WAEC SSCE - Biology - 1988 (Essay)

(a) Conservation of natural resources

Conservation of natural resources is the careful and wise management, protection and use of the earth's natural resources so that they are not wasted, destroyed or exhausted, and so that they remain available for the benefit of both present and future generations.

(b) Natural resources to be conserved and the benefits

(c) Problems often encountered in ensuring conservation

• Rapid population growth increases demand for land, food, timber and fuel, causing over-exploitation.
• Poverty and unemployment force people to exploit resources such as forests and wildlife for survival.
• Ignorance and lack of public awareness about the importance and methods of conservation.
• Poor enforcement of conservation laws and inadequate funding of enforcement agencies.
• Bush burning, illegal logging, poaching and over-grazing that destroy resources faster than they can be replaced.
• Industrialisation and urbanisation leading to pollution and destruction of natural habitats.
• Corruption and conflict of interest between economic development and conservation.

(d) Efforts employed in my country to ensure conservation

• Establishment of national parks, game reserves and forest reserves to protect wildlife and vegetation.
• Enactment and enforcement of conservation and anti-poaching laws, and a ban on indiscriminate bush burning and illegal logging.
• Afforestation and reforestation programmes (tree-planting campaigns) to replace felled trees.
• Setting up agencies such as environmental protection and forestry commissions to regulate resource use.
• Public enlightenment and environmental education in schools and the media to create awareness.
• Encouraging controlled or selective harvesting, crop rotation and use of alternative energy to reduce pressure on resources.
• Control of pollution through laws on waste disposal and treatment of industrial effluents.

(a) Four modes of nutrition in plants

• Autotrophic (holophytic) nutrition: green plants manufacture their own food by photosynthesis, e.g. most green plants.
• Saprophytic nutrition: feeding on dead organic matter, e.g. Mucor, Rhizopus, mushroom.
• Parasitic nutrition: feeding on a living host, e.g. Cuscuta (dodder), mistletoe.
• Carnivorous (insectivorous) nutrition: trapping and digesting insects, e.g. sundew, pitcher plant, bladderwort.

(Symbiotic nutrition, e.g. lichen, is also acceptable.)

(b) Light and dark reactions of photosynthesis

Light reaction (light-dependent stage):

• Takes place in the grana of the chloroplast and requires light.
• Chlorophyll absorbs (traps) light energy.
• The trapped energy splits water molecules into hydrogen and oxygen (photolysis of water); the oxygen is released as a by-product.
• The light energy is used to form energy-rich ATP, and hydrogen is picked up by a carrier (NADP) to form reduced NADP (NADPH₂).

Dark reaction (light-independent stage / Calvin cycle):

• Takes place in the stroma of the chloroplast and does not need light directly.
• Carbon dioxide is fixed (combined) using the hydrogen (from reduced NADP) and the energy (from ATP) produced in the light stage.
• This reduces carbon dioxide to form simple sugar (glucose), which may then be built up into starch.

Overall: \[ 6CO_2 + 6H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6O_2 \]

(c) Modifications and mechanisms of feeding habits

• (i) Filter feeding in mosquito larva: the larva lives in water and hangs just below the surface. It has a pair of brushes of fine hairs (mouth brushes) around the mouth. These hairs beat rapidly and set up water currents that sweep small floating food particles (plankton, tiny organisms and organic matter) in the water towards the mouth, where they are filtered out and swallowed. In this way the larva strains (filters) food from the water.
• (ii) Parasitic feeding in tapeworm: the tapeworm lives in the small intestine of its host (man), where digested food is abundant. It has a head (scolex) bearing hooks and suckers for firm attachment to the intestinal wall, so that it is not swept away. It has no mouth and no gut; instead it has a thin, flat, ribbon-like body with a very large surface area covered by a resistant cuticle that protects it from the host's digestive enzymes. It simply absorbs already-digested, soluble food from the host through its whole body surface (osmosis and diffusion). Its long, segmented body also allows it to produce large numbers of eggs for reproduction.

(a)(i) Definition of population

A population is a group of organisms of the same species living together in the same habitat at a particular time, and which are able to interbreed to produce fertile offspring.

(a)(ii) Factors affecting population growth

Biotic factors (living factors):

• Availability of food - abundant food supports growth while scarcity limits it.
• Predation, competition, parasitism and disease - these reduce numbers by increasing the death rate.

Abiotic factors (non-living factors):

• Temperature - extreme heat or cold slows reproduction and raises mortality.
• Availability of water, light and space - shortage of any of these restricts survival and reproduction.

(b) Effects of food shortage on a population

• Increased competition among members for the limited food, both intraspecific and interspecific.
• Increased death rate (higher mortality) as the weaker individuals starve.
• Reduced reproductive rate (lower natality) because underfed organisms produce fewer offspring.
• Migration or emigration of some members in search of food, reducing the local population.
• Stunted growth and poor health, making individuals more prone to disease.
• Overall the population size falls until it matches the available food (the carrying capacity).

(c) Methods and their principles

(i) Improving crop yield

• Application of fertilizers (manuring) - Principle: fertilizers add essential mineral nutrients such as nitrogen, phosphorus and potassium to the soil, which the crops need for the manufacture of proteins and healthy growth, thereby raising yield.
• Use of improved (high-yielding) varieties through selective breeding - Principle: plants with desirable genes (fast growth, disease resistance, large produce) are selected and bred so that these favourable characteristics are passed on, giving heavier harvests.

Other acceptable methods: irrigation (supplies water for photosynthesis and transport of nutrients) and pest and weed control (removes organisms that compete with or destroy the crop).

(ii) Preserving and storing food

• Refrigeration or freezing (low temperature) - Principle: low temperatures slow down the activity of enzymes and inhibit the growth and multiplication of decay-causing micro-organisms (bacteria and fungi), so the food does not spoil quickly.
• Drying (dehydration) - Principle: removing moisture from the food deprives micro-organisms of the water they need to grow and reproduce, thereby preventing decay.

Other acceptable methods: salting or sugaring (high salt/sugar concentration draws water out of microbial cells by osmosis, killing or inhibiting them) and canning/pasteurisation (heat kills micro-organisms and sealing prevents re-entry of air and microbes).