AL Biology 1990 paper 2 Marking

AL BIO 1990 Alex Lam

Paper II Section A

1. (a) Oxygen debt is a state of oxygen depletion after extreme physical exertion; measured

by the amount of oxygen required to restore the system to its original state. During

vigorous exercise, the metabolic rate and hence the demand of oxygen of the active

muscles increases greatly. The oxygen delivered to the muscle is insufficient to keep

pace with the demand and the muscle cells may undergo anaerobic respiration and

produce ATP by lactic fermentation. 2

As a result, lactic acid builds up in the muscle and the oxygen deficit resulting from this

temporary employment of anaerobic pathway is to be paid off when muscle returns to

rest and adequate oxygen is available. At this time, lactic acid is converted back into

pyruvic acid. Some of the lactic acid built up is exported to the liver and converted into

glycogen. 2

Oxygen debt enables the animal to carry out vigorous exercise beyond the capacity of

aerobic respiration AND without having an unnecessary large reserve of oxygen and

blood supply. Such exercise may be of great survival value to the animal (e.g. run

away, catching preys) 1

(5)

(b) An ectotherm may not be able to regulate its body temperature which increases with

an increase in ambient temperature. 1

With a 10C rise in body temperature, the metabolic rate (rate of enzymic reactions)

and 1

hence the respiration rate will increase / double according 1

to the Q10 rule / explanation of increasing temperature on rate of enzymic reaction. 1

(4)

(c) Aquatic ectotherms will be more affected since an increase in water temperature will

decrease the solubility of dissolved oxygen in water, making oxygen less available

(NOTE : saturated value of dissolved oxygen decreases from 6.19 to 5.27 ml / l in

freshwater and from 5.35 to 4.5 ml/l in 30% sea water when water temperature

increases from 20 to 30C) while at the same time, the respiration rate (and hence

oxygen demand) of the animal will double / increase. (2)

(d) The rate and depth of breathing are controlled by a respiratory centre in the medulls, 1

which is responsive directly or indirectly (through chemoreceptors : the aortic and

carotid bodies in the walls of major arteries) to changes in CO2, H+ and O2

concentrations in blood. 1

An increase in the concentration of CO2 and H+ in the blood stimulates the respiratory

centre which in turn, increases depth of breathing (and vice versa) 1

Lack of oxygen stimulates chemoreceptors in carotid and aortic body and stimulates

respiration. Oxygen receptors however, are sensitive only to large changes in blood

Paper II Marking Scheme P.1


PO2 (<70 mm Hg) and is little affected by slight changes of oxygen in blood. 1

Since oxygen in blood PCO2 and PO2 normally are proportional to one another,

breathing is generally regulated by CO2 in blood.

(4)

(e) The smaller the animal, the farther its oxygen dissociation curve-is shifted to the right. 1

The haemoglobin of small animals therefore unloads more of its oxygen at any given

pressure than does the haemoglobin of a larger animal. 1

Heat loss from an endotherm is proportional to its body surface area. The smaller the

animal, the larger is its S.A. / V ratio and hence its rate of heat loss 1

In order to compensate for the higher rate of heat loss to maintain a constant body

temperature, the smaller animal needs to have a higher metabolic rate 1

The shifting of the curve to the right allows small animals to obtain more oxygen to

sustain their higher metabolic rates at any given oxygen partial pressure. (NOTE :

partial pressure of oxygen in air and the lung is normally higher than 100 mm Hg, and

animals of all size should have their haemoglobin fully saturated with oxygen at such

partial pressure) 1

(5)



2. (a) No motile antherozoids / sperms so do not need free water for fertilisation. 1

Male gametes transferred inside resistant pollen. 1

Use of wind or animals for pollen transfer. 1

Ovules enclosed in ovary / carpels. 1

Embryo protected in seed. 1

(5)

(b) Sexual reproduction :

advantages － genetic variation from random assortment and crossing over

during meiosis 1

－ wide dispersal of seeds 1

disadvantages － dependent on successful pollination 1

－ seed must carry own food store 1

Vegetative reproduction :

advantages － more reliable as no need for pollination 1

－ offspring can get continued food supply from parent plant 1

disadvantages － genetically same as parent 1

－ cannot spread far from parent 1

(8)

(c) Wind-dispersed fruits are dry and typically have some sort of extension of the pericarp

(e.g. wings, pappus) to increase air resistance and reduce the rate of fall. 1

Animal-dispersed fruits are fleshy (succulent) to attract and reward animals, while the

seed or seeds are protected by a tough covering (endocarp or seed coat) to prevent

damage during consumption of the fruit. 1

Colour of wind-dispersed fruits is typically brown (cryptic). 1

Animal-dispersed is typically brightly coloured. 1

Some fruits may develop hook-like structures or produce sticky secretions such that

they attach very easily onto the fur of animals. 1

(5)

(d) Dispersal of seeds

－ reduces overcrowding (intraspecific competition) 1)

－ enables colonization of new habitats 1) any 2

－ permits escape from species-specific pests and diseases 1)

(2)



3. (a) Air movement 1)

In still air there is a boundary layer of saturated air over the leaf surface which reduces

transpiration. Air movements reduce the thickness of this boundary layer and so

increase transpiration.

Relative humidity of the atmosphere 1)

The lower the relative humidity the greater the water potential gradient between the

saturated air inside the leaf and the air outside, and thus the higher the transpiration

rate.

Light intensity 1)

The stomata of most plants open in the light and close in the dark. The transpiration

rate is much higher when the stomata are open because the cuticle acts as a barrier to

the diffusion of water vapour.

Air temperature 1)

A rise in temperature increases the water potential of the air inside the leaf because

increased evaporation from the cell walls keeps the air saturated. In contrast, a rise in

temperature will have little effect on the water potential of the external atmosphere.

Thus a rise in temperature increases the water potential gradient between the leaf and

the surrounding air and hence increases the rate of transpiration.

Soil water 1)

A reduction in the availability of water to the plant means that the mesophyll cells will

not be fully turgid and the cell walls not completely wet. Transpiration will thus be

reduced. any 4

(6)

(b) － reduced density of stomata on leaf surfaces

－ a thick cuticle and / or wax layer - which reduces the rate of water loss when the

stomata are closed

－ stomata sunken in pits below the leaf surface so that the air outside the-stomata

is more saturated than the surrounding atmosphere (or so that the thickness of

the boundary layer is increased)

－ hairs on the leaf surface - increasing the thickness of the boundary layer

－ leaf rolling with the stomata inside increases the humidity of the atmosphere

outside the stomata

－ reduction in the surface area / volume ratio of the plant - reduces water loss per

unit volume of the plant

－ C4 or CAM photosynthesis - increases water use efficiency / opening of stomata

at night time (4)



(c) Water lost by transpiration is replaced by evaporation from the mesophyll cell walls.

water is in turn drawn from the protoplasts of the mesophyll cells. The lowered water

potential of the mesophyll cells results in water being drawn from the xylem of the leaf

veins which is in turn drawn from the xylem elements in the stem. The cohesion of the

water molecules ensures that wate is pulled up the stem as long as transpiration is

continued. (4)

(d) Ringing experiments 6)

Cylinders of bark, which include the secondary phloem, are removed from woody stems and

the contents of the phloem above and below the ring are analysed. When the ring is

between a source and a sink for carbohydrates (e.g. between the leaves and the roots),

sucrose accumulates in the phloem on the source side of the ring only.

Aphid stylets (mouthparts), which penetrate the phloem elements are used to sample the

contents of the phloem. The composition of the phloem sap varies with the photosynthetic

activity of the plant. 6)

Photosynthetically active green leaves are supplied with carbon dioxide labelled with the

radioisotope 14C. The labelled products of photosynthesis spread through the plant in the

phloem. 6) any 1

(6)



4. (a) Resting neurone : 1

－ membrane is highly permeable to potassium ions, but is only slightly permeable

to sodium ions;

－ membrane potential is negative inside with respect-to outside :

Excited neurone : 1

－ membrane is highly permeable to both sodium and potassium ions;

－ membrane potential is positive inside with respect to outside

(2)

(b) (i) nerve impulse propagates along the whole length of the axon in a non-

decremental (all-or-nothing) manner. 1

Mechanism : 4

The inner surface of the excited membrane is locally positive (as a result of influx

of sodium ions).

Potential difference between this active region and the adjacent inactive

membrane.

Local current flows laterally through the axoplasm from the former to the latter

region, causing depolarization of the inactive region.

When this inactive region is depolarized to the threshold, it becomes excited

automatically with the generation of a new action potential.

(5)

(ii) Diameter of axon - conduction speed increases as axon diameter increases

Myelination - myelinated axon has higher conduction speed than unmyelinated

axon (2)

(c) Nerve action potential depolarizes axonal terminal; (5)

Migration of vesicles (contain acetylcholine) to the axonal membrane next to the

synapse;

Release of acetylcholine from presynaptic side into synaptic cleft

Acetylcholine combines with receptors on the post-synaptic membrane

Changes the ionic permeability of the post-synaptic membrane;

Post-synaptic membrane depolarizes to threshold;

Resulted in initiation of action potential in the post-synaptic nerve cell.

Rapid destruction of acetylcholine by the action of cholinesterase.



(d) Cerebrum Spinal cord (6)

Control of voluntary actions i.e. motor

execution. Motor cortex is the region

where descending motor pathways

emerge.

The site for synaptic interaction of somatic

reflexes, which are involuntary actions

The site for sensation and perception

e.g. skin sensation, vision, audition,

vestibular, taste and olfaction. Cerebral

cortex is the final station of all major

ascending sensory systems.

For passage of nerve fibres relaying nerve

impulses to and from the brain.

The site for other integrative functions

e.g. emotion, language, learning,

memory, planning, judgement,

motivation, intelligence, etc.



Paper II Section B

5. (a) regulates － blood sugar, )

(through the intermediary of insulin and glucagon) )

－ lipids, )

(removed form blood, broken down & sent to fat depot) )

－ amino acids, (deamination) )

eliminates － poisonous substances, (by detoxification) )

－ red blood cells / pathogens, )

converts － nitrogenous wastes into urea for excretion ) (2 each)

－ food molecules for storage )

production of － heat, )

－ bile, )

－ cholesterol )

－ red blood cells (in the foetus) )

－ plasma proteins, (e.g. fibrinogen, albumen & globulin) )

storage of － blood, )

－ glycogen, (provide an energy reserve) )

(10)

N.B. In any case, the answer must have a brief description of the homeostatic mechanism

concerned.

(b) carbohydrates － structural component of membranes )

nucleic acid ) any 2

and ATP )

－ as energy source 1

lipids－ as energy reserve )

－ as primary component of membrane )

－ as insulating layer )

－ as carrier of fat soluble vitamins (e.g. A & D) )

proteins : source of (essential) amino acids for

－ growth and repair / structural proteins / enzymes ;;)

－ hormones )

－ enzymes )

－ antibodies ) any 4

－ contractile proteins )

－ energy reserve in starvation )

roughage : make food more bulky to stimulate peristalsis of the gut,

prevent excessive absorption of water in colon / constipation



Discussion : Each of the four food substances has at least one unique function which

cannot be replaced by other food substance.

therefore they are all important in the diet i.e. a balanced diet must consist all

of them.

However, there is still some possible interconversion of food substance

e.g. between carbohydrates, lipids and proteins (excluding the essential a.a.)

except roughage.

(8)

(c) Ruminants do not have cellulase and yet the main bluk of their food is composed of

cellulose. 1

Symbiotic bacteria produces cellulase for cellulose digestion. 1

(2)



6. (a) Mammals maintain a constant body temperature, often one much higher than the

surroundings. This leads to continuous heat loss to the surroundings. 1

The skin plays an important role in temperature regulations as its large surface area is

a potential site for heat loss. 1

Therefore the skin must be a good heat insulator, 1

and this is fullfilled by the layer of subcutaneous fat,

and by the hairs.

(Mentioning of seasonal changes in the subcutaneous fat layer and the hairs 1)

Hair can be raised or lowered

by erector pili muscles

to trap more or less air in a shell layer over the skin

Still air is a poor conductor so more air leads to lower heat loss 1

More blood can be brought nearer the skin surface by vasoconstriction of shunt

vessels

thus opening peripheral capillaries more widely to bring

more blood to surface causing it to cool more rapidly 1

(or vice-versa-explanation of vasodilation of "shunts" 2)

Sweat glands caused to produce sweat from tissue fluid; mostly water

When sweat pours onto body surface and evaporates, heat

energy is lost to the environment 1

Processes controlled by hypothalamus which receives (core)

blood and is activated to stimulate 1

hair erection, vasoconstriction in cold temperatures ; 1

vasodilation, non-erection of hair and sweating in hot temperatures 1

(any 10 coherently expressed) max. (10)

(b) List of functions : (10)

－ mechanical protection / prevents entry of pathogens / antigens

－ light (radiation) protection

－ camouflage

－ waterproofing / dehydration protection

－ touch / sensitivity to pain, pressure, temperature

i.e. sensory

－ vitamin D conversion

－ other functions related to epidermal tissues

e.g. scent glands － for defence as in skunks

－ for sexual recognition as in dogs

－ for recognition of members of their own species

nails, hooves and claws - for defence or offence

any 5 of above (2 marks each) - a brief description of each function and its significance is

expected.



7. (a) Structure :

－ both are firm, supporting connective tissues consisting of cells surrounded by

matrix

cartilage :

－ consists of perichondrium, chondroblasts / chondrocytes and matrix of chondrin)

－ softer, normally no hard calcium salt )

－ no blood vessels & nerve endings ) any 3

－ nutrition by diffusion )

－ a diagram showing / a description of the spatial relationship of the various

components 1

e.g.

bone :

－ consists of periosteum, osteoblasts / osteocytes, canaliculi and lacuna &

Harversian system )

－ harder, with calcium deposition ) any 2

－ nutrition supplied by Harversian cannal / canaliculi )

－ a diagram showlng / a description of the spatial relationship of the various components 1

e.g.



Functions :

cartilage :

－ important supporting tissue in embryonic stage )

－ resists to compression force, provide flexibility and elasticity, serves to absorb

mechanical shocks between articular surfaces of bones ) any 2

－ collagen fibrils for resisting tension which may impose on the tissue )

bone :

－ body support, maintain body shape in adult stage (except for cartilaginous fish) )

－ protection )

－ muscle attachment and movement ) any 4

－ maintain mineral balance (Ca2+) )

－ production of red blood cells )

(8)

(b) Locomotion in mammals involve systems of leverage consisting of bones whose

relative positions in relation to one another must be somehow determined and

maintained to prevent dislocation, and this is the function of the ligaments which join

bones to bones. In addition, the elasticity of the ligament tissue also allows movement

of bones. 2

For every leverage system, there must be an effort applied to move the lever. It is

provided by muscle contraction in this case. Muscles attach to bones via tendons

which are tough, relatively inextensible connective tissue such that force produced by

the muscle can be transmitted to the bones to move them. But tendons have no

contractile ability as muscles do. 2

(4)

(c) Amoeba :

－ by amoeboid movement

－ locomotive organ : pseudopodium (flow of fluid endoplasm inside the layer of

ectoplasm to form a temporary projection of the cell)

－ endoplasm everts when it reaches the leading end of the advancing

pseudopodium, and changes into the stiffer ectoplasm. At the other end the

reverse happens. The movement is the result of changes within the protoplasm,

from gel - solution - gel.

Paramecium :

－ by ciliary movement

－ locomotive organ : cilia (successive cilia in each row beat in a synchronised way,

which propels the animal forward / backward).

－ pattern : spiral movement, so that the asymmetrical body progress on a direct

course. This is achieved by beating the cilia diagonally.

(3)



(d) The density of water (1 g dm-3 at 4C) is similar to that of protoplasm (1.028 g dm-3) and

therefore offers excellent support to most of the aquatic organisms. Air, on the contrary,

has a low density (0.0013 g dm-3 ) and offers poor support to terrestrial organisms. 1

As a result, supporting structures are required in the terrestrial environment, in order

for organisms to erect on the ground against the force of gravity (e.g. for the purposes

of obtaining sunlight for photosynthesis, walking etc.) while supporting structure /

tissue may be greatly reduced or entirely lacking in aquatic organisms since support is

already provided by water. 1

Giant kelp (or any aquatic plant) :

no supportive tissue and structure, since excellent buoyance and support are provided

by water air space inside / pneumatocyst for buoyance 1

Bauhinia (or any land plant) :

supportive tissues and structures are well developed, since poor support is offered by

air adaptive supporting tissues include

xylem

sclerenchyma

collenchyma

woody tissue (secondary thickening)

herbaceous plants rely very much on turgidity of cells for support

(5)



8. (a) Speciation is brought about by reproductive isolation

which stops gene flow between populations.

when populations are isolated, mutation / selection (and genetic drift) occur

independently

and the population can diverge.

Reproductive isolation may be brought about by one or more than one of the following

mechanisms : [with explanations offered :]

－ geographical isolation : by physical barrier / distance 2)

－ ecological isolation : different environment 2)

－ mechanical isolation : cannot interbreed due to physical non-correspondence of

genitalia 2)

－ behavioural isolation : cannot interbreed e.g. courtship behaviour of one fails to

stimulate the other 2)

－ temporal isolation : seasonal barrier : reproduce at different times of the year 2)

－ physiological isolation e.g. stigma and style of recipient flower do not provide

suitable physiological condiditons for germination of pollen grains 2) any 2

－ genetic isoliation / difference in genetic constitution prevents fertilization / zygote

development 2)

－ hybrid inviability / hybrid sterility 2)

－ Polyploidy also plays a significant part in new species formation (especially in

plants), 2)

(6)

(b) Significance :

allows the production of a variety of phenotypes,

of which some may have better survival value than the other

in a particular environment.

Natural selection may therefore be able

to select those advantageous genes and

eliminates those “lens fit” ones from the variety.

Mechanism :

Mutation 1

recombination due to : crossing over 1

independent assortment during meiosis 1

fertilization / gene flow between individuals and populations 1

(7)

(c) because natural selection selects on the overall survival value of a phenotype 1

but not on individual genes, 1

some disadvantageous genes that associate with one or more genes that are of

distinct survival values (on the same chromosome / genome). 1



Together with these advantageous genes, the disadvantageous genes, may therefore

be retained by natural selection without being weeded out. 1

recessive deleterious genes are not expressed out in heterozygous condition. 1

(d) Ecological : weed out the old and weak individuals “less fit” individuals

reduce intra-specific competition

to keep the prey population below the carrying capacity of the habitat.

evolutionary : weed out the “less fit” individual as part of the process of nature selection.

(2)



Paper II Section C

Essays to be marked as a whole and by impression.

Points to be considered in the assessment may include :

1. whether the candidate is answering the question

2. whether the essay is well written and well thought-out (e.g. in organized presentation, with

introduction, paragraphs of facts and discussion, and conclusion)

3. contents and substance of the answer, including the ability to assemble relevant general

knowledge and basic biological facts / concepts

4. some awareness of biology to the subject, ability to draw on general knowledge, and ideally

come evidence of teaching or reading 'round the subject'

(N.B. Errors in the use of English such as spelling and grammatical mistakes should not be

specifically penalized, since a candidate with good command of English will have

obvious advantage (e.g. in writing speed, in organization, in expressing his ideas, etc.)

in essay writing.

Detailed marking scheme (e.g. scope/requirement for (3) could be worked out by the examiner

and the markers after they have examined some sample scripts.)



9. Advantages / disadvantages of greenhouse effects for plants

Advantages :

－ ‘Greenhouse’ effect is a temperature rise

－ causes by increasing level of CO2 and other ‘greenhouse gases’ such as methane, in the

atmosphere

－ this increase is caused by fossil fuel burning of man and deforestation (since industrial revolution)

－ thus increased temperature leads to greater metabolism of plants.

－ may lead to greater growth rates.

－ both on land and in warmer seas

－ faster growth leads to more use of CO2 and could bring a rebalance of CO2.

－ but CO2 production is far more greater than the use even under warmer conditions.

－ furthermore higher CO2 causing ‘greenhouse effect’ will allow greater growth of plant (beneficial

up to 0.15%) only if not limited by water or nutrients + improved water use efficiency as

stomata can be fewer or open shorter time for same CO2 uptake.

－ warmer air will lead to greater transpiration of plants leads to more humid atmosphere which is

also to the advantage of plants.

－ leads to more cloud cover which means perhaps less light for photosynthesis.

－ so could be advantages or disadvantages.

Disadvantages :

－ warming leads to melting of polar ice-caps.

－ so raises water levels giving less land surface for plant growth.

－ no disadvantage for algae and marginal plants.

－ higher could cover could reduce growth through light limitation

－ also more cloud – poor conditions for insect pollinators – less seed set and colonisation.

－ warmer conditions puts pressure on temperature plants – broadleaves die

－ most rainforest is low lying – river marginal so water level rising leads to inundation – death.

－ removal of remaining rainforest reduces CO2 conversion still further so ‘grreenhouse effect’ gets

worse.

－ but by this time land surface reduced to 20% of present, mostly prime productive land.

－ so man has to stop production and return to caves !

－ so long before this man’s production will cease, CO2 will cease so CO2 will fall.

－ also greater sea plant growth may bring farming of marine algae / fresh water plants.

－ may benefit plant growth by selective breeding or disbenefit by over cropping.



10. Adverse effects of agricultural practices on the biosphere :

－ pollution problems related to the use of artificial fertilizers and pesticides, etc.

－ deforestation and its consequences

－ monoculture in relation to extinction of certain species and stability of the agro-ecosystem and its

consequences.

－ artificial breeding / selection of new species and its implications.

－ overgrazing and its implication.

－ ploughing and soil erosion.

N.B. This essay should be marked as a whole.



11. Main points :

organ transplant － kidney

heart

Tissues transplant － blood transfusion

Bone marrow transplant

Cornea replacement

Heart valve replacement

Skin grafting

Tendon and cartilage transplant

Blood vessel transplant

Concept of donor-recipient compatibility :

－ contract transplantation of tissue between genetically different individuals of the same species

(allografts); transfer of tissue between genetically identical individuals; and grafts removed from

and placed on the same organism

－ blood group compatilibility

－ hist-compartibility testing – cell / tissues & serum typing (antigen)

Host response to transplantation － rejection reaction

Immunologic tolerance

(non-vascularized tissues such as cornea normally have no

immune rejection)

Artificial devices : eye lens

Heart pacemaker － electrical

Ball and socket joint － mechanical

Heart valve － mechanical

Prosthetic limb / tooth － mechanical

Cochlear implant － electronics

Post-operative management using immunosuppressive drugs (such as corticosteriods) is directed at

blocking the expression of cell mediated immunity.

N.B. This essay should be marked as a whole.



12. Ways employed to improve food supply

－ eating low on the food chain – save energy loss during trophic transfer

－ improve genetic strain – more productive varieties.

－ better pest control technology – existing 1/3 or 1/2 of agricultural product is lost to pest.

－ better resource management and pollution control – improve soil quality, determine maximum

sustainable yield, better use of fertilizers, better pollution control and resources conservation.

－ better post-harvest technology e.g. at present 1/20 of fisheries produce is lost to spoilage.

－ culture in addition to capture – fish farming etc. would increase productivity significantly.

－ increase arable areas e.g. by reclamation, deforestation, contour farming etc.

－ biotechnological advancement e.g. SCP etc.

Current status of supply and demand

－ current supply & demand are largely unbalanced : surplus exist in some regions (e.g. USA) while

hunger in the others (e.g. Africa). A billion people suffer from starvation and malnutrition at

present.

－ although it has been predicted that food shortage will soon occur since human population will

grow exponentially, while food supply will not be able to match human population growth.

However, remarkable improvement in farming methods have exponentially increases agricultural

production, so that amount of food per person has actually remained roughly constant or

improved slightly in the last 200 years. Such trend is likely to maintain in the for see able future.

－ the main problems of economics – the insufficient buying power of under-developed countries,

rather than insufficient food production capacity in the world.


AL Biology 1990 paper 2 Marking

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Transcript of AL Biology 1990 paper 2 Marking