WASSCE Biology past questions and answers

The visking tubing bag contains 25% sugar solution. Since more sugar diffuses OUT of the bag than INTO it, the concentration of solution X must be lower than 25%.

Key concept: Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration. The net outward movement of sugar confirms solution X has a lower concentration than the bag.

This is a demonstration of OSMOSIS. The thistle funnel holds a concentrated sugar solution (low water potential) and its mouth is sealed with a semi-permeable membrane. It is dipped in a beaker of pure water (high water potential).

Water molecules move from the beaker (high water potential) across the semi-permeable membrane into the sugar solution (low water potential) by osmosis. The membrane lets water through but holds back the sugar, so the net movement of water is INTO the funnel. As water enters, the volume of liquid inside increases and the level of the solution in the narrow stem RISES.

It does not simply double (A) — the rise depends on the concentration difference and stops at equilibrium; it does not remain the same (B) because there is a water-potential gradient; and it does not fall (C) because water moves in, not out. The correct answer is D (Rise).

In a typical population growth curve, four phases are seen:

I — Lag phase: the population grows slowly at first as the organisms adjust to the new environment (this is the answer, A).

II — Log (exponential) phase: rapid growth while resources are plentiful.

III — Stationary phase: growth levels off at the carrying capacity as births balance deaths.

IV — Death (decline) phase: the population falls as resources run out and wastes accumulate.

The lag phase is the slow initial section of the curve, so it is labelled I — the answer is A.

In a cross between two heterozygous tall plants (Tt × Tt), each parent produces T and t gametes. The Punnett square gives offspring genotypes in the ratio 1 TT : 2 Tt : 1 tt. Because T (tall) is DOMINANT over t (short), TT and Tt are both tall, so the phenotype ratio is 3 tall : 1 short. The answer is D (3:1).

Why the others are wrong:
• A (4:0) — would occur only if no offspring could be short (e.g. TT × anything); here tt short plants do appear.
• B (2:1) — not a standard monohybrid ratio.
• C (1:1) — this is the ratio of a test cross (Tt × tt), not Tt × Tt.

A cell wall is a rigid structure (made mainly of cellulose in plants) that lies OUTSIDE the cell membrane and gives the cell support and shape. Cell walls are PRESENT in plant cells (and in fungi and bacteria) but ABSENT in animal cells. So for an animal cell the cell wall is absent, and for a plant cell it is present outside the cell membrane — option B. Options C and D are wrong because a cell wall is never inside the membrane; option A is wrong because it reverses the animal and plant cells.

The biological hierarchy of organisation is: cell → tissue → organ → organ system → organism. The wall of a villus is made of similar cells working together (mainly epithelial tissue), so it is at the TISSUE level. The small intestine is made of several different tissues working together to carry out digestion and absorption, so it is an ORGAN. Therefore the levels are villus wall = tissue and small intestine = organ — option D. Options calling the villus wall a single 'cell' (A, B) or the small intestine an 'organ system' (A, C) are wrong; an organ system is several organs working together, e.g. the whole digestive system.

A good surface for diffusion is large, thin and well supplied with blood. The ILEUM (part of the small intestine) is folded into millions of finger-like villi, each covered in microvilli, giving an enormous surface area for the absorption of digested food by diffusion and active transport. So the answer is D.

Why the others are wrong: • A (bronchioles) — small airways that CONDUCT air to the alveoli; it is the alveoli, not the bronchioles, that form the lung's diffusion surface. • B (trachea) — the windpipe; it carries air but has no role in diffusion. • C (atrium) — a chamber of the heart that receives and pumps blood; it is not a diffusion surface.

The cell membrane (plasma membrane) is a thin, partially permeable layer that controls what enters and leaves the cell; it carries out active transport, moving substances across the membrane against a concentration gradient using energy (ATP). The cell wall is a rigid layer of cellulose outside the membrane that gives the plant cell its shape and provides mechanical support — it does not carry out active transport.

So the cell membrane → active transport and the cell wall → support, which is option C. Option C is the only one where both are correct: A swaps the two functions; B wrongly gives the cell wall an active-transport role; D wrongly assigns "support" to the membrane.