WAEC SSCE - Biology - 2000 (Essay)

Bayanin Amsa

(a) What is a sense organ?

A sense organ is a specialised organ made up of a group of cells or tissues that contain receptors sensitive to a particular kind of stimulus (light, sound, heat, chemicals, pressure). The receptors detect the stimulus and convert it into nerve impulses which are transmitted along sensory nerves to the central nervous system for interpretation. Examples: the eye, ear, skin, nose and tongue.

(b)(i) Two main functions of the ear

• Hearing - the detection of sound.
• Balance (equilibrium) - the maintenance of body posture and balance.

(b)(ii) Mechanism of hearing

Sound waves in the air are collected and concentrated by the pinna and passed along the external auditory canal (meatus) to the ear drum (tympanic membrane), causing it to vibrate. These vibrations are transmitted across the middle ear and amplified by the three ear ossicles - the malleus, incus and stapes. The stapes presses on the oval window, setting up pressure waves in the fluid of the inner ear. The perilymph in the cochlea vibrates and this in turn makes the endolymph vibrate, stimulating the sensory hair cells of the organ of Corti on the basilar membrane. These cells generate nerve impulses which travel along the auditory nerve to the brain, where they are interpreted as sound.

(c) The eye defect of short-sight (myopia) and its correction

In short-sight the eyeball is too long from front to back (or the eye lens is too strongly converging). Light rays coming from a distant object are therefore brought to a focus in front of the retina; by the time the rays reach the retina they have spread out again, so a blurred image is formed and distant objects cannot be seen clearly. The defect is corrected by placing a concave (diverging) lens in front of the eye. This lens first diverges the incoming parallel rays, so that after passing through the eye lens they are brought to a focus exactly on the retina, forming a sharp image.

Bayanin Amsa

(a) Five classes of food other than carbohydrates

1. Proteins
2. Fats and oils (lipids)
3. Vitamins
4. Mineral salts
5. Water

(Roughage/dietary fibre is also accepted as a class.)

(b) Why glucose rather than corn meal is taken by an athlete ready for a race

Glucose is a simple sugar (monosaccharide) that requires no digestion. It is absorbed directly and quickly through the wall of the small intestine into the blood, so it provides instant energy for the race. Corn meal, on the other hand, is a complex carbohydrate (starch) that must first be digested (broken down by enzymes into glucose) before it can be absorbed. This digestion takes time, so the energy from corn meal is released too slowly to be useful for an athlete about to run.

(c) Test for starch

1. Place a small quantity of the food sample on a white tile or in a test tube.
2. Add a few drops of iodine solution to the sample.
3. Observation: the iodine changes from brown/yellow to a blue-black colour.
4. Conclusion: the blue-black colour confirms that starch is present. (If the colour remains brown, starch is absent.)

Bayanin Amsa

(a) The water cycle

The water cycle is the continuous circulation of water between the earth's surface and the atmosphere. The main stages are:

1. Evaporation: heat from the sun evaporates water from seas, rivers, lakes and the soil into water vapour. Plants add water vapour by transpiration and animals by respiration and sweating.
2. Condensation: the rising water vapour cools at higher altitudes and condenses into tiny droplets that form clouds.
3. Precipitation: when the droplets become large and heavy they fall as rain, hail or snow.
4. Collection / run-off: the water flows over the ground as run-off into rivers, lakes and seas, or sinks into the soil as ground water, and the cycle repeats.

(An annotated diagram should show the sun, the sea, arrows for evaporation and transpiration rising to a cloud, condensation in the cloud, precipitation as rain falling, and run-off returning to the sea.)

(b) Importance of water to living organisms

1. It is the medium in which all metabolic reactions and enzyme actions take place inside cells.
2. It is a universal solvent for transporting dissolved food, minerals, gases and waste products.
3. It is a raw material for photosynthesis in green plants.
4. It helps to regulate body temperature (through sweating and transpiration).
5. It maintains turgidity in plant cells and gives them support.
6. It aids digestion, absorption and the removal of excretory waste, and forms a large part of protoplasm.

Bayanin Amsa

(a) Five methods of vegetative propagation with an example of each

Also acceptable: corm (cocoyam), runner/stolon (grass), budding and grafting (mango, citrus), and leaf propagation (Bryophyllum).

(b)(i) Five advantages of vegetative propagation

1. Offspring are genetically identical to the parent, so desirable qualities are retained.
2. Reproduction is faster; the plant matures and yields earlier than from seed.
3. Seedless plants and those that produce few or no viable seeds can still be propagated.
4. Only one parent is needed; no pollination or fertilization is required.
5. The new plant is well nourished from the parent's food store, so it establishes quickly.

(b)(ii) Five disadvantages of vegetative propagation

1. There is no variation, so the plants cannot readily adapt to changes in the environment.
2. Diseases present in the parent are passed on to all the offspring.
3. Overcrowding occurs because the new plants develop close to the parent.
4. The plants lack the dormancy and dispersal advantages of seeds.
5. The clones weaken over successive generations and may lose vigour.

Bayanin Amsa

(a)(i) The four levels of organisation in living organisms

1. Cell
2. Tissue
3. Organ
4. System (organ system)

These build up in increasing complexity, from the cell as the smallest unit to systems which together form the whole organism.

(a)(ii) Explanation of two of the levels

Cell: The cell is the basic structural and functional unit of life. It is the smallest living part of an organism and can carry out all the processes of life such as respiration, feeding, excretion and reproduction. Examples include the amoeba (a single-celled organism) and individual cells such as red blood cells or muscle cells.

Tissue: A tissue is a group of similar cells that have the same origin and work together to perform a particular function. The cells are specialised for that function. Examples are muscle tissue (for contraction and movement), epithelial tissue (for covering and protection) in animals, and xylem or phloem tissue (for transport) in plants.

Bayanin Amsa

(a) Osmosis

Osmosis is the movement of water (solvent) molecules from a region of higher water potential (dilute / more water) to a region of lower water potential (concentrated / less water) across a semi-permeable (selectively permeable) membrane, until the concentrations on both sides are equal.

(b) Experiment to show that xylem conducts water from root to shoot

1. Take a young, leafy balsam or Impatiens plant (or a white flower such as a Coleus) with its roots intact, or a freshly cut leafy shoot.
2. Place it in a beaker containing water to which a red dye (eosin) has been added.
3. Leave it in a well-lit place for a few hours so that transpiration draws the coloured water up.
4. Observation: the leaf veins and the flower become tinged red, showing water has risen up the stem.
5. Cut a thin transverse section of the stem and examine it. Only the xylem tissue is stained red, while the phloem and other tissues remain unstained.
6. Conclusion: since only the xylem is coloured, it is the xylem that conducts water from the root up to the shoot.

(c)(i) Red blood cell in a hypotonic solution

A hypotonic solution has a higher water potential than the cell contents. Water therefore enters the red blood cell by osmosis. The cell swells, becomes spherical, and finally its membrane bursts. This bursting is called haemolysis.

(c)(ii) Three conditions that may cause haemolysis

1. Placing the cells in a hypotonic solution or in distilled water.
2. Action of certain toxins, venoms (e.g. snake venom) or bacterial poisons.
3. Incompatible blood transfusion (mismatched blood groups) or certain infections such as malaria.