WAEC SSCE - Biology - 2007 (Essay)

(a) Definitions

• (i) Gene: A gene is a unit of hereditary material located at a specific position (locus) on a chromosome. It is a segment of DNA that carries the coded information for the synthesis of a particular protein and thus controls the expression of a particular characteristic (trait) which is transmitted from parents to offspring.
• (ii) Gene mutation: A gene mutation is a sudden, permanent and heritable change in the base sequence of the DNA of a single gene. It alters the genetic message (for example by substitution, insertion or deletion of a base) so that a different protein, or no functional protein, is produced.

(b) Structures where the processes take place

(c) Four advantages of cross breeding two different breeds of cows

1. It combines the desirable qualities of both breeds in the offspring (for example high milk yield of one breed with the heat/disease tolerance of the other).
2. It produces offspring showing hybrid vigour (heterosis), giving faster growth, greater size and higher productivity than either parent.
3. It can produce animals more resistant to local diseases, ticks and harsh climatic conditions, reducing losses.
4. It improves the economic value of the stock, giving better beef and/or milk quality, higher fertility and greater profit to the farmer.

(d) Three applications of the knowledge of genetics in medicine

1. Genetic counselling: Prospective parents (for example carriers of the sickle-cell gene) are advised on the risk of transmitting hereditary diseases to their children.
2. Diagnosis and screening of hereditary diseases: Conditions such as sickle-cell anaemia, haemophilia, colour blindness and Down syndrome can be detected before or after birth.
3. Blood group and tissue matching: Knowledge of inheritance of blood groups guides safe blood transfusion and organ transplantation (tissue compatibility).

(Also acceptable: production of medicines such as insulin and vaccines by genetic engineering; and forensic use in determining paternity.)

(a) What is a balanced diet?

A balanced diet is a diet that contains all the classes of food (carbohydrates, proteins, fats and oils, vitamins, mineral salts, water and roughage) in the correct proportions and in the right amounts needed by the body for good health.

(b) Classes of food, with the importance of two

The classes of food are: carbohydrates, proteins, fats and oils, vitamins, mineral salts, water and roughage (fibre).

• Carbohydrates: provide energy for body activities and warmth.
• Proteins: provide materials for growth and for the repair of worn-out body tissues.

(Fats and oils give a concentrated store of energy and insulation; vitamins and mineral salts protect the body and keep it working properly.)

(c) Source, substrate and end products of the enzymes

(a) Tropic movement versus nastic movement

(b) How light brings about tropic movement in the stem and root

Light coming from one side causes the growth hormone auxin to move to the shaded (dark) side of the shoot tip. In the stem, the extra auxin on the dark side makes those cells elongate faster than the lit side, so the stem bends and grows towards the light: the stem is positively phototropic. In the root, the same higher concentration of auxin on the dark side slows down growth there (roots are inhibited by high auxin), so the lit side grows faster and the root bends away from the light: the root is negatively phototropic.

(c) Observation when a growing seedling is placed horizontally

Under the influence of gravity, auxin gathers on the lower side of both stem and root. The stem tip bends and grows upward (away from gravity, negatively geotropic) because the lower side grows faster, while the root tip bends and grows downward (towards gravity, positively geotropic) because the extra auxin slows the lower side of the root. The seedling therefore turns so the shoot points up and the root points down.

(d) Five functions of the pituitary gland

1. Secretes growth hormone that controls growth of the body.
2. Secretes hormones that control the thyroid gland (TSH).
3. Secretes hormones that control the adrenal cortex (ACTH).
4. Secretes hormones (FSH and LH) that control the gonads and reproduction.
5. Secretes anti-diuretic hormone (ADH) that controls water reabsorption in the kidney, and prolactin/oxytocin for milk production and birth.

(e) Two differences between growth in plants and in animals

(a) Characteristic features of the vegetation of the tropical rain forest

• Trees are very tall, close together and evergreen, with buttress roots supporting the tall trunks.
• The vegetation shows distinct layers (stratification): an emergent top layer, a continuous canopy, an under-canopy and a sparse shrub/ground layer.
• The dense canopy allows little light to reach the forest floor, so undergrowth is scanty.
• Plants are of many different species (high biodiversity) with broad leaves bearing drip-tips.
• Abundant epiphytes, climbers (lianas) and parasites grow on the trees.
• The climate supporting it is hot and wet all year with high rainfall and high humidity.

(b) Two types of savanna in Nigeria with a town in each

(Also acceptable: Sahel savanna - Maiduguri/Nguru; derived savanna - Enugu.)

(c) Adaptation of animals in arid habitats

(i) Adaptation to drought (water shortage)

1. They excrete concentrated urine and dry faeces to conserve water.
2. They obtain water from metabolic (oxidation) water and from the food they eat, and store fat (for example in the camel's hump) which yields water on oxidation.
3. They have thick, dry, impermeable skin or cuticle to reduce water loss by evaporation.

(ii) Adaptation to high temperatures

1. They are nocturnal, resting in burrows or shade during the hot day and being active at night when it is cool.
2. They have pale/light-coloured body surfaces that reflect heat, and long limbs raising the body above the hot ground.
3. They lose heat through large exposed surfaces (for example large ears) and by panting/sweating where possible.

(a)(i) Five birth control methods

1. Oral contraceptive pill (hormonal method).
2. Male condom / sheath (barrier method).
3. Intra-uterine device (IUD) / coil / loop.
4. Tubal ligation (female sterilization) or vasectomy (male sterilization).
5. Rhythm / safe-period (calendar) method, or the withdrawal method (coitus interruptus).

Other acceptable methods: diaphragm / cervical cap, spermicides / foam, injectable hormones (Depo-Provera), and abstinence.

(a)(ii) Five reasons why family planning is important to a nation

1. It prevents population explosion by controlling rapid, uncontrolled population growth.
2. It promotes maternal and child health, and so reduces maternal and infant mortality rates.
3. It makes child spacing possible, giving mothers time to recover and children better care.
4. It raises the standard of living because the nation's resources (food, jobs, housing, schools, hospitals) can adequately serve the population.
5. It reduces the pressure on social amenities and infrastructure, and helps to prevent indiscriminate abortion, unemployment and poverty, allowing planned national development.

(b) Drawing of the human female reproductive system

The diagram below is a full front (anterior) view of the human female reproductive system, about 8-10 cm long, with every part clearly labelled. The two ovaries lie one on each side; each is linked to the top of the uterus by an oviduct (Fallopian tube) whose funnel-shaped opening carries finger-like fimbriae close to the ovary. The thick, muscular, pear-shaped uterus (womb) is lined internally by the endometrium. The lower narrow neck of the uterus is the cervix, which opens into the vagina.

Path of the egg and functions of the labelled parts: An ovary releases a mature egg (ovulation); the fimbriae sweep it into the oviduct, where fertilization by a sperm normally occurs. The cilia and muscular wall of the oviduct move the egg (or embryo) into the uterus, whose endometrium receives and nourishes the implanted embryo during pregnancy. The cervix closes to protect the developing foetus and dilates at birth, while the vagina serves both as the birth canal and as the receptacle for sperm during mating.

(a)(i) Meaning of pollution

Pollution is the introduction into the environment (air, water or soil) of harmful or unwanted substances (pollutants) in quantities that are injurious to living organisms, degrade the quality of the environment and upset the balance of nature.

(a)(ii) Four pollutants of water

1. Sewage and other domestic organic wastes.
2. Industrial effluents containing toxic chemicals and heavy metals (for example lead, mercury).
3. Crude oil and petroleum products (oil spillage).
4. Agricultural chemicals: fertilizers and pesticides/herbicides washed into water.

(Also acceptable: hot water/thermal discharge, detergents.)

(b) Effects of each pollutant on aquatic organisms

• Sewage and organic waste: It is decomposed by bacteria which use up the dissolved oxygen. The resulting low oxygen (high biochemical oxygen demand) suffocates fish and other aquatic animals, killing them. It also spreads water-borne disease organisms and encourages eutrophication (excessive algal growth).
• Industrial effluents / heavy metals: Toxic chemicals such as mercury and lead poison aquatic organisms directly, killing fish and invertebrates. The poisons accumulate along the food chain (bioaccumulation) and may kill top consumers and humans that eat the fish.
• Crude oil (oil spillage): The oil forms a film on the water surface that prevents oxygen from dissolving into the water, so aquatic organisms are suffocated. It coats the gills of fish and the feathers/bodies of aquatic birds, and blocks light, killing plankton and reducing photosynthesis.
• Fertilizers/agricultural chemicals: Excess nitrates and phosphates cause eutrophication, leading to rapid algal blooms; when the algae die and decay, oxygen is depleted and aquatic animals die. Pesticides directly poison fish and other aquatic organisms.