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 10°C 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 30°C) while at the same time, the respiration rate (and hence oxygen demand) of the animal will double / increase. (2)

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AL BIO 1990 (d)

Alex Lam

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 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)

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AL BIO 1990 2.

(a)

Alex Lam

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)

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AL BIO 1990 (d)

Alex Lam

Dispersal of seeds －

reduces overcrowding (intraspecific competition)

1)

－

enables colonization of new habitats

1)

－

permits escape from species-specific pests and diseases

1)

any 2 (2)

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AL BIO 1990 3.

(a)

Alex Lam

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 not be fully turgid

means that the mesophyll cells will

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

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AL BIO 1990

Alex Lam

at night time (4) (c)

Water lost by transpiration is replaced by evaporation water is in turn drawn

from the mesophyll cell walls.

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

14

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

phloem.

6)

any 1 (6)

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AL BIO 1990 4.

(a)

Alex Lam

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 nondecremental (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.

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AL BIO 1990 (d)

Alex Lam

Cerebrum

Spinal cord

Control of voluntary actions i.e. motor

The site for synaptic interaction of somatic

execution. Motor cortex is the region

reflexes, which are involuntary actions

where

descending

motor

(6)

pathways

emerge. The site for sensation and perception

For passage of nerve fibres relaying nerve

e.g. skin sensation, vision, audition,

impulses to and from the brain.

vestibular, taste and olfaction. Cerebral cortex is the final station of all major ascending sensory systems. The site for other integrative functions e.g.

emotion,

memory,

language,

planning,

learning, judgement,

motivation, intelligence, etc.

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AL BIO 1990

Alex Lam

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)

)

－

poisonous substances, (by detoxification)

)

－

red blood cells / pathogens,

)

－

nitrogenous wastes into urea for excretion

)

－

food molecules for storage

)

－

heat,

)

－

bile,

)

－

cholesterol

)

－

red blood cells (in the foetus)

)

－

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

)

－

blood,

)

－

glycogen, (provide an energy reserve)

)

－ －

eliminates converts production of

storage of

(2 each)

(10) N.B. In any case, the answer must have a brief description of the homeostatic mechanism concerned. (b)

carbohydrates －

－ lipids －

structural component of membranes

)

nucleic acid

)

and ATP

)

as energy source

any 2

1

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

)

－

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

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;;)

any 4

 

P.9

AL BIO 1990

Alex Lam

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)

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AL BIO 1990 6.

(a)

Alex Lam

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

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AL BIO 1990

Alex Lam

vasodilation, non-erection of hair and sweating in hot temperatures 1 (any 10 coherently expressed) (b)

max. (10)

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.

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AL BIO 1990

Alex Lam

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

Paper II Marking Scheme

)

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AL BIO 1990 －

Alex Lam

muscle attachment and movement 2+

)

－

maintain mineral balance (Ca )

)

－

production of red blood cells

)

any 4

(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)

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AL BIO 1990 (d)

Alex Lam

The density of water (1 g dm-3 at 4°C) 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)

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AL BIO 1990 8.

(a)

Alex Lam

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)

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AL BIO 1990 (c)

Alex Lam

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 Marking Scheme

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AL BIO 1990

Alex Lam

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.)

Paper II Marking Scheme

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AL BIO 1990 9.

Alex Lam

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.

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AL BIO 1990

Alex Lam

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.

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P.20

AL BIO 1990 11.

Alex Lam

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.

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AL BIO 1990

Alex Lam

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.

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