WAEC SSCE - Biology - 2008 (Essay)

Bayanin Amsa

(a) Main stages of mitotic division

Mitosis is a type of nuclear division that produces two daughter cells each having the same number of chromosomes as the parent cell. Before it begins (interphase) the chromosomes replicate so that each consists of two chromatids joined at a centromere. Mitosis then proceeds through four main stages:

1. Prophase - The chromosomes shorten, thicken and become visible, each seen as two chromatids joined at a centromere. The centrioles move to opposite poles and spindle fibres begin to form. Towards the end, the nuclear membrane and nucleolus break down and disappear.
2. Metaphase - The spindle is fully formed. The chromosomes line up singly along the equator (metaphase plate) of the spindle, each attached to a spindle fibre by its centromere.
3. Anaphase - The centromere of each chromosome splits, and the two chromatids (now called daughter chromosomes) are pulled apart to opposite poles of the spindle by the shortening spindle fibres.
4. Telophase - The daughter chromosomes reach the poles, lengthen and become indistinct. A new nuclear membrane forms round each group and the nucleolus reappears, giving two daughter nuclei. This is followed by cytokinesis, the division of the cytoplasm into two daughter cells.

(b) Four ways in which mitosis is important to living organisms

• It brings about growth by increasing the number of body (somatic) cells.
• It is used in the repair of worn-out tissues and healing of wounds by replacing damaged or dead cells.
• It is the basis of asexual reproduction in many organisms (for example binary fission, budding and vegetative propagation).
• It ensures genetic stability, because each daughter cell receives exactly the same number and type of chromosomes as the parent cell, maintaining the chromosome number of the species.

Bayanin Amsa

(a) Modes of nutrition in plants

• (i) Autotrophic nutrition - This is a mode of nutrition in which an organism manufactures its own organic food (such as glucose) from simple inorganic raw materials. In green plants this is done by photosynthesis, using carbon dioxide and water in the presence of sunlight and chlorophyll.
• (ii) Chemosynthetic nutrition - This is a form of autotrophic nutrition in which an organism synthesises its own food from inorganic materials using energy obtained from chemical reactions (the oxidation of inorganic substances) rather than from sunlight.
• (iii) Carnivorous (insectivorous) nutrition - This is a mode of nutrition in certain green plants that trap and digest small animals, especially insects, to obtain nitrogenous nutrients. Such plants photosynthesise but grow in nitrogen-poor soils, so they supplement their nitrogen supply by digesting the trapped animals.

(b) One example of a plant for each mode

• Autotrophic: maize (or any green plant such as hibiscus).
• Chemosynthetic: nitrifying bacteria such as Nitrosomonas (or Nitrobacter).
• Carnivorous: pitcher plant (Nepenthes) or sundew (Drosera); Venus fly trap.

(c)(i) Three ways nitrogen is added to the soil for plant use

• Nitrogen fixation - free-living and symbiotic nitrogen-fixing bacteria (for example Rhizobium in the root nodules of legumes) convert atmospheric nitrogen into nitrogenous compounds.
• Decomposition - decomposers (bacteria and fungi) break down dead plants, animals and their waste, releasing ammonium compounds which are nitrified to nitrates.
• Lightning and rainfall - electrical discharge during thunderstorms combines atmospheric nitrogen and oxygen into oxides that dissolve in rain and reach the soil as nitrates (application of nitrogenous fertilisers and manure is another way).

(c)(ii) Two ways nitrogen is lost from the soil

• Denitrification - denitrifying bacteria convert soil nitrates back into free nitrogen gas which escapes into the atmosphere.
• Leaching - soluble nitrates are washed down beyond the reach of roots by percolating rain water (uptake by plants and removal at harvest, and loss by soil erosion, are others).

Bayanin Amsa

(a) One function of each structure

• (i) Arteries: carry blood at high pressure away from the heart to the body organs.
• (ii) Capillaries: allow exchange of materials (oxygen, food, carbon dioxide, wastes) between the blood and the body cells.
• (iii) Veins: carry blood at low pressure back to the heart (their valves prevent backflow).
• (iv) Xylem: conducts water and dissolved mineral salts from the roots up to the leaves (and gives support).
• (v) Phloem: conducts (translocates) manufactured food such as sucrose from the leaves to other parts of the plant.

(b) Changes in the composition of blood as it passes through the

• (i) Lungs: blood gains oxygen and loses carbon dioxide (becomes oxygenated).
• (ii) Liver: excess glucose is removed and stored as glycogen so blood sugar is regulated; amino acids are deaminated so urea is added; toxins are removed; blood may gain heat.
• (iii) Kidneys: urea, excess salts and water are removed, so the blood leaving is purified of nitrogenous waste.
• (iv) Small intestine: blood gains digested food such as glucose, amino acids, vitamins and mineral salts (it becomes rich in nutrients after a meal).

(c) Cellular components of blood

1. Red blood cells (erythrocytes)
2. White blood cells (leucocytes)
3. Platelets (thrombocytes)

Bayanin Amsa

(a) Explanation of terms

• (i) Population - A population is the total number of organisms of the same species living together in a given habitat at a particular time, and which can interbreed. Example: all the tilapia fish in one pond.
• (ii) Competition - Competition is the struggle among organisms for the same limited resources such as food, water, light, space and mates. It may be intraspecific (between members of the same species) or interspecific (between members of different species).
• (iii) Succession - Succession is the gradual and orderly replacement of one community of organisms by another in a given area over a period of time, until a stable community (the climax community) is finally established.

(b)(i) Three sources of noise pollution

• Vehicles and heavy traffic (cars, lorries, motorcycles) and aircraft engines.
• Industrial machinery in factories.
• Loud music from public address systems, loudspeakers and entertainment houses (also generators and construction equipment).

(b)(ii) Two ways of reducing noise pollution

• Fitting machines, engines and vehicles with silencers or mufflers, and servicing them regularly.
• Enforcing laws that ban or limit loud noise, and planting trees or building walls that act as sound barriers around noisy areas.

(c)(i) Three effects of smoke on humans

• It irritates the respiratory tract and causes coughing and breathing difficulty (bronchitis, asthma).
• It reduces the oxygen-carrying capacity of the blood (carbon monoxide in smoke combines with haemoglobin).
• It can cause lung cancer and other lung diseases, and irritates the eyes.

(c)(ii) Three effects of sewage on humans

• It contaminates drinking water and spreads water-borne diseases such as cholera, typhoid and dysentery.
• It provides a breeding ground for disease vectors such as houseflies and mosquitoes.
• It produces offensive smells and can cause food poisoning and infection by parasitic worms.

Bayanin Amsa

(a) Hormones

Hormones are chemical substances (organic compounds) produced in very small quantities by the endocrine (ductless) glands of the body, and carried by the bloodstream to specific target organs where they regulate and coordinate the body's activities, growth and development.

(b) Four differences between nervous and hormonal coordination

(c)(i) Three effects of over-secretion of thyroxin

• Raised metabolic rate leading to loss of body weight despite increased appetite.
• Rapid or irregular heartbeat, restlessness, nervousness and excitability.
• Protrusion of the eyeballs and enlargement of the thyroid gland (exophthalmic goitre / Grave's disease).

(c)(ii) Three effects of under-secretion of thyroxin

• Lowered metabolic rate leading to gain in body weight and sluggishness.
• In children, retarded physical and mental growth (cretinism).
• In adults, swelling of the tissues, dry skin and mental dullness (myxoedema); enlargement of the thyroid (simple goitre) where iodine is lacking.

(d) Diagram of the reflex arc

A labelled diagram, 10 - 12 cm long, should show a spinal-cord reflex arc with these labelled parts arranged in order: receptor in the skin, sensory (afferent) neurone with its cell body in the dorsal root ganglion, dorsal root, the grey matter and white matter of the spinal cord with a relay (intermediate) neurone, synapses, motor (efferent) neurone leaving by the ventral root, and the effector (muscle). Arrows should show the direction of the impulse from receptor to effector.

Bayanin Amsa

(a) Transpiration

Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves, but also through the cuticle and lenticels. The evaporation of water from the moist surfaces of the mesophyll cells into the air spaces, followed by diffusion of the vapour out through the stomata, creates a suction pull that draws water up the plant.

(b)(i) Two types of transpiration

• Stomatal transpiration - loss of water vapour through the stomata (the most important, accounting for the greatest proportion).
• Cuticular transpiration - loss of water vapour directly through the cuticle of the leaf and stem surface.

(Lenticular transpiration, through the lenticels of woody stems, is a third minor type.)

(b)(ii) Two factors affecting the rate of transpiration

• Temperature - a rise in temperature increases evaporation and so raises the rate.
• Humidity - high atmospheric humidity lowers the rate because the vapour-pressure gradient between the leaf and the air is reduced.

(Others include light intensity, wind or air movement, and available soil water.)

(c) Mechanism of opening and closing of the stomata

Each stoma is bordered by two guard cells whose inner walls (next to the pore) are thick and inelastic while the outer walls are thin and elastic. Opening and closing depend on changes in the turgor of these guard cells, controlled by movement of ions and sugars.

Opening (usually in daylight): In light, guard cells photosynthesise and potassium ions (K⁺) are actively pumped into the guard cells. This lowers the osmotic potential of the guard cells, so water enters them by osmosis from the neighbouring epidermal cells. The guard cells become turgid; because the thin outer wall stretches more than the thick inner wall, the guard cells curve outward and the pore between them opens.

Closing (usually at night or when water is scarce): K⁺ ions move out of the guard cells, raising their osmotic potential, so water leaves them by osmosis. The guard cells become flaccid, lose their curvature, straighten and come together, closing the pore.

(d)(i) Two similarities between transpiration and sweating

• Both involve the loss of water from the body of the organism to the environment.
• Both bring about cooling of the organism by the evaporation of water.

(d)(ii) Two differences between transpiration and sweating