1995 Biology Paper I Marking Scheme
Transcript of 1995 Biology Paper I Marking Scheme
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CE BIO 1995 Alex Lam
Paper I Marking Scheme & Year Report P.1
1. (a) (i) It contains enzyme / protease that ----------------------------------------------------------------------- 1
digests the proteins in egg stains to small molecules -------------------------------------------------1
which can dissolve in water / can be removed by water ---------------------------------------------1
Communication skill (C) -----------------------------------------------------------------------------------1
(ii) - to allow time for the action of enzyme -------------------------------------------------------------------1
(iii) Soak clothes in warm water ------------------------------------------------------- )
because the enzyme action is faster at a higher temperature ------------ )
OR --- any ONE set 1+1
Use more washing powder -------------------------------------------------------- )
because the enzyme action is faster a higher enzyme concentration -- )
(iv) because the enzyme would be denatured above 60°C -----------------------------------------------1
(v) because protein in silk and wool will be digested by the enzyme ----------------------------------1
Total : 8 + 1 marks
1. (a) (i) Some candidates simply stated that protease in the washing powder can digest the egg
stain but did not mention that the egg stain contains protein. Most candidates failed to point
out that the proteins are broken down into small, soluble molecules which can then be
removed by water.
(iii) To shorten the soaking time, many candidates suggested using water at 37 ° C as this is the
optimum temperature for enzyme action. This is wrong because for most enzymes,including many human enzymes, the optimum temperature lies between 45-50 ° C. Only a
small proportion of candidates gave the correct reason to explain why soaking the clothes in
warm water can shorten the soaking time: that enzyme action is faster at a higher
temperature.
(iv) To explain why it is not advisable to use the washing power with water at temperatures
above 60 ° C, many candidates stated wrongly that ' enzymes are killed or destroyed by
heating '. Enzymes are simply protein molecules; they are not living organisms. It is
therefore wrong to say that they are killed by heating Enzyme function is dependent on the
specific configuration of the enzyme molecule which is largely maintained by relatively weak
bondings such as electrovalent bonds and hydrogen bonds. Heating the molecule above the
optimum temperature will disrupt these weak bondings, leading to a loss of the natural
configuration of the molecule. If the molecule is not subjected to too high a temperature, this
process is reversible. As only the weak bondings but not the covalent bonds are broken, the
molecule will resume its normal shape when it is returned to a lower temperature. In this
process, the molecule has not been destroyed, but is denatured, which simply means that
the molecule has lost its natural shape. It is essential that students should not simply
memorize the term 'denaturation' by rote, as an understanding of the meaning of this term
will help them to gain a deeper understanding of the characteristics of enzyme action.
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1. (b) (i) Blood group A is dominant -----------------------------------------------------------------------------------1
Reasons:
Both individuals 3 and 4 have blood group A therefore each of
them must possess at least one allele for blood group A ---------- )
One of their children (individual 12) has blood group O,
that means at least one of the parents must also possessone allele for blood group O ----------------------------------------------- )
At least one of the parents is heterozygous --------------------------- )
In a heterozygous condition, only the dominant allele
will be expressed ------------------------------------------------------------- )
OR ----------------------------------------------------------------------------------- any ONE set 1+1+1+1
Both individuals 3 and 4 have blood group A / are of the
same phenotype and ------------------------------------------------------ )
produce offspring who arc of two phenotypes ------------------------ )
indicating that both parents are heterozygous ------------------------ )Only the dominant allele expresses itself / recessive allele is
masked in heterozygous state -------------------------------------------- )
Communication skill (C) -----------------------------------------------------------------------------------1
(ii) Defined symbols e.g. : A = allele for blood group A
a = allele for blood group O -------------------------------------------------1
No mark for defining symbols if not using capital letter for dominant allele and small
letter for recessive allele.
(1) Individual 7 : Aa --------------------------------------------------------------------------------------1
(2) Individual 13 : AA , Aa ---------------------------------------------------------------------------- 1,1(iii) (1) 5 and 6 ---------------------------------------------------------------------------------------------- 1 or 0
(2) 4, 5, 6 and 7 ---------------------------------------------------------------------------------------- 1 or 0
Total : 11 + 1 marks 1. (b) (i) By asking the students to explain their conclusion without using genetic diagrams, this
question is set in such a way as to probe students' understanding of genetics. While mostcandidates stated correctly that blood group A is the dominant character, many failed topresent a logical explanation, showing that they had not yet mastered the deductivereasoning needed for interpreting crossing results. First of all, they had to look for thecrossing that gave a clue to the nature of the two blood groups in relation to dominance orrecessiveness. Some candidates wrongly explained that blood group A is dominant becauseit is the blood group of a majority of individuals in the family. Many candidates pointed outcorrectly that the cross results between individuals 3 and 4 could indicate which blood groupis dominant, but their explanation was far from complete and logical. The following illustrateshow to arrive at a logical conclusion from the result of the cross :1. Both individuals 3 and 4 are of blood group A, so each of them must possess at least
one allele for blood group A.2. One of their children (individual 12) is of blood group O. This means that at least one
of the parents must also possess one allele for blood group O.3. Therefore at least one of the parents is heterozygous.4. In a heterozygous condition, only the dominant allele will be expressed while the effect
of the recessive allele will be masked. Since both individuals 3 and 4 are of blood group A, blood group A should be the dominant character.
(ii) The performance of candidates in this question shows that many of them were not familiarwith the basic rules of defining allele symbols used in genetic crossings. A common errorwas to use ‘ A for the dominant character or gamete ’, showing that these students had apoor concept of alleles. Another mistake was using A for the dominant allele and O for therecessive allele.
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Paper I Marking Scheme & Year Report P.3
(iii) Some candidates wrongly stated that individuals 3 to 13 were all children of individuals 1 and2.
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1. (c) (i) (1) diameter = 53 (50 to 55) × 0.03 mm --------------------------------------------------------------1
= 0.159 mm (0.150 mm to 0.165 mm)
(no unit, no mark) -----------------------------------------------------------------------1
(2) because cell A contains food store -----------------------------------------------------------------1
(ii) (1) * ovary -----------------------------------------------------------------------------------------------------1
(2) * testis -------------------------------------------------------------------------------------------------------1
(iii) (1) fertilization --------------------------------------------------------------------------------------------------1
(2) It forms a zygote -------------------------------------------------------- )
which will develop into a new individual ------------------------- )
OR
It combines the genes from both parents ----------------------- )
to result in genetic variation in the offspring -------------------- ) ------- any TWO point 1+1
OR
It restores the diploid chromosome number -------------------- )
which has been reduced to half by meiosis during
gamete formation / so as to maintain the chromosome
number of the species constant ----------------------------------- )
(iv) The penis becomes erect ------------------------------------------------------------------------------------1
and it is inserted into the vagina ----------------------------------------------------------------------------1Semen is then ejaculated ------------------------------------------------------------------------------------1
Total : 11 marks
1. (c) (i) (1) The performance on this question was good, although some candidates did not show
their calculation.
(2) A considerable number of candidates wrongly wrote that cell A is large so as to provide
a large area for meeting cell B. Some candidates did not seem to understand the word
‘account ’ and attempted to calculate the size of cell A.
(ii) Some candidates did not read the question carefully. They gave the names of the cells
instead of the organs that produce them.
(iv) A number of candidates misinterpreted the question. They elaborated on how semen is
produced and failed to describe the process of semen transfer.
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2. (a) (i) Ventilation rate = 1000 × 36 cm 3 per minute -----------------------------------------------------------1
= 36000 cm 3 per minute (no unit, no mark)
(ii) To supply more oxygen to the skeletal muscles ----------------------------------------------------- 1,1
for faster respiration / for more energy supply ----------------------------------------------------------1
To remove carbon dioxide more rapidly ------------------------------------------------------------------1
(iii) Dust particles are trapped by the hair in the nose and ------------------------------------------------1
mucus along the lining of the air passage / nasal cavity / trachea ---------------------------------1
(iv) large, clear, accurate diagram (D) ------------------------------------------------------------------- 1 or 0
labels and title (any four) (L) ----------------------------------------------------------------- × 4 each
graduated
plastic bottle
exhaled air
rubbertubingwater
trough
A set-up used to estimate the volume of air exhaled in one breath
Total : 12 marks
2. (a) (ii) Many candidates only stated that the increase in ventilation rate during exercise is to supply
more oxygen to body cells, without specifically mentioning the muscle cells which are more
active and so need more energy during exercise.
(iii) A considerable number of candidates stated wrongly that hairs are found in the nasal cavity
and that dust particles are trapped by the cilia
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Paper I Marking Scheme & Year Report P.6
2. (b) (i) The fluid in the boils contains the cowpox virus / pathogen / germ --------------------------------1
which cannot be killed by the body on its first exposure to the pathogen ------------------------1
The virus multiplies/damages body cells -----------------------------------------------------------------1
so James became ill.
After a few days, sufficient antibodies / phagocytes (white blood cells) are produced ---1
to kill the pathogen, so James got well again.
Communication skill (C) -----------------------------------------------------------------------------------1
(ii) The previous exposure to the cowpox antigen was
'memorized' by certain white blood cells -----------------------------------------------------------------1
The pathogens / antigens of smallpox and cowpox are similar -------------------------------------1
It can stimulate the production of a lot of antibodies / phagocytes in a short time ---------- 1,1
and kill the smallpox virus.
(iii) No ----------------------------------------------------------------------------------------------------------------1
because his immunity is specific to the antigen / pathogen -----------------------------------------1
OR because antibodies are specific in action
Total : 10 + 1 marks
2. (b) (i) Many candidates did not explain why James caught cowpox. Some stated wrongly that
cowpox was caused by an antigen rather than a pathogen, showing that they did not have a
clear concept of immunity and infection.
(ii) To explain why James was immune to smallpox, many candidates just stated that it was
because smallpox was similar to cowpox. They should point out more specifically that it was
due to the similarity of the antigens of smallpox and cowpox. Some also failed to state that
the immunity against smallpox is due to the fast and large production of antibodies.
(iii) A considerable number of candidates answered ‘ yes ’ which was wrong, but gave an
explanation that was correct. The likely reason for this contradiction is that they did not
understand the meaning of ‘ immune to other infectious diseases ’.
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2. (c) (i) (1) cell type 1 - D --------------------------------------------------------------------------------------------1
(2) cell type 2 - C --------------------------------------------------------------------------------------------1
(ii) (1) * Mitosis / cell division -----------------------------------------------------------------------------------1
(2) to increase the cell number ---------------------------------------------------------------------------1
for growth / repair of the root -------------------------------------------------------------------------1
(iii) It has an outgrowth / root hair -------------------------------------------------------------------------------1
to provide a large surface area -----------------------------------------------------------------------------1
for the absorption of water/minerals from the soil ------------------------------------------------------1
Communication skill (C) -----------------------------------------------------------------------------------1
Total : 8 + 1 marks
2. (c) (i) Many candidates wrongly stated that cell type 1 is found in region B, which is the zone ofmaturation of the root.
(ii) (2) Most candidates stated correctly that mitosis is for growth or repair of the root, but
failed to point out that this role resulted from an increase in the number of cells in the
root.
(iii) Some candidates stated wrongly that the root hair is used for anchorage or for penetrating
the soil.
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Paper I Marking Scheme & Year Report P.8
3. (a) (i) Bacteria break down the food -------------------------------------------------------------------------------1
releasing acid ---------------------------------------------------------------------------------------------------1
(ii) The acid dissolves the enamel / dentine of the tooth -------------------------------------------------1
(iii) - avoid eating snacks between meals )
to reduce the chance of acid formation )
- rinse the mouth / brush the teeth after eating ) any ONE --------------------------------- 1 + 1
to remove food debris / acid / plaque )
- use alkaline toothpaste )
to neutralize the acid )
(accept other reasonable answers)
(iv) large and clear diagram (D) ----------------------------------------------------------------------------- 1 or 0labels and title (any five) (L) ------------------------------------------------------------------------- 5 ×
sign of early decay (S) -------------------------------------------------------------------------------------
A vertical section of a tooth at the early stage of decay
Total : 9 marks
3. (a) (i) This part was well attempted, but some candidates answered very imprecisely by saying
that 'the bacteria react with the food to form acid'; some even stated that bacteria secrete
acid or that saliva is acidic.
(ii) Many candidates just stated that tooth decay is caused by the action of acid on teeth,
without specifically pointing out that the hard material of teeth (enamel or dentine) is being
attacked.
(iii) Some candidates gave answers that were not based on the information provided, e.g.
seeing the dentist and drinking water with fluoride.
(iv) The standard of drawing was poor and very few candidates drew a clear and accurate
diagram. Most candidates did not give a title to their drawings. Some wrongly labelled the
blood capillaries in the pulp cavity as “blood vessels” and the nerve endings as ‘nerves’.
There were many spelling mistakes, such as 'plup' for pulp , 'enamal' for enamel and 'dentin'
for dentine .
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Paper I Marking Scheme & Year Report P.9
3. (b) (i) (1) 5 - 3 - 4 --------------------------------------------------------------------------------------------- 1 or 0
(2) 1 - 2 - 4 ---------------------------------------------------------------------------------------------- 1 or 0
(ii)
Reflex actions Voluntary actions
cerebrum is not involved cerebrum is involved
stereotyped response is produced different response are produced
a receptor is involved a receptor may not be involved / may
be initiated spontaneously
inborn not inborn
usually faster usually slower
any TWO ------------------------------------------------------------------------------------------------------ 1,1
(iii) A receives / integrates sensory impulses to )
make an appropriate decision for action )OR any ONE set ------------------------- 1+1
A sends nerve impulses to the skeletal mussels )
to bring about the movement )
B coordinates the action of the skeletal muscles / receives
impulses from semi-circular canals etc. ----------------------------------------------------------1
to maintain balance of the body ----------------------------------------------------------------------------1
C increases / controls the rate of heart beat / the rate and depth of breathing ----------------1
to supply more oxygen to the skeletal mussels ---------------------------------------------------------1
Total : 10 marks
3. (b) (i) (2) This part was poorly answered. Many candidates simply did not understand that
voluntary action can be initiated by the cerebrum without involving stimulation of
receptors in the fingers. A wrong answer given by many candidates was as follows :
5 3 2 1 2 4
(ii) Some candidates still had the misconception that reflex actions do not involve the brain. In
fact, reflex actions do not involve only the cerebrum. Many candidates stated that reflexactions are faster than voluntary actions. Although this answer was accepted, it should be
pointed out that such a statement is an over-generalization and may not be correct in some
situations. Voluntary actions can sometimes be very fast.
(iii) This part was answered poorly. Most candidates only regurgitated statements about the
general roles of the cerebrum, cerebellum and medulla without relating them to the situation
when a man is riding a bicycle.
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3. (c) (i) Sea water contains a higher concentration
of salts / Water potential of sea water is lower than that of the root cells -----------------1
As a result, water will be drawn out of the cells by osmosis ---------------------------------------1
and the plants will become dehydrated / wilted --------------------------------------------------1
Communication skill (C) -----------------------------------------------------------------------------------1
(ii) Crops absorb minerals from soil / minerals are leached away by irrigation --------------------1
When the crop is harvested, minerals are removed from the field ---------------------------------1
As minerals are essential for crop growth, -------------------------------------------------------------1
fertilizers are added to replenish the lost minerals -----------------------------------------------------1
Communication skill (C) -----------------------------------------------------------------------------------1
(iii) Conservation of rain forests helps
- to maintain a diversity of species -------------------------------------------------------------------------1
- to conserve the natural habitat ----------------------------------------------------------------------------1
- to maintain a constant level of carbon dioxide / oxygen in the atmosphere -------------------1
- to prevent soil erosion ---------------------------------------------------------------------------------------1
(accept other reasonable answers and give credit for elaboration)
Total : 11 + 2 marks
3. (c) (i) Although many candidates correctly stated that the water potential of the sea water is lowerthan that of the root cells and that the plant would become dehydrated, very few mentioned
that water is lost from the root cells by osmosis.
(ii) Many candidates failed to give a good and comprehensive explanation for the statement
given in this question. Some did not even mention that fertilizers contain minerals for plant
growth
(iii) Most candidates mentioned the importance of tropical rain forests in maintaining the balance
of atmospheric carbon dioxide and oxygen, but they failed to state other important roles
such as maintaining a diversity of species, conservation of natural habitats and prevention of
soil erosion.
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4. (a) (i) (1) B, C (E optional) (all or no mark) ------------------------------------------------------------ 1 or 0
(2) E ------------------------------------------------------------------------------------------------------------1
(ii) (1) Water moves from cell to cell ------------------------------------------------------------------------1
by osmosis ------------------------------------------------------------------------------------------------1
(2) Carbon dioxide in air diffuses ------------------------------------------------------------------------1
through stomatal pores --------------------------------------------------------------------------------1
into the air spaces to cell type B --------------------------------------------------------------------1
(iii) Water is split into hydrogen and oxygen ----------------------------------------------------------------1
Hydrogen combines with carbon dioxide -----------------------------------------------------------------1
to form carbohydrate inside cell type B -------------------------------------------------------------------1
Communication skill (C) -----------------------------------------------------------------------------------1
Total : 10 + 1 marks
4. (a) (ii) (1) Most candidates simply mentioned that water moves from cell G to cell B by osmosis,
without pointing out that water is transported across a number of adjoining cells before
reaching cell B.
(2) A common mistake was saying that carbon dioxide passes through cell E, which is theguard cell, instead of through the stomatal pore.
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4. (b) (i) (1) A - * blood capillary -------------------------------------------------------------------------------------1
B - * sweat gland ----------------------------------------------------------------------------------------1
(2) More blood will reach A --------------------------------------------------------------------------------1
As A is close to the surface / provides a larger surface area ---------------------------------1
it will lead to faster heat loss by radiation / conduction / convection -----------------------1
B will produce more sweat to the skin surface -------------------------------------------------1
to increase heat loss by the evaporation of water ----------------------------------------------1
which absorbs body heat ----------------------------------------------------------------------------1
Communication skill (C) ----------------------------------------------------------------------------1
(ii) This damage may lead to excessive loss of water from the body ----------------------------------1
and bacterial infection -----------------------------------------------------------------------------------------1
Total : 10 + 1 marks
4. (b) (i) Structure A shows blood capillaries but some candidates named it inaccurately as blood
vessels. When explaining the function of structure A, many incorrectly stated that it
undergoes vasodilation when the body temperature rises. Actually, vasodilation takes place
in the arterioles located in the deeper part of the dermis. As a result of the vasodilation of
these arterioles, more blood flows into structure A which presents a large area close to the
skin surface for rapid heat loss by radiation and convection.
Many candidates did not realize that sweat is produced by structure B most of the time, and
when the body temperature rises, more sweat is produced. A considerable number of
candidates also failed to point out that sweating is effective in promoting heat loss from the
body only when the sweat evaporates.
(ii) Only a small number of candidates were able to point out that with a large area of C being
damaged, excessive water would be lost through the skin, and that this might endanger the
person‘s life.
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4. (c) (i) (1) site B -------------------------------------------------------------------------------------------------------1
(2) Pig farm discharges animal waste ------------------------------------------------------------------1
which contains a lot of bacteria / which promotes the growth of bacteria -----------------1
(ii) A large number of bacteria / decomposers at site B consumes a lot of oxygen ----------------1
Thus the dissolved oxygen content is lower at site B than at site A -------------------------------1
(iii) Ammonia is converted to nitrate ----------------------------------------------------------------------------1
(iv) Algal population increases ---------------------------------------------------------------------------------1
due to an increasing level of nitrate ----------------------------------------------------------------------1
which is a nutrient for the growth of algae ---------------------------------------------------------------1
(v) To treat the waste before discharging it )
To reuse the waste as fertilizer ) any ONE ------------------------------------1
To set up laws to control the discharge of waste )
Total : 10 marks
4. (c) The performance in this question is satisfactory in general. For part (ii), however, many
candidates did not relate the decrease in ammonia concentration to the conversion of ammonia
to nitrate, which is an important process in the nitrogen cycle. Some even said that ammonia is
used up by the bacteria.