Report > 1996

1.

(a)

(i)

(1)

Some candidates thought that both pine and bread mould do not possess chlorophyll or seeds.

(2)

The sub-group to which pine belongs should be gymnosperm, as stated in the syllabus, and not conifer. Many candidates could not spell this term correctly.

(ii)

(1)

Some candidates stated that the seed could be dispersed by wind without pointing out that the role of the seed is to help in the dispersal of the offspring in order to avoid overcrowding or to exploit new habitats.

(2)

A large number of candidates wrongly stated that the pollen grain is the male gamete of the plant and that it fertilizes the egg directly. Candidates should be aware that the role of the pollen grains in reproduction is to produce the male gametes and to carry them to the female part of the flower for fertilization.

(iv) When answering this part, many candidates did not refer to the features shown in the diagram that are responsible for obtaining nutrients from the substrate, such as the 1.

(b)

1.

(c)

branching hyphae that form the rhizoids. Candidates made a lot of spelling mistakes, such as 'artery' for the 'artery', 'capillary' for 'capillary', and 'think wall' for 'thick' or 'thin' wall. (i) (2) Many candidates did not refer to the information given in the photograph and wrongly stated that blood vessel B was a vein. This mistake indicates that many candidates simply answered the question by rote and regurgitated what had been memorised without thinking. (ii) Many candidates did not correctly relate the observable feature of the artery wall to its functional significance. Some of the common misconceptions shown by the candidates are given below : the thick wall maintains a high blood pressure ; the muscular wall helps to pump the blood under high pressure. Such answers indicate that many of the candidates simply memorised information without understanding. Some candidates erroneously described vessel A/B as having a thick/thin cell wall. A considerable number of the answers were ambiguous or inaccurate, such as 'the thick wall enables the artery to accept or adapt to the high blood pressure'. (iii) Most candidates correctly stated that the biconcave shape of the red blood cells provides a relatively large surface area. But instead of relating this feature to facilitating the diffusion of gases, many of them wrongly asserted that it enables the red blood cell to carry more oxygen. (iv) Some candidates misinterpreted the question by describing the possible effects of blocking the vessel on other parts of the body instead of on the heart. (i)

Most candidates answered this part correctly.

(ii)

The ray diagrams showed a number of mistakes which are also found in some local textbooks. It is likely that some candidates might have memorised such errors from the textbooks without genuine understanding. Some of the common mistakes made by the candidates are illustrated in the following figures :

(1)

(2)

(3)

(1)

The two parallel light rays come from different points on the object.

(2)

The light rays stop after converging in front of the retina.

(3)

The incident rays from the distant object are diverging instead of being nearly parallel. The quality of drawing was rather poor; many drawings were too small, untidy and not in proportion. Most candidates failed to give a title to their drawings.

(iv) Some answers were quite concise and accurate. Many candidates, however, confused the functions of the rods and of the cones. For example, some stated that rods cannot detect colour but failed to mention the role of the cones. A small number of candidates wrongly related the phenomenon to night-blindness. 2.

(a)

(ii)

Most candidates calculated the rate of water loss correctly and presented their results in a table. Many, however, did not put down the unit for light intensity.

(iii)

Many candidates did not give a satisfactory explanation. They did point out the increase in stomata! size at higher light intensity, but failed to state that this resulted in a faster rate of diffusion of water vapour from the leaf. Instead, they wrongly asserted that the increase in stomatal size led to a higher rate of photosynthesis or evaporation, which therefore caused a more rapid water loss from the plant.

2.

(b)

(ii)

There were some good answers which were accurate and organised descriptions. A considerable number of candidates, however, did not distinguish between antibodies and antigens, or pathogens and the disease.

2.

(c)

(iii)

Most candidates mentioned the presence and action of protease in the pancreatic juice, but only a small number indicated the alkaline nature of the juice and its significance.

(iv) This part was generally well answered, showing that the candidates could apply their knowledge to explain a novel situation. (v)

A common misconception of the candidates was that deamination of proteins or amino acids forms urea. This indicates that the candidates just memorised the term 'deamination' without understanding its exact meaning. For the CE level, it is sufficient and appropriate for the candidates to understand that the liver breaks down excess proteins or amino acids to form urea. Introducing the term 'deamination' does not help their understanding and, in fact, may result in misconceptions.

3.

(a)

(i)

Most candidates did not make a sound, logical deduction based on the results of the genetic crosses. In general, candidates' communicative skills, as shown in this part, was far from satisfactory.

(ii)

Many candidates did not give a proper definition of the symbols used for the alleles. The most common error was using a letter to represent the character instead of the allele for that character.

(iii)

The quality of most candidates' genetic diagram was satisfactory.

(iv) Some candidates stated wrongly that the self-cross was a kind of asexual reproduction because it involved only one parent. Some answered correctly that the self-cross was a sexual process but wrongly explained that variation occurred in the offspring. This was not true in the context of this question. 3.

(b)

(i)

Some candidates could not distinguish diffusion from osmosis and answered that 'urea goes

out of the blood by osmosis'. (ii)

(1)

Most candidates did not point out precisely that the kidney machine could only treat a small volume of blood of the body at a time. Many wrongly stated that 'a long time was required for urea to diffuse across the dialysis tubing in the kidney machine'.

(iii)

(2)

Some candidates answered this question accurately by comparing the solute concentrations of the circulating solution and plasma, showing an understanding of the mechanism of the kidney machine. A number of candidates wrongly stated that 'the circulating solution was isotonic with the normal plasma', or 'no osmosis occurs'.

3.

(c)

(i)

(1)

Some candidates did not follow the instruction; they wrote a chemical equation for respiration instead of using a word equation.

(2)

Although the question referred to a standard experiment in biology, most candidates wrongly stated that the set-up measured the change in the amount of carbon dioxide instead of oxygen.

(ii)

Most candidates answered this part correctly, although many of them asserted that the setup measured change in the amount of carbon dioxide in (i)(2). This suggests that these candidates actually did not understand the principle of the investigation, but simply answered from rote memory using information given in textbooks or by teachers. Some common mistakes were detected in the candidates' explanations, e.g. the consumption of oxygen by respiration results in a drop in air volume in flask A; heat released from respiration of the grasshoppers causes the air in flask A to expand.

(iii)

Most candidates mentioned that flask B was a control but did not explain its use. Very few candidates correctly stated that the presence of flask B was to eliminate the error caused by changes in environmental temperature or pressure during the experiment.

4.

(a)

(ii)

The poor performance on this question shows that the candidates were still quite weak in drawing conclusions by comparing the results of two set-ups with reference to the different set-up conditions.

(iii)

The quality of drawing was poor. Various cellular features were not drawn in the right way, e.g. the cell wall wrongly represented as a single line, the cell membrane in a turgid cell incorrectly shown as detached from the cell wall, and the vacuole and cytoplasm incorrectly positioned. There were a lot of spelling mistakes on the labels. Most candidates did not give an appropriate title for their drawings.

(iv) Many answers were incomplete with the candidates simply saying that the boiled potato was no longer selectively permeable, without pointing out specifically that the tissue became freely permeable after boiling. Some candidates erroneously stated that 'the sucrose solution (rather than sucrose) diffuses out to the distilled water'. 4.

(b)

(i)

(2)

Most candidates mentioned predation but left out competition as a type of relationship between the squid and the sea bird.

(iii)

Most answers failed to include a clear, organised explanation for the concentration effect of DDT along the food chain. Some candidates did not even mention the role of the algae or the non-biodegradable nature of DDT There was a general lack of communicative skills shown by the candidates in answering this question.

4.

(c)

(i)

(2)

Some candidates did not seem to understand what was meant by physiological

change. Some wrongly stated that vasodilation occurs in the capillaries instead of the arterioles of the skin. Many did not point out the significance of vasodilation of the skin in causing a rise in skin temperature. (ii)

(2)

Although most candidates knew that heat loss from the body could occur in different ways, they did not understand the significance of these ways in different situations. For instance, heat cannot be lost from the body by conduction or radiation when the room temperature is higher than the body temperature. As a result, many candidates did not point out correctly that sweating is the main way to lose body heat under the specified condition.

(3)

Some candidates erroneously stated that sweating is very slow when the relative humidity of a room becomes high. As body heat cannot be removed efficiently by the evaporation of sweat in such a situation, sweat production actually increases. Only a small number of candidates mentioned the consequence of a rise in body temperature. Very few pointed out that the body actually absorbs heat from the surroundings in such a situation.