c) Three observable differences between specimens A and C

(d) Drawing of specimen C (Waterleaf / Talinum sp.)

The drawing below, 8–10 cm long, shows specimen C in correct proportion with all parts fully labelled using ruled label lines that do not cross.