WAEC SSCE - Biology - 2006 (Essay)

Bayanin Amsa

(a) Short notes

• (i) Ecosystem: An ecosystem is a natural unit made up of all the living organisms (the biotic community) in a given area together with the non-living (abiotic) components of their environment, interacting as a functional whole through the flow of energy and the cycling of nutrients. Examples are a pond, a forest or a grassland.
• (ii) Population: A population is a group of organisms of the same species living together in the same habitat at a particular time and able to interbreed, for example all the tilapia fish in a pond.
• (iii) Ecological succession in plants: This is the gradual and orderly replacement of one plant community by another over a period of time until a stable, self-perpetuating community (the climax community) is reached. It begins with pioneer species (for example lichens and mosses) that colonise bare land, which change the conditions and are succeeded by grasses, then shrubs, and finally trees.

(b) Five ways nitrogen can be made available to the soil for plant growth

1. By nitrogen-fixing bacteria in the root nodules of legumes and free-living soil bacteria (for example Rhizobium, Azotobacter) fixing atmospheric nitrogen into nitrates.
2. By nitrifying bacteria converting ammonium compounds to nitrites and then to nitrates (nitrification).
3. By lightning (thunderstorms) causing nitrogen and oxygen to combine, forming oxides that dissolve in rain as nitrates.
4. By decay of dead plants and animals and their waste, releasing ammonia which is nitrified (action of decomposers/ammonifying bacteria).
5. By addition of nitrogenous fertilizers and organic manure by farmers.

Bayanin Amsa

(a)(i) Three features of a typical vertebra

• It has a solid, weight-bearing block of bone called the centrum (body).
• It has a neural arch enclosing a neural canal through which the spinal cord passes.
• It bears projecting processes (a neural spine and a pair of transverse processes) for the attachment of muscles, and articulating facets (zygapophyses) for joining with the next vertebrae.

(a)(ii) Three types of skeleton with an animal in each case

Type of skeleton	Animal that possesses it
Hydrostatic skeleton	Earthworm (or Hydra)
Exoskeleton	Cockroach (or crab, grasshopper)
Endoskeleton	Man (or any mammal, fish, toad)

(b)(i) Four functions of the mammalian skeleton

1. It supports the body and gives it a definite shape.
2. It protects delicate internal organs (for example the skull protects the brain, the ribs protect the heart and lungs).
3. It provides a framework for the attachment of muscles and, with the muscles and joints, brings about movement/locomotion.
4. It manufactures blood cells in the red bone marrow and stores minerals such as calcium and phosphate.

(b)(ii) Three types of skeleton with an animal in each case

Type of skeleton	Animal that possesses it
Hydrostatic skeleton	Earthworm
Exoskeleton	Grasshopper (or crab)
Endoskeleton	Dog (or any mammal, fish)

Bayanin Amsa

(a)(i) Functions of a typical foliage leaf

• Manufacture of food by photosynthesis.
• Gaseous exchange (intake of carbon dioxide and oxygen) through the stomata.
• Loss of water vapour by transpiration, which helps in the upward pull of water.
• Storage of food and water in some leaves.

(a)(ii) Three types of leaf modification with one example each

Modification	Example
Tendril (for climbing/support)	Passion flower (or pea)
Spine (for protection/reduced water loss)	Cactus (or Opuntia)
Storage leaf (fleshy, for food/water storage)	Onion (bulb scale leaves) or Aloe vera

(Also acceptable: insect-trapping leaf - pitcher plant; bract - Bougainvillea.)

(b) Diagram of the female reproductive part of a flowering plant

The female part is the pistil (carpel/gynoecium). A clear drawing (8-10 cm) should show and label, from top to bottom: the stigma (sticky, receives pollen); the style (stalk connecting stigma to ovary); and the swollen ovary cut to show the ovules attached to the placenta by the funicle, with the micropyle and ovule wall (integuments) indicated.

(c) Three plants pollinated by insects

1. Hibiscus (flamboyant/showy flower).
2. Flamboyant (Delonix regia).
3. Crotalaria (or sunflower, Bougainvillea, Ixora).

Bayanin Amsa

(a)(i) Respiration

Respiration is the biochemical process by which food substances (chiefly glucose) are broken down within living cells to release energy, which is stored as ATP for the various activities of the organism. It usually involves the intake of oxygen and the release of carbon dioxide and water.

(a)(ii) Two types of respiration with balanced equations

Aerobic respiration (in the presence of oxygen):

\[ C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy} \]

Anaerobic respiration (in the absence of oxygen):

In plants/yeast (alcoholic fermentation):

\[ C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + \text{energy} \]

In animal muscle (lactic acid formation):

\[ C_6H_{12}O_6 \rightarrow 2C_3H_6O_3 + \text{energy} \]

(b) Mechanisms of breathing in human beings

(i) Inhalation (breathing in)

• The muscles of the diaphragm contract so that the diaphragm flattens and moves downward.
• The external intercostal muscles contract, pulling the ribs upward and outward.
• These increase the volume of the thoracic cavity and lower the pressure inside the lungs below atmospheric pressure.
• Air therefore rushes in through the nose and trachea into the lungs, which inflate.

(ii) Exhalation (breathing out)

• The muscles of the diaphragm relax so that the diaphragm curves upward (dome-shaped) again.
• The external intercostal muscles relax, allowing the ribs to move downward and inward.
• These decrease the volume of the thoracic cavity and raise the pressure inside the lungs above atmospheric pressure.
• Air is therefore forced out of the lungs through the trachea and nose.

Bayanin Amsa

(a) What is a hormone?

A hormone is a chemical substance (chemical messenger) secreted in small quantities directly into the blood by an endocrine (ductless) gland, and carried by the blood to a target organ where it regulates or coordinates a particular body activity.

(b) Role of adrenaline in an emergency situation

Adrenaline is secreted by the adrenal (suprarenal) glands during fright, anger or excitement to prepare the body for 'fight or flight'. It:

• Increases the heart rate and force of heartbeat, and raises blood pressure, so more blood reaches the muscles.
• Raises the blood glucose level by converting stored glycogen in the liver to glucose, supplying energy.
• Increases the rate and depth of breathing, supplying more oxygen.
• Dilates the pupils and diverts blood from the gut and skin to the skeletal muscles, making the body alert and ready for vigorous action.

(c) Five differences between hormonal and nervous coordination

Hormonal (endocrine) coordination	Nervous coordination
Message is carried as chemicals (hormones)	Message is carried as electrical impulses
Transmitted through the blood stream	Transmitted through nerves (neurones)
Response is usually slow	Response is very fast
Effect is usually long-lasting	Effect is usually short-lived
Response is usually widespread (affects many organs)	Response is usually localised to a specific part

(d) Three animal hormones and two plant hormones

• Animal hormones: adrenaline, insulin, thyroxine (also acceptable: oestrogen, testosterone).
• Plant hormones: auxin, gibberellin (also acceptable: cytokinin, abscisic acid, ethylene).

Bayanin Amsa

(a) Conservation of natural resources

Conservation of natural resources is the wise and careful use, protection and management of natural resources (soil, water, forests, wildlife, minerals) so that they are not wasted, degraded or destroyed, but are preserved and can be used continuously by both the present and future generations.

(b)(i) Seven methods of soil conservation

1. Crop rotation.
2. Cover cropping / mulching.
3. Contour ploughing (ploughing across the slope).
4. Terracing of hillsides.
5. Afforestation and planting of wind breaks/shelter belts.
6. Addition of manure and fertilizers (maintaining fertility).
7. Avoiding overgrazing and bush burning; strip cropping.

(b)(ii) Four other natural resources that may be conserved

1. Water.
2. Forests (timber/vegetation).
3. Wildlife (animals).
4. Minerals (for example petroleum, coal).

(c) Three factors that negatively affect conservation of forest in West Africa

1. Indiscriminate felling of trees (logging) for timber and fuel wood.
2. Bush burning and clearing of forest for farming (shifting cultivation) and settlement.
3. Overgrazing by animals and rapid population growth increasing demand for land.

(d) The denitrification stage of the nitrogen cycle

Denitrification is carried out by denitrifying bacteria (for example Pseudomonas, Thiobacillus) living in poorly aerated (anaerobic/waterlogged) soils. During this stage the bacteria break down nitrates in the soil into free gaseous nitrogen and oxides of nitrogen, which are released back into the atmosphere. This reduces the amount of nitrates available to plants and returns nitrogen to the air.