WAEC SSCE - Biology - 1998 (Essay)

Answer Details

(a)(i) Two stages of photosynthesis

The light stage (light-dependent reaction) and the dark stage (light-independent reaction / Calvin cycle).

(a)(ii) Equation for the first (light) stage

In the light stage, water is split by light energy (photolysis) to release oxygen, hydrogen and energy:

\[ 2H_2O \xrightarrow{\text{light energy / chlorophyll}} 4H^+ + 4e^- + O_2 \]

(In words: water is split in the presence of light and chlorophyll to give hydrogen ions, electrons and oxygen. The energy is trapped as ATP.)

(b)(i) Six modes of nutrition

Autotrophic (holophytic), heterotrophic (holozoic), saprophytic, parasitic, symbiotic, and chemosynthetic nutrition (carnivorous/insectivorous also acceptable).

(b)(ii) Feature that makes feeding possible

• I. Paramecium: cilia (which sweep food into the oral groove / gullet).
• II. Spirogyra: chloroplasts (containing chlorophyll for photosynthesis).
• III. Rhizopus: rhizoids/hyphae which secrete enzymes and absorb digested food (saprophytic feeding).
• IV. Tapeworm: hooks and suckers (scolex) for attachment, and a thin body surface for absorbing already-digested food from the host.

(c) Balanced diet

A balanced diet is a diet that contains all the classes of food (carbohydrates, proteins, fats and oils, vitamins, mineral salts and water) in the correct proportions needed by the body for good health.

(d)(i) Three characteristics of enzymes

• They are proteins and act as biological catalysts.
• They are specific in their action.
• They are affected by temperature and pH (denatured by high temperature).

(d)(ii) Ptyalin and pepsin

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(a) Four floral parts of a flower

Sepals (calyx), petals (corolla), stamens (androecium, the male part), and carpels/pistil (gynoecium, the female part).

(b)(i) Differences between wind- and insect-pollinated flowers

(b)(ii) Examples

• Wind-pollinated: maize (also grasses, guinea corn).
• Insect-pollinated: Hibiscus (also flamboyant, Crotalaria).

(c)(i) Three agents of fruit and seed dispersal

Wind, water, animals (and explosive/self mechanism).

(c)(ii) Examples dispersed by each

• Wind: Tridax / cotton seed (light, winged or hairy fruits and seeds).
• Water: coconut (buoyant fruit).
• Animals: Desmodium / cactus (hooked or fleshy fruits); also mango eaten by animals.
• Explosive mechanism: Crotalaria / oil bean (pods that burst open).

(d)(i) Hormones

Hormones are chemical substances produced in small quantities by an organism which are transported to other parts of the body where they control and coordinate growth, development and other activities.

(d)(ii) One plant hormone

Auxin (others: gibberellin, cytokinin, abscisic acid, ethylene/ethene).

(d)(iii) Two effects of plant hormones

• They promote cell elongation and growth of stems and roots (and control tropisms, that is, growth movements towards or away from a stimulus).
• They control the ripening of fruits, formation of fruits, and flowering.
• They control leaf and fruit fall (abscission) and seed/bud dormancy.

Answer Details

(a) Four castes in a termite nest

Queen, king, workers, and soldiers.

(b)(i) Role of each caste

• Queen: the only fully fertile female; she lays all the eggs and so reproduces to build up the colony.
• King: the fertile male that mates with the queen to fertilise her eggs.
• Workers: sterile insects that build and repair the nest, gather food, feed the queen, king and young, and keep the nest clean.
• Soldiers: sterile insects with large heads and strong jaws that defend the colony against enemies.

(b)(ii) Two economic importances of termites to man

• Harmful: they destroy wooden buildings, furniture, books, crops and paper, causing great loss.
• Useful: they help to break down (decompose) dead wood and plant matter, enriching and aerating the soil; their mounds provide fertile soil; and they serve as food (source of protein) for people and animals.

(c)(i) Two types of courtship behaviour in lower animals

Sound/vocal display (for example croaking in male toads, chirping in crickets), colour/visual display (for example nodding and colour change in the Agama lizard), release of scent (pheromones), and nuptial dance/movement display. (Any two.)

(c)(ii) Two importances of courtship behaviour in reproduction

• It attracts a mate of the same species and of the opposite sex, ensuring successful mating.
• It prepares the partners and synchronises the release of gametes, so that fertilization succeeds and interbreeding between different species is avoided.

(d) Courtship behaviour of the Agama lizard

The male Agama lizard (rainbow lizard) attracts a female by a display of colour and movement. During courtship the male develops bright colours, with an orange or reddish head and a bluish body. He carries out repeated up-and-down movements of the head and the front part of the body (head-bobbing or press-up movements), often on a raised surface, to attract the attention of the female. This display also warns off rival males and marks his territory. When a female is attracted and becomes receptive, the two mate.

Answer Details

(a) Organic evolution

Organic evolution is the gradual change in living organisms from simple earlier forms to more complex and better-adapted forms over a very long period of time; that is, the process by which present-day organisms have descended, with modification, from earlier ancestral forms.

(b) Contributions to organic evolution

• (i) Lamarck: proposed the theory of the inheritance of acquired characteristics. He held that characters an organism develops during its lifetime through use and disuse of parts (for example the long neck of the giraffe from constantly stretching to reach high leaves) are passed on to its offspring, and that in this way organisms change over generations. He was the first to put forward a clear theory of how evolution could occur.
• (ii) Darwin: proposed the theory of evolution by natural selection (survival of the fittest). He observed that organisms produce more offspring than can survive, that there is variation among individuals, and that there is a struggle for existence. Individuals with favourable variations are better adapted, survive and reproduce, passing on their favourable characters, while the less-adapted die out. Over many generations this leads to new, better-adapted species.

(c) Three evidences of organic evolution

• Fossil records (palaeontology), which show a graded series of extinct forms leading to modern ones.
• Comparative anatomy, especially homologous organs (for example the pentadactyl limb of different vertebrates) showing common ancestry.
• Comparative embryology, in which the early embryos of different vertebrates are strikingly similar.
• Vestigial structures (such as the appendix in man) and geographical distribution of organisms. (Any three.)

(d) Role in the formation of new species

• (i) Mutation: a mutation is a sudden heritable change in the genetic material (genes or chromosomes). It produces new inheritable variations in a population. If a mutation is favourable, it is preserved by natural selection and spreads, so that over time the population becomes different from the original and a new species may arise.
• (ii) Isolation: isolation (for example by a geographical barrier such as a mountain, river or sea) separates a population into groups that can no longer interbreed. Each isolated group accumulates its own variations and adapts to its own conditions. In time the groups become so different that even when brought together they can no longer interbreed, and thus new species are formed.