WAEC SSCE - Biology - 1995 (Essay)

Détails de la réponse

(a) What is a cell?

A cell is the basic structural and functional unit of life; it is the smallest unit of a living organism that can carry out all the processes of life. All living things are made up of one or more cells.

(b) Structure of DNA

DNA (deoxyribonucleic acid) is the hereditary material that carries genetic information. Its structure is:

• It is a very long molecule made up of two strands twisted round each other to form a double helix (like a twisted ladder).
• Each strand is a chain of units called nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
• There are four bases: adenine (A), thymine (T), guanine (G) and cytosine (C).
• The two strands are held together by hydrogen bonds between the bases. Bases pair specifically: adenine with thymine and guanine with cytosine (complementary base pairing).
• The sugar and phosphate groups form the sides (backbone) of the ladder, while the paired bases form the rungs.

(c) Process of mitosis

Mitosis is a type of cell division in which a parent cell divides to produce two daughter cells, each with the same number of chromosomes as the parent. It occurs in stages:

1. Interphase: the cell grows and the DNA (chromosomes) is copied (replicated).
2. Prophase: chromosomes shorten, thicken and become visible; each appears as two chromatids joined at the centromere; the nuclear membrane begins to break down and spindle fibres form.
3. Metaphase: chromosomes line up singly along the equator (middle) of the cell, attached to the spindle fibres.
4. Anaphase: the centromeres split and the two chromatids of each chromosome are pulled apart to opposite poles of the cell.
5. Telophase: the chromatids reach the poles; a new nuclear membrane forms around each group; the chromosomes uncoil.
6. Cytokinesis: the cytoplasm divides, forming two identical daughter cells.

(d) Importance of mitosis

• It brings about growth by increasing the number of cells.
• It repairs damaged tissues and replaces worn out or dead cells (healing of wounds).
• It is the basis of asexual reproduction in many organisms.
• It ensures that daughter cells receive an exact and constant number of chromosomes, keeping the species true to type.

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(a) Labelled diagram of a vertical section of a mammalian tooth

(b) Roles in mammalian nutrition

1. Teeth: They cut, bite, tear and grind food during mastication. This breaks food into small particles, increases its surface area for enzyme action and makes swallowing easier.
2. Tongue: It moves food around the mouth during chewing, mixes it with saliva, moulds the food into a bolus and pushes the bolus into the pharynx for swallowing. It also bears taste buds for tasting food.
3. Saliva: It moistens and lubricates food, making chewing and swallowing easy. It acts as a solvent for soluble food substances and contains salivary amylase (ptyalin), which begins the digestion of cooked starch to maltose. It is slightly alkaline.
4. Stomach: It temporarily stores food and churns it, thereby mixing it with gastric juice. Gastric juice contains hydrochloric acid, which provides an acidic medium and kills many bacteria, and pepsin, which starts protein digestion. In young mammals, rennin coagulates milk protein.

(c) Test for protein: Biuret test

1. Prepare an aqueous suspension or solution of the food sample in a clean test tube.
2. Add about 2 cm³ of dilute sodium hydroxide solution and shake.
3. Add a few drops of dilute copper(II) sulphate solution and mix gently.

Observation: A violet or purple colour indicates the presence of protein. If protein is absent, the mixture remains blue.

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(a) Sensory cell

A sensory cell (receptor cell) is a specialised cell that is sensitive to a particular stimulus (such as light, sound, heat, touch or chemicals) and converts the stimulus into a nerve impulse which is sent to the central nervous system.

(b) Functions of parts of the eye

• (i) Lens: focuses (refracts) light rays onto the retina and, by changing its shape (accommodation), forms a clear image of both near and distant objects.
• (ii) Vitreous humour: the jelly-like fluid that fills the eyeball behind the lens; it keeps the eyeball firm and in shape and helps to refract light onto the retina.
• (iii) Optic nerve: carries nerve impulses from the retina to the brain, where the impulses are interpreted as sight.

(c)(i) Four structures that protect the eye

Eyelids, eyelashes, eyebrows, and tears (with the conjunctiva/bony eye socket also acceptable).

(c)(ii) How each performs its function

• Eyelids: close over the eye to shut out excess light and to prevent dust and foreign bodies from entering; blinking spreads tears over the eye.
• Eyelashes: hairs on the eyelids that trap dust and small particles, preventing them from reaching the surface of the eye.
• Eyebrows: hairs above the eye that prevent sweat and rain from running down into the eye.
• Tears (from the tear glands): wash away dust and contain an enzyme (lysozyme) that kills bacteria, keeping the eye clean and moist.

(d) Why a person is dazzled in bright sunlight

In a dark room the pupil is widely dilated (opened) to let in as much light as possible, and the light-sensitive pigment in the retina is fully built up. When the person is suddenly exposed to bright sunlight, a large amount of light floods into the eye through the still-wide pupil and over-stimulates the retina, so the person is dazzled and cannot see clearly. After a few seconds the circular muscles of the iris contract, the pupil becomes small (constricts) to reduce the amount of light entering, and the retina adjusts; normal vision then returns.

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(a) Roles in the food chain

• (i) Green plants: they are the producers. Using chlorophyll they trap sunlight and manufacture their own food (glucose) from carbon dioxide and water by photosynthesis. They form the first trophic level and are the source of food and energy for all other organisms in the chain.
• (ii) Herbivores: they are the primary consumers. They feed directly on green plants and convert plant material into animal tissue, passing energy on to the next level.
• (iii) Carnivores: they are the secondary and tertiary consumers. They feed on herbivores (and on other carnivores), transferring energy along the food chain and helping to control the population of the animals they eat.
• (iv) Decomposers: bacteria and fungi that break down dead plants, dead animals and waste matter, releasing simple substances and mineral nutrients back into the soil and air, so that nutrients are recycled for use by green plants.

(b) The carbon cycle

The carbon cycle is the continuous circulation of carbon between the atmosphere, living organisms and the environment. A diagram should show carbon dioxide in the atmosphere at the centre with arrows into and out of it. The main processes are:

1. Photosynthesis: green plants remove carbon dioxide (CO₂) from the atmosphere and fix it into organic food (carbohydrates).
2. Feeding: carbon passes from plants to animals when herbivores eat plants and carnivores eat herbivores.
3. Respiration: plants and animals break down food and release CO₂ back into the atmosphere.
4. Death and decay: when plants and animals die, decomposers (bacteria and fungi) break down their bodies and release CO₂ during decomposition.
5. Combustion: burning of wood and fossil fuels (coal, petroleum, natural gas) releases CO₂ into the atmosphere.
6. Fossilisation: over millions of years the remains of dead organisms form fossil fuels which store carbon.

These processes keep the amount of carbon dioxide in nature roughly in balance.