WAEC SSCE - Biology - 2009 (Essay)

Maelezo ya Majibu

(a) Growth is the permanent (irreversible) increase in size, mass and volume of an organism resulting from cell division, cell enlargement and the addition of new living material.

(b)(i) Two differences between growth in plants and in animals

(b)(ii) Two differences between metamorphosis in housefly and in toad

(c) & (d) Hormones and their effects

(i) Plant growth hormones

• Auxin: promotes cell elongation of shoots, controls phototropism and geotropism, and promotes root formation in cuttings.
• Gibberellin: promotes stem elongation, breaks seed dormancy and stimulates germination and flowering.

(ii) Animal growth hormones

• Growth hormone (somatotrophin, from the pituitary): stimulates general body growth, especially of bones and muscles; excess causes gigantism and deficiency causes dwarfism.
• Thyroxine (from the thyroid gland): controls the rate of metabolism and is necessary for normal growth and development (in amphibians it brings about metamorphosis).

(e) Type of germination

• (i) Monocotyledonous plants (for example maize): hypogeal germination.
• (ii) Dicotyledonous plants (for example bean/cowpea): epigeal germination.

(f) Differences between epigeal and hypogeal germination

Maelezo ya Majibu

(a) A sense organ is a specialised organ or structure that contains receptor cells sensitive to a particular kind of stimulus (such as light, sound, chemicals, heat or touch) and which converts the stimulus into nerve impulses sent to the brain for interpretation.

(b) Four sense organs found in mammals

• Eye (sight)
• Ear (hearing and balance)
• Nose (smell)
• Tongue (taste)
• Skin (touch, temperature, pain)

(c) Why a person who has been spinning feels dizzy

Balance is detected by the semicircular canals of the inner ear, which are filled with fluid (endolymph) and contain sensory receptors. When the person spins, the fluid in the canals is set in motion and stimulates the receptors, which send impulses to the brain telling it that the body is turning. When the person suddenly stops, the fluid keeps moving (by inertia) for a short while and continues to stimulate the receptors. The brain therefore still receives messages that the body is turning even though the eyes report that the body is now still. This conflict of information between the ear and the eyes confuses the brain, and the person feels giddy or dizzy and loses balance until the fluid comes to rest.

(d) Two functions each

(i) Cerebellum

• Controls and coordinates voluntary muscular movements so that they are smooth and accurate.
• Maintains balance and posture of the body.

(ii) Cerebrum

• Centre of intelligence, memory, reasoning, thought and will.
• Controls voluntary actions and interprets sensations (sight, hearing, smell, touch).

(iii) Medulla oblongata

• Controls involuntary vital activities such as heartbeat and breathing rate.
• Controls reflex actions such as coughing, sneezing, swallowing and vomiting, and regulates blood pressure.

Maelezo ya Majibu

(a) The water cycle (annotated diagram required)

The diagram should show a continuous cycle with labelled arrows and brief annotations:

• Evaporation: heat from the Sun evaporates water from oceans, lakes, rivers and moist soil, forming water vapour.
• Transpiration: plants give off water vapour from their leaves into the air.
• Condensation: the rising water vapour cools at high altitude and condenses into tiny droplets forming clouds.
• Precipitation: the droplets join and fall back to the earth as rain (or hail/snow).
• Run-off and drainage: the water flows over the land into streams, rivers and back into the sea, and some sinks into the ground (infiltration) to form ground water, completing the cycle.

The arrows should form a closed loop: sea/land \( \rightarrow \) evaporation/transpiration \( \rightarrow \) condensation (cloud) \( \rightarrow \) precipitation \( \rightarrow \) run-off \( \rightarrow \) back to sea.

(b) Experiment to show that water is conducted in the xylem tissue

• Materials: a fresh leafy herbaceous shoot (for example balsam or Impatiens) with a soft translucent stem, a beaker, water and red ink (eosin), a sharp blade.
• Method: Put water in the beaker and add enough red ink (eosin) to colour it deeply. Cut the base of the shoot cleanly under water and stand it in the coloured water. Leave the set-up in a warm, well-lit place for a few hours.
• Observation: the veins of the leaves and the upper parts of the stem gradually become stained red.
• Now cut a thin transverse (cross) section of the stem and examine it. It is seen that only the xylem tissue is stained red; the other tissues are not stained.
• Conclusion: since the red dye rose only through the xylem, this shows that water (with dissolved dye) is conducted upward through the xylem tissue of the plant.

Maelezo ya Majibu

(a) Why an athlete is given a glucose drink rather than a piece of bread

Glucose is a simple (reducing) sugar that is already in its final absorbable form. It does not need to be digested, so it is absorbed directly and quickly from the alimentary canal into the blood and is at once available to the muscles for respiration to release energy rapidly. Bread contains starch, a complex carbohydrate, which must first be digested (broken down) into glucose by enzymes over a long time before it can be absorbed and used. During a race the athlete needs energy immediately, so glucose supplies it faster than bread.

(b) Test for starch in a food substance

• Put the food substance in a test tube or on a white tile.
• Add a few drops of iodine solution (iodine in potassium iodide).
• Observation: if the yellow/brown iodine turns blue-black, starch is present. If the colour remains yellow/brown, starch is absent.

(c) Five reasons why water is important in the human diet

• It is the medium (solvent) in which digestion and other chemical reactions of the body take place.
• It is needed for the transport of digested food, oxygen, hormones and waste materials in the blood and lymph.
• It helps in the regulation of body temperature through sweating (evaporation of sweat cools the body).
• It is needed for the removal (excretion) of waste products such as urea and excess salts in urine and sweat.
• It keeps body tissues and cells moist and turgid, and replaces water lost through urine, sweat and breathing; it also aids the passage of food and prevents constipation.

Maelezo ya Majibu

(a) Meaning of the term "Food chain"

A food chain is a linear feeding relationship among the organisms in a habitat, showing the transfer of food energy in one direction from the producer (green plant) through a series of organisms, each of which feeds on the one before it (the preceding organism) and in turn provides food for the one after it (the succeeding organism). The energy passes from one trophic level to the next along a straight line.

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(b) A food chain with four trophic levels in a terrestrial habitat

In this chain:

• Grass – producer (1st trophic level)
• Grasshopper – primary consumer / herbivore (2nd trophic level)
• Lizard – secondary consumer / carnivore (3rd trophic level)
• Hawk – tertiary consumer / top carnivore (4th trophic level)

The arrows point from the organism that is eaten to the organism that eats it, showing the direction in which food energy flows.

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(c)(i) Flow of energy through the food chain

The energy flow is unidirectional (non-cyclic). The original source of energy is the Sun. Light energy from the sun is absorbed by the chlorophyll in the grass and used in photosynthesis to build carbohydrates, so light energy is converted into chemical energy stored in the plant's tissues. When the grasshopper feeds on the grass, this chemical energy is passed on to it; when the lizard eats the grasshopper the energy is passed along to the lizard; and finally to the hawk when it eats the lizard. Thus the chemical energy is passed down the chain in one direction only from producer to top consumer; at each level the energy is either utilised (for life activities) or stored in body tissues, and it is not recycled back along the chain.

(c)(ii) How energy in the chain is lost to the environment

Energy is lost at each trophic level, so that only a small fraction (about 10 percent) is passed on to the next level. Energy is lost in the following ways:

• Not all parts of the organism are eaten (roots, bones, uneaten and undigested parts), so not all the energy at a level is consumed by the next.
• Energy is released as heat during respiration.
• Energy is used up in movement, growth and other metabolic (life) activities.
• Energy is lost in excretory products and faeces (egestion and excretion).
• Energy in dead bodies and uneaten remains passes to decomposers rather than to the next consumer.

Because of these repeated losses, the amount of usable energy decreases sharply along the chain, which is why a food chain rarely has more than four or five trophic levels.

Maelezo ya Majibu

(a) Variation is the differences in characteristics (structural, physiological or behavioural) that exist among individuals of the same species, or between the offspring and their parents.

(b) Five advantages of variation within a species

• It enables some members of the species to survive changes in the environment (adaptation and survival of the fittest).
• It provides the raw material for natural selection and evolution, leading to the formation of new species.
• It enables the species to resist diseases and pests, since some individuals will be resistant.
• It allows the species to colonise and exploit different or new habitats.
• It makes selective breeding and improvement of crops and livestock possible by choosing desirable individuals.

(c) Differences between continuous and discontinuous variation

(d) Explanation of terms

• (i) Sex-linked characters: characters whose controlling genes are carried on the sex chromosomes (usually the X chromosome) and are therefore inherited along with sex; they appear more often in one sex (mostly males), for example colour blindness and haemophilia.
• (ii) Co-dominance: a condition in which two contrasting alleles of a gene are both fully expressed in a heterozygote, so neither is dominant over the other, for example the AB blood group where both A and B antigens show, and roan colour in cattle.
• (iii) Hybrid vigour (heterosis): the increased strength, size, yield, fertility or vigour shown by the offspring (hybrid) of a cross between two genetically different (pure-breeding) parents, which is greater than that of either parent.