Report > 1992

1.

(a)

(i)

When asked to construct a food web showing the feeding relations most candidates could draw the connecting lines linking the organisms. Some, however, forgot to put in the relevant arrows. A number of candidates put the higher level consumers at the bottom of the food web which is not the conventional representation of a food web.

(ii)

Many candidates tailed to stress that the longer the food chain, the more energy lost during the transfer between successive trophic levels.

(iv) Many candidates mentioned the presence of scales as a noticeable external feature of snakes but failed to point out that they are "dry". 1.

(b)

(i)

Many candidates’ descriptions of the function of an artery did not match with the structural feature stated in the answer. Some candidates thought that the thick and elastic wall of the artery was to pump or to create a high blood pressure rather than to withstand or resist it.

(ii)

Many candidates did not read the question carefully and so drew diagrams of the kidney and its related blood capillaries instead of a flowchart showing a double circulation of blood in relation to the kidney. It seems that they had no idea of what a flowchart is.

(iii)

A number of candidates were confused about the relative concentrations of carbon dioxide and glucose in the pulmonary artery when compared with those in the pulmonary vein.

1.

(c)

(i)

Most candidates failed to plot all the points for the import of sucrose into the leaf as given in the question, in particular for those points which lie along the horizontal axis (X-axis). Many even failed to label the axes or mark the origins of both axes.

(ii)

Many candidates did not read the questions thoroughly, only indicating the difference between the import and export of sucrose rather than explaining it. Others failed to point out that while the leaf size changed from 0% to 20% there was little photosynthesis (but not no photosynthesis) due to the small leaf area and hence insufficient chlorophyll.

(iv) Candidates were not used to suggesting simple ----------workable methods to extend a given investigation. When asked to estimate the leaf area, many simply suggested measuring both the length and width of the -----leaf. Others suggested a weighing method which in fact would not lead to any estimate of the leaf area. 2.

(a)

(i)

Many candidates were confused about the meaning of the terms "urea" and "urine".

(iii)

Some candidates were uncertain about the function of the kidney and wrongly thought that it could produce urea (a function of the liver).

2.

(b)

(i)

Many candidates misunderstood the question, giving only the changes in amount of water in the plant from 1300 hours to 2100 hours instead of relating how such changes could be linked to the relative size of the stomatal pores. Many candidates misread the question as requiring them to explain changes in transpiration ---------------rate with respect to changes in temperature and light intensity. Only a few candidates correctly pointed out the relationship between transpiration pull and water --------absorption. In this regard candidates* concept of transpiration pull was very vague.

(ii)

Many candidates wrongly interpreted areas A and B as the amount of water transpired or absorbed respectively. These are actually the net gain and net loss of water in the plant respectively.

(iv) Many candidates gave the function of the sweat pore in the human skin as only one of temperature regulation without specifying that it is mainly for preventing over- heating. Many

incorrectly cited the hair / hair follicle / hair pore / hair gland (rather than the sweat pore or sweat gland) as the ‘counterpart’ (in human skin) of the plant stoma. 2.

(c)

(i)

Many candidates lacked the competence to deduce the genotypes of parent plants without using a genetic diagram. This inability was clearly aggravated by the poor standard of expression in English.

(ii)

Some candidates did not use the genetic symbols as ------given in the question while others gave the genotypic ratio of 1:2:1 without referring to the genotypes -------------involved. Many candidates failed to give a logical explanation of how they could deduce the parental genotypes from what is given in the question.

3.

(a)

(i)

The term "chlorophyll" was often misspell. Many candidates mistook "chloroplast" as the pigment responsible for the green colour of the green pepper.

(iii)

Some candidates confused excretion with egestion of undigested food substances.

(iv) Some candidates were not aware that the presence of vitamin C could still be deduced although the decolorisation of DCPIP was masked by the pale green colour of the original filtrate. A considerable number of candidates wrongly thought that Benedict’s test was a test for the presence of glucose. They failed to note that Benedict’s test is only a test for the presence of reducing sugar. 3.

(b)

(i)

Some candidates confused the terms "iris" and "pupil". They failed to realize that the pupil is an aperture or opening at the centre of the iris which is a circular muscular sheet at the front of the eye ball. Also quite a few candidates confused the terms: "eye muscles", "ciliary muscles" and "iris muscles" in their descriptions.

(ii)

Many candidates did not realize that in the eye, the ---relaxation of the ciliary muscles would lead to an increase in tension of the suspensory ligament. As a ---------result the suspensory ligament would become tightened or taut but it would not contract on its own.

(iii)

Some candidates did not complete the ray diagram fully -----whilst others forgot to put in the arrows to indicate the direction of the light rays all the way onto the retina.

3.

(c)

This was the most well answered part of the paper indicating that candidates were generally well prepared for topics related to fundamental human physiological processes like the breathing mechanism in the human body. In contrast, however, questions set in other parts of the paper on plant physiological processes like tropism and transpiration led to vague answers which were often filled with misconceptions.

4.

(a)

(i)

Some candidates incorrectly deduced from the graph that digestion of proteins started in the small intestines.

(iii)

Many candidates gave the names of enzymes found in the small intestines rather than the names of the digestive juices. Some candidates did not realize that "bile" (not "bile juice") is a digestive juice found in the small intestines.

(v)

Some candidates incorrectly believed that the caecum was responsible for reabsorption of water from the undigested food.

4.

(b)

(i)&(ii)

Many candidates could not satisfactorily explain phototropism and geotropism in terms of the action of plant hormones. It should be cautioned that up to now no one definitive theory can fully and conclusively explain the mechanism causing phototropism. Nevertheless, recent findings tend to support the "Light-induced Lateral Transfer of

Auxin Model" as a more plausible explanation of phototropism although the "Lightinduced Destruction or Inhibition of Auxin Model" has not been conclusively disproved. (iv) Candidates were generally weak in the provision of experimental designs slightly extended from the conventional ones on geotropism and phototropism. This was particularly so when they were asked to consider the joint effects of gravity and light on the growth response of a plant seedling. 4.

(c)

(ii)

While the suggestion of reallocating factories to a remote site was accepted in candidates* answers, more ecologically sound and environmentally friendly long term solutions for reducing air pollution in Kwai Chung may include considering sites with a good air dispersion pattern and reducing pollutants emitted from the nearby power station, incineration plant and heavy container truck traffic.