AS 2023 — CCEA AS Biology Practice Questions

Question 1

Describe how you would use a quadrat and a transect to investigate the distribution of a plant species across a sand dune.

Accepted Answer

I would lay a tape measure as a belt transect from the strandline inland, along the environmental gradient of the dune. Quadrats would be placed at regular intervals, for example every 5 metres, because this is systematic sampling along a gradient rather than random sampling. In each quadrat I would record the percentage cover, or the frequency, of the plant species. I would repeat the procedure along several parallel transects and calculate a mean for each distance to improve reliability. At each quadrat position I would also measure abiotic factors such as soil moisture content and pH, so the distribution of the species can be related to the changing conditions along the dune.

Question 2

Explain how the structure of the mammalian heart ensures that oxygenated and deoxygenated blood are kept separate and that blood flows in one direction only.

Accepted Answer

A muscular septum divides the heart so that the deoxygenated blood on the right side never mixes with the oxygenated blood on the left. Deoxygenated blood from the body enters the right atrium through the vena cava, passes into the right ventricle and is pumped through the pulmonary artery to the lungs, while oxygenated blood returns in the pulmonary vein to the left atrium, passes into the left ventricle and is pumped out through the aorta. One-way flow is ensured by valves: the atrioventricular valves close as soon as ventricular pressure rises above atrial pressure, preventing backflow into the atria, and the semi-lunar valves close when the pressure in the arteries exceeds that in the emptying ventricles, preventing backflow from the arteries. The wall of the left ventricle is much thicker than that of the right because it must generate a far higher pressure to force blood around the whole body.

Question 3

Describe how you would test a solution for the presence of reducing sugar, non-reducing sugar, starch and protein.

Accepted Answer

To test for a reducing sugar I would add an excess of Benedict's solution and heat the mixture in a water bath at about 80 degrees Celsius; a brick-red precipitate is a positive result, while the solution staying blue is negative. If that test is negative I would test for a non-reducing sugar by boiling a fresh sample with dilute hydrochloric acid to hydrolyse it, neutralising with sodium hydrogencarbonate, and then repeating the Benedict's test - a brick-red precipitate now shows a non-reducing sugar was present. For starch I would add a few drops of iodine in potassium iodide solution to the sample at room temperature; a blue-black colour is positive and orange-brown is negative. For protein I would add Biuret reagent, and a purple or lilac colour indicates protein. The intensity of the Benedict's colour, or a colorimeter reading, can be used to compare concentrations semi-quantitatively.

AS 2023: CCEA Biology practice questions

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Describe how you would use a quadrat and a transect to investigate the distribution of a plant species across a sand dune.

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