Report > 1991

CE BIO 1991 1.

(a)

(i)

1.

(b)

(ii)

Alex Lam

Some candidates produced an answer without referring to the photomicrograph supplied. For example they described the shape of the white blood cell as ‘irregular’ or ‘amoeboid’. This was a clear reflection of mere rote-memory that the white blood cells must be amoeboid. However, in the photomicrograph, the white blood cell could only be described as round or circular. Other candidates thought that all plant cells must have chloroplasts which led them to describe the presence of chloroplasts in the onion epidermal cells. In the photomicrograph, however, the chloroplasts are obviously absent. (ii) (1) Many candidates could not satisfactorily explain the lighter colour at the central part of the red blood cells. They failed to note that there was less cytoplasm and less haemoglobin there. (2) Very few candidates could precisely point out the importance of a large surface area to volume ratio in facilitating gaseous exchange. (iv) Many candidates wrongly explained that the red blood ----------------cells after immersion in a concentrated sugar solution would undergo ‘plasmolysis’. They failed to note that ‘plasmolysis’ and ‘flaccid’ are terms commonly used to describe the condition of a plant cell when they loose excessive water resulting in the cell membrane being detached from the cell wall. In the case of crenation in animal cells, the cells simply shrink but do not ‘plasmolyse’. (v) Most candidates could produce a drawing of a plant cell showing plasmolysis. Many candidates, however, could not produce an accurate drawing of the onion epidermal cell with the general shape as shown in the photomicrograph. Others used a single line to denote the plant cell wall. It is encouraging to see that most candidates could correctly extract numerical data from the question and could readily perform simple calculations to obtain the right answer. (iii)

Many candidates did not refer to the table given and so gave the presence of antibodies as an advantage of cow’s milk over breast feeding.

(iv) This was the most well answered part of the whole question paper showing students’ increased awareness of the daily applications of human nutrition. 1.

(c)

(i)

Many candidates could succeed in concluding that albino is the recessive character, but not many of them could explain their deduction in sentences.

(ii)

(1)

Candidates showed a marked improvement in - -their ability to draw a genetic diagram. However, some did not use the given symbols.

2.

(a)

(i)

Some candidates incorrectly explained the lack of digestive activities in saliva after boiling as due to the enzymes being “killed” by heating.

(iii)

Many candidates’ answers were too brief and vague, e.g. saying that saliva (rather than the enzymes within it) than the enzymes within couid break down starch.

(iv) The term "sporangiophore" was often misspell as "sporangiospore" or "sporangiopore". Drawings made by candidates of the bread mould were generally of poor quality and the labels were far from complete. 2.

(b)

(i)

The graph was well plotted by most candidates although a few still plotted the curve without using graph paper :

(iii)

Some candidates mixed up the deaminating function of the liver with the excretory function of the kidneys.

(iv) Quite a number of candidates wrongly gave the production of insulin as a function of the liver 2.

(c)

(i)

(1)

Some candidates obviously overlooked the phrase “these supplies are maintained“

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CE BIO 1991

Alex Lam in the question and wrongly stated that to maintain a supply of respiratory gases in the enclosed system, carbon dioxide was absorbed by green plants for photosynthesis and oxygen was absorbed for respiration.

(2)

Many candidates neglected the fact that, within the enclosed system, the major source of water vapour came from the transpiration of the green plants rather than from the moist soil.

(3)

Very few candidates correctly mentioned that the dead bodies of plants would undergo decay to give inorganic nutrients.

(iii)

Despite comments in previous examiners* reports, many ---------------candidates still held the misconception that humus is a direct source of inorganic fertilizer rather than dead decaying organic matter.

3.

(a)

(ii)

This part of the question was set to test candidates’ ability to design an experimental set-up for hypothesis testing. Many candidates failed to draw a workable set-up either because they wrongly positioned the relevant reagents or they blocked the delivery tube at the outlet.

3.

(b)

(i)

"Contractile" was often misspell as "contratile" or "contracile". "Vacuole" was often misspell as "vacule". Some candidates in the Chinese version mistook structure Y as a food vacuole.

(iii)

(2)

Candidates’ explanations involving water potential to explain the different rates of discharge of contractile vacuoles under different external media revealed their good grasp of the concept.

3.

(c)

(i)

"Cartilage" was sometimes wrongly spelt as "cartinage".

(ii)

(1)

"Pleural" was often misspell as "pleual" or "pleral". "Membrane" was often misspell as "membrance". Many candidates were unsure about the function of the pleural cavity.

(2)

Most candidates could correctly explain the collapse of the lungs when punctured by pointing out the difference in pressure on both sides of the pleura. Some merely explained this in terms of the leaking out of pleural fluid or simply the leakage of air. Most candidates failed to note that during expiration, the lungs will recoil or collapse due to their own elasticity.

(iv) (1)

Few candidates could relate the one-cell thick feature to the short distance needed for diffusion of respiratory gases.

4.

(a)

(i)

(1)

Many candidates suggested that setting the initial water level near the middle of the glass tubing could make it easier to observe the results, without stating what the results were i.e. they did not mention any changes in the water level within the tubing.

(2)

Rather than saying 'to suck up' a column of water, many candidates tended to say 'to pump up'. Still far too many candidates omitted the very important step of closing the screw clip after sucking the water column up.

(ii)

This part of the question was well answered showing candidates' improvement in explaining the phenomenon of gaseous exchange during seed germination. Many candidates, however, could not clearly point out that the rise in water level inside the glass tubing was due to the greater atmospheric pressure on the outside.

4.

(b)

(iv) "Aorta" was often wrongly spelt as "arota". "Pulmonary" was often wrongly spelt as "pulmonry".

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CE BIO 1991 4.

(c)

(ii)

(2)

Alex Lam Many candidates could not fully explain the significance of meiosis in terms of maintaining constant chromosome numbers over successive generations. Many candidates wrongly claimed that meiosis gives rise to haploid offspring rather than haploid gametes. Others wrongly stated that meiosis could reduce the chromosome numbers of the “offspring” by half.

(v) 5.

(2)

Many candidates described the embryo or foetus as the baby.

The highly disappointing results in this question demonstrated that candidates are still weak in applying their knowledge in the nervous system to explain neural responses. (a)

(ii)

Most candidates were weak in understanding the location and functions of the different types of neurones (sensory, relay and motor) in the nervous system. Many candidates mixed up the motor or sensory areas of the brain with the sensory or motor neurones in the neural pathway and failed to distinguish nerve fibres from nerves. Others were confused about the meanings of reflex actions and voluntary actions. Many did not realise that a nerve impulse would be sent to the brain from the reflex arc involved in the withdrawal action.

5.

(b)

(ii)

(1)

Some candidates could not relate growth to an increase in dry mass.

(iii)

Many candidates could not realise the importance of -----using large samples, particularly of biological specimens, to arrive at more reliable results. They only stated that a large sample size could give more accurate results' but could not explain why.

(iv) (1)

The functions of cotyledons during the initial stage of germination was not clear to many candidates. They did not realise that removal of cotyledons on the 4th day of germination could lead to "retarded growth" but not "zero growth".

5.

(c)

(iii)

Many candidates did not refer to the bar chart given in the question and simply regurgitated what they had learnt from textbooks.

(iv) Many candidates could grasp the idea of blockage of respiratory surfaces by particles while a few could point out the "irritating" function of particles to the respiratory system.

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