WAEC SSCE - Biology - 2004 (Essay)

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(a) What is a habitat?

A habitat is the particular place or type of environment in which an organism naturally lives, feeds and reproduces, and which provides all the conditions and resources it needs for survival. Examples are a pond, a forest floor and the seashore.

(b) Energy flow in a freshwater habitat

Energy enters the freshwater habitat as sunlight, which is trapped by the producers (green plants such as algae, Spirogyra and water weeds) during photosynthesis and stored as chemical energy in food. This energy then flows along a food chain from one feeding level (trophic level) to the next:

\[ \text{Sun} \rightarrow \text{Producers} \rightarrow \text{Primary consumers} \rightarrow \text{Secondary consumers} \rightarrow \text{Tertiary consumers} \]

For example: algae \(\rightarrow\) water flea \(\rightarrow\) small fish \(\rightarrow\) big fish. The primary consumers (herbivores such as tadpoles and water fleas) eat the producers; secondary consumers (small carnivorous fish) eat the herbivores; and tertiary consumers (larger fish) eat the smaller fish. At each transfer, much of the energy is lost as heat during respiration and in movement, so the amount of usable energy decreases along the chain. When plants and animals die, decomposers (bacteria and fungi) break down their bodies, releasing the remaining energy as heat and returning nutrients to the water. Energy flow is therefore one-way (non-cyclic).

(c)(i) Three biotic factors affecting plants in a freshwater habitat

1. Grazing/feeding by aquatic animals (herbivores).
2. Competition among plants for light and space.
3. Decomposers (bacteria and fungi) acting on dead plants.

Also: disease-causing organisms (parasites).

(c)(ii) Five abiotic factors affecting plants in a freshwater habitat

1. Light (intensity and penetration).
2. Temperature of the water.
3. Dissolved oxygen and carbon dioxide.
4. Dissolved mineral salts (nutrients).
5. pH (acidity or alkalinity) of the water.

Also: water current and turbidity.

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(a) Six protective adaptations used by animals against predation, with one example each

1. Camouflage (protective/cryptic colouration): body colour blends with the surroundings so the animal is not easily seen. Example: the green grasshopper on green grass.
2. Warning (aposematic) colouration: bright colours warn predators that the animal is poisonous or distasteful. Example: the brightly coloured wasp.
3. Mimicry: a harmless animal resembles a dangerous or distasteful one and so is avoided. Example: the hoverfly mimicking a bee.
4. Possession of a protective covering/armour: a hard shell or scales shields the body. Example: the tortoise withdrawing into its shell.
5. Chemical/offensive defence: release of poison, sting or foul-smelling substance. Example: the bee's sting (or the millipede's repellent fluid).
6. Behavioural defences such as feigning death or autotomy: the animal pretends to be dead, or sheds a body part. Example: the lizard casting off its tail (autotomy) to escape.

Others: living in groups/herding, and burrowing/hiding.

(b)(i) What is pollution?

Pollution is the release into the environment (air, water or soil) of substances or forms of energy in quantities harmful to living organisms, thereby making the environment dirty, unhealthy or unsuitable for life.

(b)(ii) Six human activities which cause pollution

1. Burning of fossil fuels by vehicles and factories, releasing smoke and gases into the air.
2. Discharge of untreated sewage into water bodies.
3. Dumping of industrial effluents and chemical wastes into rivers.
4. Indiscriminate disposal of refuse and solid waste.
5. Excessive use of fertilizers and pesticides on farms.
6. Oil spillage from tankers and pipelines.

Also: mining activities and noise from machines.

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(a) Experiment to show that green plants require sunlight for photosynthesis

• Take a potted green plant and place it in a dark cupboard for about 48 hours to destarch its leaves (so that any starch found later must have been newly made).
• Cover part of one leaf on both surfaces with a strip of black paper (or aluminium foil), leaving the rest of the leaf exposed. This screens the covered part from light.
• Place the plant in bright sunlight for several hours so that photosynthesis can take place in the exposed parts.
• Remove the covered leaf, take off the black paper, and test the whole leaf for starch.

Observation: the part of the leaf that was exposed to light turns blue-black with iodine (starch present), while the part that was covered does not (no starch).

Conclusion: starch (food) was made only where light reached the leaf, showing that sunlight is necessary for photosynthesis.

(b) Other conditions necessary for photosynthesis

1. Chlorophyll (in chloroplasts).
2. Carbon dioxide.
3. Water.
4. A suitable (warm) temperature.

(c) Part of a dicotyledonous leaf where most food is manufactured, with reasons

Most food is manufactured in the palisade mesophyll (palisade layer). This is because its cells are packed with numerous chloroplasts containing chlorophyll to trap light, and they are arranged closely at the upper part of the leaf, directly beneath the transparent epidermis, where they receive the most light.

(d) What happens to the manufactured food

The glucose made is used in respiration for energy; some is converted to starch and stored temporarily in the leaf; the rest is changed to sucrose and translocated through the phloem to other parts of the plant, where it is used for growth or stored (as starch, oils or proteins) in roots, stems, seeds and fruits. Some is also used to build cellulose and, with mineral salts, to form proteins.

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(a) Distinction between pollination and fertilization

(b)(i) Five features which ensure cross-pollination, with (ii) a plant example each

1. Dioecism - male and female flowers on separate plants (e.g. pawpaw).
2. Protandry - anthers mature before the stigma of the same flower (e.g. sunflower).
3. Protogyny - stigma matures before the anthers of the same flower (e.g. maize).
4. Self-sterility (self-incompatibility) - pollen cannot fertilise the stigma of the same flower (e.g. Passion flower).
5. Heterostyly - different lengths of styles and stamens in different flowers (e.g. Primula).

Unisexual flowers on the same plant (monoecy, e.g. maize) is also acceptable.

(c)(i) Three agents of pollination

1. Wind.
2. Insects.
3. Water (or birds/animals).

(c)(ii) Three features of wind-dispersed fruits/seeds

1. They are small and very light, so they are easily carried by wind.
2. They have wing-like or membranous extensions (e.g. in Combretum) that help them glide.
3. They possess hairs, tufts or parachute-like structures (e.g. the pappus of dandelion) that increase buoyancy in air.

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(a)

(i) Phenotype: the observable external appearance or characteristics of an organism, resulting from the interaction of its genotype and the environment.

(ii) Hybrid: an offspring produced by crossing two genetically different parents.

(iii) Dominant character: a trait that is expressed in an organism even when its contrasting allele is present.

(b) Key: Y = allele for yellow grains (dominant); y = allele for purple grains (recessive).

(c) Benefits of genetics in agriculture:

1. It enables the selection and breeding of crop varieties and livestock with desirable qualities such as high yield, early maturity and resistance to diseases.
2. It permits the production of hybrids with hybrid vigour, improved quality, drought tolerance or pest resistance.

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(a)(i) A joint is the place where two or more bones meet or articulate.

(a)(ii) Types of movable joints and their locations are:

Type of joint	One location in the mammalian body
Ball-and-socket joint	Hip or shoulder
Hinge joint	Elbow or knee
Pivot joint	Between the atlas and axis vertebrae in the neck

(b)(i) Labelled diagram of a typical movable (synovial) joint, drawn to a length of about 9 cm:

(b)(iii) The joint is adapted for movement as follows:

1. The ends of the bones are covered with smooth articular cartilage. This prevents direct rubbing of bone on bone, reduces friction and absorbs shock.
2. The synovial membrane secretes synovial fluid into the synovial cavity.
3. The synovial fluid lubricates the articulating surfaces, reducing friction and wear and allowing easy movement.
4. The tough fibrous joint capsule encloses the joint, retains the synovial fluid and provides support.
5. Ligaments bind the bones firmly together, stabilise the joint and prevent excessive movement or dislocation while allowing the required movement.