WAEC SSCE - Biology - 2018 (Essay)

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(a) Explanation of terms

• (i) Gene: a unit of hereditary material, a length of DNA on a chromosome, that carries the code for a particular characteristic and is passed from parent to offspring.
• (ii) Hybrid: the offspring produced from a cross between two genetically different (unlike) parents; in genetics a heterozygote carrying two different alleles of a gene.
• (iii) Trait: an observable inherited characteristic or feature of an organism, such as flower colour or height.

(b) Cross between two heterozygous yellow flowers

Let \(Y\) = allele for yellow (dominant) and \(y\) = allele for the recessive colour. Each heterozygous parent is \(Yy\).

Parental genotypes: \(Yy \times Yy\)

Gametes: \(Y\) and \(y\) from each parent.

Genotypic ratio of F1: \(1\,YY : 2\,Yy : 1\,yy\).

Phenotypic ratio of F1: 3 yellow : 1 recessive (non-yellow), i.e. \(3:1\).

(c) Four transmitted (inherited) characters in plants

1. Flower colour.
2. Seed shape (round or wrinkled).
3. Height of the plant (tall or short).
4. Fruit or seed colour. Leaf shape is also acceptable.

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None

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(a) Explanation of terms

(i) Renewable natural resources: natural resources that can be replaced by natural processes or re-grown after use, so they are not permanently exhausted, for example forests, fish and water.

(ii) Non-renewable natural resources: natural resources that are present in fixed amounts and cannot be replaced within a human lifetime once used up, for example crude oil, coal and mineral ores.

(b) Two examples each

• (i) Renewable: forests (timber) and fish (wildlife). Water and solar energy are also acceptable.
• (ii) Non-renewable: crude oil (petroleum) and coal. Natural gas and metallic ores are also acceptable.

(c) Five ways of conserving forests

1. Afforestation and reafforestation (planting new trees to replace those felled).
2. Selective felling of only mature trees instead of clear felling.
3. Preventing and controlling bush burning.
4. Establishing and protecting forest reserves.
5. Enacting and enforcing laws against indiscriminate cutting of trees.

(d) Four effects of adding animal manure to garden soil

1. It adds humus and improves the fertility of the soil.
2. It improves the soil structure and aeration.
3. It increases the water-holding (moisture-retaining) capacity of the soil.
4. It increases the population of useful soil micro-organisms and releases nutrients as it decays.

(e) Possible genotypes in humans

Taking a single gene pair with a dominant allele and a recessive allele (for example alleles A and a), the three possible genotypes are:

• \(AA\) - homozygous dominant
• \(Aa\) - heterozygous
• \(aa\) - homozygous recessive

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(a)(i) Identity of the organisms: A is a butterfly (Papilio sp.); B is a housefly (Musca sp.).

(a)(ii) Parts labelled I to X:

• I - head
• II - antenna
• III - thorax
• IV - forewing
• V - abdomen
• VI - antenna
• VII - head
• VIII - compound eye
• IX - wing
• X - leg

(b) Function of parts VI, VIII, IX and X

• VI (antenna): for feeling, and sensitivity to touch, smell and vibration.
• VIII (compound eye): for sight or vision.
• IX (wing): for flight and balancing during flight.
• X (leg): for walking, crawling and running.

(c) Specific habitats

A (butterfly) lives in gardens and farms among flowering plants and trees. B (housefly) lives around latrines, refuse dumps, sewage, exposed food and decaying organic matter.

(d) Three structural adaptations of organism A (butterfly)

• Large, broad wings for flight.
• A long, thin proboscis for sucking nectar from flowers.
• Long clubbed antennae and large compound eyes for sensitivity and wide vision.

(e)(i) Three structural similarities between A and B: both possess wings; both have the body divided into head, thorax and abdomen; both have a pair of antennae and a pair of compound eyes.

(e)(ii) Three structural differences between A and B

(f)(i) Class of A and B: Insecta.

(f)(ii) Three reasons: the body is divided into head, thorax and abdomen; there are three pairs of jointed legs on the thorax; there is one pair of antennae and a pair of compound eyes (with wings present).

(g) One biological importance of organism A (butterfly): it pollinates flowers (and also serves as a source of food for other organisms).

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(a) Five processes involved in the water cycle

1. Evaporation (of water from seas, rivers and land).
2. Transpiration (loss of water vapour from plants).
3. Condensation (water vapour forming clouds).
4. Precipitation (rain, hail or snow).
5. Run-off (and infiltration into the ground).

(b) Instruments for measuring each physical factor

(c) Contributions to the cell theory

• Robert Hooke: using an early microscope he examined thin slices of cork and observed and named the tiny box-like compartments \'cells\', the first observation of cells.
• Theodore Schwann: he proposed that all animals are made of cells and, with Schleiden, stated that all living things are composed of cells, so the cell is the basic unit of life.

(d) Meaning of terms

• Nastism (nastic movement): a non-directional response of a plant part to a diffuse (non-directional) stimulus, for example the folding of Mimosa leaves when touched.
• Taxism (taxis): the movement of a whole organism towards or away from a directional stimulus, for example a Euglena swimming towards light.

(e) A diagram 6 to 8 cm long of the human tongue is required, showing and labelling the four taste areas: sweet (tip), salt (front sides), sour (back sides) and bitter (back of the tongue).

(f)(i) Four ways microorganisms enter the human body

1. Through the mouth in contaminated food and water.
2. Through the nose in inhaled air (droplets).
3. Through cuts, wounds or breaks in the skin.
4. Through the reproductive tract (sexual contact) or insect bites.

(f)(ii) Three harmful effects of microorganisms to humans

1. They cause diseases such as cholera, tuberculosis and malaria.
2. They cause food spoilage and food poisoning.
3. They produce toxins that poison the body and cause decay of body tissues (sepsis).

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(a) Explanation of terms

Ecosystem: a natural unit made up of all the living organisms (the biotic components) in a given area together with the non-living surroundings (the abiotic components) with which they interact to form a stable, self-supporting system through which energy flows and nutrients are recycled.

Abiotic environment: the non-living physical and chemical part of a habitat, that is, factors such as light, temperature, water, dissolved oxygen, wind, pH and soil, which affect the organisms living in that habitat.

(b) Three ecological factors specific to each habitat

Aquatic habitats: salinity (of the water); turbidity (cloudiness of the water); water current (tidal/wave movement). Dissolved gases and density are also acceptable.

Terrestrial habitats: wind; humidity (of the air); topography (relief of the land). Soil (edaphic factors) and altitude are also acceptable.

(c) Relationship between green grass and a rabbit in a given habitat

The relationship is a feeding (food) relationship that also involves an exchange of gases:

• The green grass is the producer; it photosynthesises to manufacture its own food.
• The rabbit is a herbivore (primary consumer); it cannot make its own food and so feeds on the grass, thereby obtaining energy from it.
• During respiration the rabbit gives out carbon dioxide, which the grass uses for photosynthesis.
• During photosynthesis the grass releases oxygen, which the rabbit uses for respiration.
• When the rabbit defecates or dies, bacteria and fungi decompose the waste, forming humus/nutrients that the grass absorbs from the soil.

Energy therefore flows from the grass to the rabbit, while gases and nutrients are cycled between them.

(d) Pyramid of energy using four feeding levels

A pyramid of energy is drawn as horizontal bars stacked on one another, widest at the base (producers) and narrowing upwards, because usable energy is lost (mainly as heat during respiration) at each successive trophic level. The values shown (in kJ m^-2 yr^-1) are typical and confirm that each higher level contains far less energy than the one below it.

The base (producers) contains the greatest amount of energy and each higher feeding level contains progressively less, giving the upright, narrowing pyramid shape.

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(a) Five vertebrae in mammals and their locations

(b) One function each in a dicotyledonous leaf

• (i) Palisade: contains many chloroplasts and carries out most of the photosynthesis.
• (ii) Vascular bundle (vein): conducts water and mineral salts to the leaf (xylem) and food away from it (phloem), and supports the leaf.
• (iii) Epidermis: protects the inner tissues and (with its cuticle) reduces water loss.

(c) One function each in the stem of a flowering plant

• (i) Sieve tube: conducts (translocates) manufactured food such as sugars through the plant.
• (ii) Cortex: stores food and water and gives support to the stem.
• (iii) Pith: stores food and water in the centre of the stem.

(d) Stem versus root of a maize (monocot) seedling

(i) Two external structural differences

(ii) Two internal structural differences

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(a)(i) Identity of the diagrams

• D = Sweep net (hand net) - a long-handled net with a mesh bag.
• E = Pooter (insect aspirator) - a jar carrying two tubes through its lid.
• F = Quadrat - a square frame of side 1 m divided into smaller squares.

(a)(ii) Names of the parts I to VIII

(a)(iii) One function each of I, III, IV, V and VII

• I (handle): provides a grip so the net can be held and swung to sweep up organisms.
• III (mouth tube): it is sucked to create a partial vacuum inside the jar, drawing the insect in through the inlet tube.
• IV (gauze): prevents the insect and debris from being sucked out of the jar into the collector's mouth.
• V (collecting jar): receives and holds the trapped insects for observation.
• VII (frame): marks out a fixed, known area (1 m by 1 m) within which organisms are counted.

(b) One use each of D, E and F

• D (sweep net): catching small, fast-moving animals such as flying insects (or aquatic organisms when swept through water).
• E (pooter): picking up small, delicate insects and other tiny arthropods from the ground or vegetation without harming them.
• F (quadrat): sampling to estimate the population density or percentage cover of plants and slow-moving/sessile animals in a habitat.

(c) Brief description of how each tool is used

D - Sweep net: The net is swung to and fro (or in a figure-of-eight) through vegetation, air or water while walking along a chosen line. Small animals are trapped in the mesh bag (II); the mouth of the bag is then closed and the catch is transferred into a container for counting and identification.

E - Pooter: The inlet tube (VI) is held close to or directly over the insect. The collector sucks sharply through the mouth tube (III); the suction draws the insect through the inlet tube into the jar (V). The gauze (IV) over the mouth tube keeps the insect inside the jar and stops it entering the mouth. The trapped insect is then examined or preserved.

F - Quadrat: The frame is thrown or placed at random points on the ground. At each placing, the organisms rooted or resting inside the frame are counted (or their cover estimated using the grid squares VIII). Several random placings are made and averaged. The area sampled is

\[ A_q = 1\,\text{m} \times 1\,\text{m} = 1\,\text{m}^2 \]

If \(N\) organisms are counted over \(n\) quadrats, the mean number per quadrat is \(\dfrac{N}{n}\), and since each quadrat covers \(1\,\text{m}^2\) the population density is

\[ \text{Density} = \frac{N}{n}\ \text{organisms per } \text{m}^2 \]

Multiplying this density by the total area \(A\) of the habitat gives an estimate of the whole population:

\[ \text{Estimated population} = \frac{N}{n} \times A \]

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(a) Gases involved in photosynthesis: carbon dioxide (CO₂) and oxygen (O₂).

(b) Role of each gas

• Carbon dioxide: it is a raw material that is taken in and used to manufacture carbohydrate (glucose).
• Oxygen: it is the gas given off (released) as a by-product of photosynthesis.

(c)(i) What is a variegated leaf?

A variegated leaf is a leaf that has both green and non-green (white or yellow) patches, because chlorophyll is present only in the green parts.

(c)(ii) Only the green part of the leaf tests positive (turns blue-black) with iodine solution.

(c)(iii) Two reasons

• Only the green part contains chlorophyll, which traps light energy for photosynthesis.
• Therefore only the green part manufactures and stores starch; the non-green part has no chlorophyll and cannot make food.

(c)(iv) Mineral element needed to form chlorophyll: magnesium (nitrogen is also acceptable).

(d) Classification of the organisms by mode of nutrition

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(a) How the structure of each cell relates to its function

(i) Sperm cell

• It has a long, whip-like tail (flagellum) which lashes from side to side to propel the cell so that it can swim to reach the egg (ovum).
• Its head bears an acrosome filled with digestive enzymes that break down and penetrate the membrane of the egg at fertilisation.
• The head contains a haploid nucleus which fuses with the nucleus of the ovum during fertilisation to form the zygote.
• The middle piece is packed with mitochondria that release the energy (ATP) needed to power the beating of the tail during swimming.

(ii) Palisade cell

• It has a thin cell wall which allows easy penetration of light and rapid diffusion of gases.
• It contains numerous chloroplasts arranged in the peripheral cytoplasm for maximum absorption of sunlight for photosynthesis.
• It is long and column-shaped and closely packed just below the upper epidermis so that many chloroplasts are exposed to light.
• It has a large central vacuole containing cell sap which pushes the chloroplasts to the edges near the light and stores the products of photosynthesis temporarily.

(b) Drawing of the front (anterior) view of the human female reproductive system

The diagram below shows the front view of the female reproductive system, about 8-10 cm long when drawn, fully labelled.

(c) Two differences between reproduction in mammals and in amphibians

(d) Two methods of birth control in humans

• Use of contraceptive pills (hormonal method).
• Use of condoms (barrier method).

Answer Details

(a) Roles in respiration

(i) Glucose

• It is the respiratory substrate (fuel) that is broken down (oxidised) to release energy.
• Its breakdown yields ATP (energy), carbon dioxide and water for the cell's activities.

(ii) Oxygen

• It is used to oxidise glucose completely (it is the final acceptor of hydrogen/electrons).
• It makes aerobic respiration possible, releasing the maximum amount of energy and producing carbon dioxide and water.

(b) Two types of cellular respiration: aerobic respiration and anaerobic respiration.

(c) Structures of gaseous exchange

(d) Diagram of an open stoma

Draw the stoma 6 cm to 8 cm long showing two bean-shaped guard cells enclosing the open stomatal pore (aperture), with the following labels: guard cell, chloroplasts (inside the guard cells), nucleus, stomatal pore (opening), thick inner wall and thin outer wall of the guard cell, and the neighbouring epidermal cells.

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(a) Conversion of food to chyme in the stomach

When food (the bolus) reaches the stomach, the gastric glands in the stomach wall secrete gastric juice, which contains hydrochloric acid, the enzyme pepsin(ogen) and rennin. The hydrochloric acid provides an acidic medium, kills harmful bacteria and activates pepsinogen into active pepsin. Pepsin digests proteins into shorter chains (peptones and polypeptides), while rennin curdles the soluble milk protein (caseinogen) into insoluble casein. At the same time the muscular wall of the stomach contracts rhythmically (churning / peristalsis), thoroughly mixing the food with the gastric juice. The food is gradually turned into a semi-liquid, acidic, partly digested paste called chyme, which is then released in small amounts into the small intestine.

(b) Completed table

(c) Experiment to show that carbon dioxide is necessary for photosynthesis

1. Destarch a potted plant by keeping it in the dark for about 48 hours, then test one leaf to confirm no starch is present.
2. Enclose one leaf (still attached) in a flask containing a little potassium hydroxide (KOH) or soda lime, which absorbs all the carbon dioxide inside; seal the neck with a split cork and vaseline so it is air-tight.
3. Set up a control: enclose another leaf in a flask containing sodium hydrogen carbonate solution (which supplies carbon dioxide).
4. Expose the whole plant to bright sunlight for several hours.
5. Remove both leaves and test each for starch (boil in water, decolourise in alcohol, then add iodine solution).

Result: the leaf deprived of carbon dioxide shows no starch (iodine stays brown), while the control leaf supplied with carbon dioxide turns blue-black, showing starch. Conclusion: carbon dioxide is necessary for photosynthesis.

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(a)(i) Chemosynthesis

Chemosynthesis is a mode of nutrition in which certain autotrophic bacteria manufacture their own organic food using energy obtained from the oxidation of inorganic chemical substances, instead of using light energy as in photosynthesis.

(a)(ii) Two examples of chemosynthetic organisms: nitrifying bacteria (Nitrosomonas and Nitrobacter) and sulphur bacteria (Thiobacillus).

(b) Three atmospheric gases and their percentage composition

(c)(i) Four characteristics of a salt marsh habitat

• The water and soil have a high salt content (high salinity).
• The soil is soft, muddy and waterlogged with little oxygen (anaerobic).
• It is periodically flooded and drained by the tides.
• It is exposed to strong winds and wide changes in temperature.

(c)(ii) Modification of plants for anchorage

Salt marsh plants (e.g. mangroves) develop strong, widely spreading and prop / stilt roots that grow into the soft mud. These extensive roots anchor the plant firmly and prevent it from being swept away by the tides.

(d)(i) Four bacterial diseases linked to poor food hygiene: cholera, typhoid fever, bacterial food poisoning (salmonellosis) and bacillary dysentery.

(d)(ii) Three effects of poor food hygiene: contamination of food and food poisoning; spread of communicable diseases; ill-health and possible death, with economic loss.

(e) How lost blood is restored

The blood is restored by blood transfusion: blood of a compatible (matching) group is collected from a healthy donor and introduced into a vein of the accident victim. Intravenous fluids (a drip) may first be given to restore blood volume, and over time the body itself replaces the lost plasma and blood cells.

(f) Three differences between tillage and bush burning

Answer Details

(a)(i) Type of teeth

• A: Canine.
• B: Molar.
• C: Incisor.

(a)(ii) Teeth found in positions I, II and IV

• I: Incisor.
• II: Canine.
• IV: Molar.

(a)(iii) One function of each

• A (canine): for tearing flesh and piercing food.
• B (molar): for breaking, crushing, chewing and grinding food.
• C (incisor): for cutting and biting food.

(a)(iv) One reason for each answer

• A (canine): it is conical, sharp and pointed.
• B (molar): it is broad with ridges (cusps) on its surface.
• C (incisor): it has a flat, sharp, chisel-shaped edge.

(b) Two factors that may cause decay of tooth B

• Accumulation of acid produced by bacteria acting on food remnants.
• Frequent eating of sweet, sugary or sticky foods with poor cleaning of the teeth.

(c) Total number in the dentition of an adult human

• A (canines): 4.
• B (molars): 12.
• C (incisors): 8.

(d) Tooth absent in a rabbit: the canine (A). A rabbit has a gap (diastema) in place of the canines.

(e)(i) Tooth in position III of diagram D: premolar.

(e)(ii) Part where a tooth is not found in an infant: position IV (the molar).

(f) Five ways of caring for the teeth

1. Brush the teeth regularly, after meals and before going to bed.
2. Reduce the intake of sweet, sticky and sugary foods.
3. Eat foods rich in vitamins and minerals, such as fruits, for strong teeth.
4. Visit the dentist at least twice a year (every six months) for check-ups.
5. Avoid using the teeth on hard objects (such as cracking bones or opening bottle tops) and avoid smoking and excessive alcohol.

Answer Details

(a)(i) Mendel's laws of inheritance

• Law of Segregation (First Law): the two alleles (factors) controlling a character separate during the formation of gametes, so that each gamete carries only one allele of the pair.
• Law of Independent Assortment (Second Law): the alleles of different genes (characters) are distributed to the gametes independently of one another during gamete formation.

(a)(ii) Five applications of genetics in medicine

1. Genetic counselling of couples to predict the risk of hereditary diseases in their children.
2. Diagnosis of inherited disorders such as sickle-cell anaemia and haemophilia.
3. Blood group determination for safe blood transfusion.
4. Production of medicines such as insulin and vaccines by genetic engineering.
5. Determination of paternity and identification through DNA (fingerprinting).

(a)(iii) Three disadvantages of inbreeding

1. It brings together harmful recessive genes, so hereditary defects appear more often.
2. It reduces genetic variation (leads to loss of hybrid vigour).
3. It can reduce fertility, vigour and resistance to disease in the offspring.

(b)(i) What is evolution?

Evolution is the gradual change in the characteristics of living organisms over a long period of time, so that new species arise from pre-existing ancestral forms.

(b)(ii) Convergent versus divergent evolution

Answer Details

(a)(i) Type of teeth in A, B and C:

• A = Canine
• B = Molar
• C = Incisor

(a)(ii) Teeth found in positions I, II and IV:

• I = Incisor
• II = Canine
• IV = Molar

(a)(iii) One function each

• A (canine): for tearing flesh and piercing food.
• B (molar): for breaking, crushing, chewing and grinding food.
• C (incisor): for cutting and biting food.

(a)(iv) One reason each

• A (canine): it is conical, sharp and pointed.
• B (molar): it is broad with ridges (cusps) on its surface.
• C (incisor): it has a flat, sharp, chisel-shaped edge.

(b) Two factors that may cause decay of tooth B

• Accumulation of acid together with bacterial (microbial) activity on the tooth.
• Food remnants from sweet, sugary or sticky foods, and lack of proper dental care.

(c) Total number in the dentition of an adult human

• A (canines): 4
• B (molars): 12
• C (incisors): 8

(d) Tooth absent in a rabbit: the canine (tooth A). A rabbit has a gap (diastema) in its place.

(e)(i) Tooth in position III of diagram D: premolar.

(e)(ii) Position where a tooth is not found in an infant: position IV (the molar), because molars are absent in the milk (deciduous) dentition.

(f) Five ways of caring for the teeth

1. Brush the teeth regularly, after meals and before going to bed.
2. Reduce the intake of sweet, sticky and sugary foods, and avoid smoking and excessive alcohol.
3. Eat foods rich in vitamins and minerals, such as fruits, for strong teeth.
4. Remove food particles with a tooth pick and floss the teeth.
5. Visit the dentist at least twice a year (every six months) for check-ups.