Report > 1994

1.

(a)

(i)

In the Benedict’s test, the formation of red precipitate indicates the presence of reducing sugar. However, many candidates incorrectly deduced that glucose was present. Some even came to the wrong conclusion that starch could not pass through the dialysis tubing. A considerable number of candidates stated wrongly that glucose passed out of the dialysis tubing by osmosis. These answers showed that some candidates were rather weak in their logical thinking and they tended to rely on factual recall to explain experimental results.

(ii)

(1)

Many candidates wrongly concluded that starch was digested into glucose. The result of the Benedict’s test only suggested that reducing sugar was formed from the breakdown of starch. In fact, the two digestive juices in humans or other mammals that can break down starch, i.e. saliva and pancreatic juice, produce maltose instead of glucose through the action of amylase.

(2)

Many candidates gave the name of the enzymes involved, i.e. salivary and pancreatic amylase, instead of the names of the digestive juices.

(iii)

Some candidates stated the purpose of the precautions instead of indicating the precautions themselves, which was required by the question. This was probably due to a misinterpretation of the question, which could have been avoided by more careful reading.

1.

(b)

(i)

Many candidates failed to read correctly from the graph the number of yeast cells at hour 20 for flask A. This was mainly due to careless calculation from the scale of the axis given. In explaining the different results of flasks A and B, many candidates just described the conditions in one flask instead of comparing the conditions of the two flasks to account for the different results. Some candidates mistakenly thought that, under anaerobic conditions, yeast could not carry out respiration and would die due to lack of an energy supply.

(ii)

The candidates in general could relate the slope of the curve to the reproductive rate of the yeast. Most candidates, however, did not point out that toxic waste would accumulate in the solution after a period of time.

(iii)

This question assessed students' ability in experimental design and their knowledge of practical skills. The rather poor performance on this question reflects a general weakness in this area. Very few candidates could present a design that satisfied the conditions required : presence of an insulating layer, a thermometer immersed in the yeast and glucose mixture, and tubes for aeration. In general, the quality of drawing was poor; clear, well labelled and accurate diagrams were very rare.

1.

(c)

(i)

This question asked for a concise account of the functioning of the semicircular canals in detecting body movements. However, most answers did not show a mastery of the concept involved and the description was often incomplete and incomprehensive. It should be noted that the endolymph inside the semicircular canal does not actually move backward. As the head moves forward, the endolymph lags behind due to inertia, i.e. it shows a backward movement relative to the head. Many candidates did not point out that the sensory hair cells in the semicircular canal are stimulated as a result of the relative movement of the endolymph.

(ii)

Most candidates answered correctly that very few people were willing to donate their cornea after death, so that only a small number of patients could receive this treatment in Hong

Kong. This shows that candidates have a good social awareness of the problems regarding this treatment. 2.

(a)

(ii)

(2)

Most answers simply stated that the rise in blood ----glucose level led to an increase in the respiration rate of body cells. Candidates should have specified that the effect was on skeletal muscle, thus more energy would be released for muscle contraction during exercise.

(iii)

Some candidates gave an account of the changes in blood insulin level by rote without relating them to the changes in blood glucose level as shown in the graph. A considerable number of candidates wrongly stated that insulin secretion was under the control of the brain or hypothalamus, and that glucose was converted to glycogen by insulin.

2.

(b)

(i)

Many candidates did not state clearly that the reason most leaves are thin is because this allows light and carbon dioxide to reach the mesophyll cells efficiently, thus speeding up the photosynthetic process. Some candidates wrongly remarked that a thin structure would present a large surface area for photosynthesis. Most candidates could relate the green colour of the leaf to the presence of chlorophyll, but many failed to point out that chlorophyll is important in photosynthesis because it absorbs solar energy. Some wrongly stated that chlorophyll was responsible for the reduction of carbon dioxide in the formation of carbohydrate.

(iii)

When comparing the water loss through leaves between daytime and night, some candidates failed to refer to the different conditions between day and night. Very few candidates realized that under daytime conditions, e.g. higher temperature, a steeper concentration gradient of water vapour is established across the stomatal pore, and thus water vapour is lost faster during the daytime.

2.

(c)

Although some of the questions in this pan were open-ended and could not be answered by simple recall, the performance on this pan was generally good. This shows that many candidates can apply their biological knowledge to explain unfamiliar situations related to conservation. (ii)

Some candidates of the English version paper mistakenly used the word ‘fertilization’ for fertilizers.

(iii)

The candidates in general showed a good understanding of the concept of conservation, which involved reducing the use of fossil fuels and artificial fertilizers, and recycling the waste into useful matter.

3.

(a)

(ii)

Many candidates did not seem to understand the precise --------requirement of the question. Instead of explaining the importance of a rich blood supply, they elaborated on other structural features of the air sacs and villus, such as the presence of a folded surface and a thin epithelial wall, which are irrelevant to the question. Very few answers mentioned the point that the transport of absorbed substances can maintain a steep concentration gradient across the wall of the air sacs or villus.

(iii)

The answers concerning the flow chart were satisfactory, -------but some candidates wrongly stated the hepatic artery supplied blood to the small intestine. There were a lot of spelling

3.

(b)

(i)

mistakes of the names of blood vessels and heart chambers. The plotting of graphs was generally satisfactory. However, many candidates did not choose a suitable scale. As a result, the changes in the length of the potato strip could not be shown clearly on the graph. This is a weakness shown by the candidates every year. For the first

(ii)

(iii)

3.

(c)

(i)

time, the candidates were asked to give a suitable title to the graph. Most candidates who attempted this question failed to give a suitable title such as “changes in the length of the potato strip with time”. Many Title, such as “the relationship between the length of the potato strip and time”, were considered acceptable, but not so appropriate. Titles such as *the changes in the length of the potato strip in a sugar solution* were incorrect. Most candidates could use the concept of water potential correctly to explain the change in the length of the potato strip in the first hour. A small number of candidates, however, concluded wrongly that the potato cells were hypotonic to the sugar solution. Many failed to explain that the decrease in the length of the potato strip was a result of the cells shrinking or becoming flaccid after losing water by osmosis. (2) To account for the constant length of the potato strip, some candidates stated wrongly that no water moved in or out of the cells. Actually water molecules moved into and out of the cells all the time but at the some rate, so that the net rate of water movement into or out of the cells was zero. It was equally acceptable to state that no osmosis or diffusion occurred during this period. A considerable number of candidates could not give a correct sequence of mitosis. This suggested that they lacked a basic understanding of the process of mitosis. This weakness was also reflected in the poor performance in Q.3(c)(ii).

(ii)

(2)

Most candidates showed a poor understanding of the changes in chromosomal materials in a dividing cell during mitosis. They often missed out the point that the chromosomes in cell (4) would separate into two equal groups for the formation of two daughter cells. They also failed to mention the relationship between chromosomes and the DNA content of a cell.

(iii)

Most candidates did not have the basic understanding of how DNA may act as a genetic

material by controlling the typos of proteins produced. To explain why they thought that seed colour was a kind of discontinuous variation, some candidates explained that seed colour was controlled by a small number of genes. This point could not be derived from the given data. The correct reason was that seed colour could be grouped into a few distinct classes with no intermediate phenotypes. (ii) (1) This appeared to be a very straight forward and simple question ad any environmental factor that affects plant growth could be cited as the answer. ----This was not, however, the case. The candidates had to consider critically what factors would cause variation in the size of seeds in the same ------plant. Such a consideration would rule out factors such as temperature, water supply and mineral supply which would affect the plant as a whole. Light and carbon dioxide supply would have s more localized effect on seed growth in the same plant. (2) Many candidates did not realize that mutation could not be a main cause for the genetic variation that leads to the difference in the mass of seeds under normal conditions. (iii) This question assessed the candidates* ability to make deductions from the results of genetic crossing. This ability has been frequently assessed in recent years. Despite this, many candidates did not seem to have mastered the skill and could not make logical deductions from the information given. The following illustrates how to arrive at a logical conclusion from the result of the cross : The parent plant war developed from a yellow seed so it should contain alleles yellow seed. It produces some green seeds which must have received alleles for green seeds from the parent plant. The parent plant should therefore contain alleles for both yellow seed and green seed. It is therefore heterozygous for the seed colour. (iv) (2) Some candidates did not understand that for the offspring to be affected, it would be necessary for mutation to have occurred in the parts of the flower that are directly involved in the production of the seed, such as the anther, pollen grain, ovary or ovule.

4.

(a)

(i)

4.

(b)

(i)

Although this was a straightforward question, many candidates were unable to explain clearly how nutrients from maternal blood could reach the foetus. The descriptions of the role of placenta in which nutrients diffused from maternal blood into foetal blood were

particularly confusing and ambiguous. (ii)

Most candidates managed to state the functions of the amniotic fluid correctly and accurately. However, some mentioned that the fluid served as a lubricant during the birth of the baby. This is a function of the fluid at the birth of the baby, but not during the development of the foetus.

4.

(c)

(i)

This part was well-answered. Most candidates correctly pointed out that the section was taken from a root because the vascular tissue was located at the centre of the section.

(ii)

Most candidates failed to mention the point that the thick cell wall conferred rigidity to the cell, and then to relate this property to the functions of cell type A.

(iii)

Many candidates wrongly stated that cell type B was responsible for the transport of starch. They failed to realize that starch is insoluble and is mainly used as a form of food storage.