Report > 1990

1.

(a)

(i)&(ii)

Most candidates were able to answer the question correctly with reference to the features shown in the diagram. A small number of candidates, however, included features not shown in the diagram such as the presence of chloroplast in cell A or myelin sheath in cell B. This suggests they had answered by rote memory. Some candidates did not realise that vacuoles are present in both plant and animal cells, although the vacuoles in plant cells are usually larger and central in position.

(iii)

Many candidates could give a clear and accurate account of the changes in cells C and D using the concept of water potential. This shows that water potential is a useful concept in understanding and explaining osmosis in living cells.

1.

(b)

(i)

Quite a number of candidates stated wrongly that air moves out of the lungs when the pressure in the lungs is increasing. They failed to see that air is breathed out only when the lung pressure exceeds the atmospheric pressure.

(ii)

This part was well answered, showing that the candidates were familiar with the mechanism of breathing movements.

(iii)

Some candidates did not express the rate of breathing as number of breaths per unit time.

(iv) Many candidates failed to provide a useful reference to indicate that the curve sketched shows an increase in rate and depth of breathing during exercise in comparison with those at rest. 1.

(c)

(ii)

Many candidates had difficulty explaining their deductions in words instead of using a genetic diagram. This may be due to the fact that some candidates could not express their deduction logically while others may have learned and used genetic diagrams in a stereotyped manner without understanding the principles involved. A small number of candidates were still unable to define the allele symbols they used in their explanation.

2.

(a)

(iii)

Many candidates could apply their knowledge on enzyme activity in answering this question.

(iv) This question assesses candidates’ ability in experimental design. The performance on this question was not satisfactory. A common mistake is to use the Biuret test to check the disappearance of the egg white cube rather than the presence of digested proteins in the solution. (vi) Most candidates were able to point out that a control can be set up by replacing the enzyme solution with boiled enzyme solution or distilled water. However, many failed to realise that, for this particular experiment, a control should be set up for each of the given pH values. 2.

(b)

(ii)

(1)

Many failed to show in their diagrams that bending of the radicle takes place at the region of elongation.

(iii)

Many candidates could give a reasonable prediction on the possible changes on the radicle in a space shuttle. Some candidates, however, considered wrongly that *no geotropism* had the same meaning as "no gravity".

2.

(c)

(i)

Many candidates had mistaken “structure A”. the vena cava. as the renal vein.

(ii)

(2)

Many candidates failed to provide a complete explanation for this question. They missed some points although they seemed to understand the mechanisms involved. A common misconception is that protein can pass into the Bowman’s capsule and be

reabsorbed later. Some candidates failed to point out that the increase in urea concentration is due to the reabsorption of a large amount of water. 3.

(a)

(i)

Some candidates did not point out that dry scale is a characteristic feature of reptiles. It is surprising to note that quite a number of candidates wrongly classified crows as mammals. This reflects a very weak knowledge of the general classification of living organisms in the study of Biology.

(ii)

Many candidates answered wrongly that the toadstool carries out asexual reproduction while the fruit tree carries out sexual reproduction. They did not realise that both the toadstool and the fruit tree, like most plants, can carry out both asexual and sexual reproduction.

(iii)

Many candidates failed to examine the food web diagram carefully and made mistakes in the identification of carnivores and omnivores.

3.

(b)

(ii)

Although most candidates pointed out that there is less predation on the springtails, very few realised that the rapid multiplication or reproduction of the springtails is also a contributing factor to the subsequent increase in their number.

(iii)

This part is quite demanding in that it requires the candidates to organise their ecological concepts in order to give a clear and accurate account of the concentration effect of DDT along the food chain. Many candidates, for example, did not state the correct feeding relationship of the food chain. The performance on this part was rather unsatisfactory.

3.

(c)

(i)

(1)

Many candidates did not present their calculations as required by the question.

(iv) Many candidates could relate the greater demand for iron by women to the loss of blood during menstruation. However, some failed to pinpoint that iron is required for the formation of red blood cells or haemoglobin. 4.

(a)

(i)

Graph plotting was satisfactory in general but some candidates failed to label the axes with correct units. Some candidates plotted the data on wrong axes while a small number of candidates plotted their graphs on the answer book instead of on graph paper.

(ii)

This part was poorly answered as many candidates gave the net carbon dioxide uptake by a leaf of 1 cm2 area. They failed to read the question carefully which states that the leaf has an area of 10 cm2. For the second Part of this question, many failed to consider the effect of respiration when calculating the net carbon dioxide uptake by the leaf.

(iii)

(2)

Some candidates failed to compare the leaf with the stem and did not stress the point that the leaf contains more chloroplasts and stomata than the stem.

4.

(c)

(iii)

Many candidates were able to apply their knowledge of mammalian reproduction to explain the process of “in vitro fertilization” although this topic is not explicitly mentioned in the syllabus.

5.

(a)

(i)& (iii)

Some candidates wrongly related the changes in weight to photosynthesis instead of transpiration.

(ii)

The question demands clarity of thought as it involves the comparison of conditions in four different set ups.

5.

(b)

(iii)

While this part was well attempted, some candidates did not present a complete nervous pathway to indicate the coordinating role of the brain in sensation and in voluntary action.

5.

(c)

For part (iii), however, while many candidates stated the effect of bacteria in causing diseases, very few pointed out the effect of microorganisms on the quality of food.