WAEC SSCE - Biology - 2010 (Essay)

Answer Details

(a)(i) Placentation

Placentation is the formation and manner of attachment of the placenta, the organ that develops in the wall of the uterus and connects the developing foetus (through the umbilical cord) to the mother, allowing exchange of materials between the foetal and maternal blood during pregnancy. (In botany, placentation refers to the arrangement of ovules on the placenta within the ovary.)

(a)(ii) Two organs associated with the human placenta

The uterus (its wall, to which the placenta is attached) and the umbilical cord (which links the placenta to the foetus).

(b) Five functions of the human placenta

• It allows oxygen to pass from the mother's blood to the foetus and carbon dioxide to pass from the foetus to the mother for removal.
• It allows soluble food materials (glucose, amino acids, mineral salts, vitamins) to pass from mother to foetus for nourishment.
• It allows the excretory waste (such as urea) of the foetus to pass into the mother's blood for removal.
• It secretes hormones (such as progesterone and oestrogen) that maintain the pregnancy.
• It acts as a barrier, preventing the mixing of the maternal and foetal blood and keeping out many harmful microorganisms, while allowing some antibodies through to give the foetus immunity.

(c) Short notes

• (i) Viviparity - A mode of reproduction in which the developing young is retained and nourished inside the mother's body (through a placenta) and is born alive, as in most mammals.
• (ii) Territoriality - The behaviour by which an animal establishes, marks out and defends a particular area (its territory) against others, especially members of the same species, for feeding, breeding or shelter.
• (iii) Seasonal migration - The regular, periodic movement of animals from one region to another and back, in response to seasonal changes in the environment, usually in search of food, favourable climate or breeding grounds (for example the migration of birds).

Answer Details

(a) Adaptation

Adaptation is the possession of special structural, physiological or behavioural features that enable an organism to survive, feed, escape enemies and reproduce successfully in its particular habitat or mode of life.

(b) How each structure adapts the organism to its mode of life

• (i) Succulent leaves in Aloe sp. - The thick, fleshy leaves store large amounts of water, enabling the plant to survive in dry (xerophytic) habitats where water is scarce.
• (ii) Succulent stems and reduced leaves in cactus plants - The swollen green stem stores water and also carries out photosynthesis, while the leaves are reduced to spines to cut down water loss by transpiration and to protect the plant from being eaten. This adapts the cactus to survive in deserts.
• (iii) Short, strong beaks in fowls - The short, hard, pointed beak is suited for pecking and picking up grains and seeds and for scratching, adapting the fowl to a grain-feeding (graminivorous) habit.
• (iv) Counter shading in fish - The dark upper (dorsal) surface and light lower (ventral) surface act as camouflage: viewed from above the fish blends with the dark water below, and from below it blends with the bright surface, helping it to escape predators and to approach prey.
• (v) Tiny scale leaves on needle-like branches of pine trees - The very small, needle-like leaves greatly reduce the surface area exposed, cutting down water loss by transpiration and enabling the pine to survive in cold or dry conditions.

(c)(i) Population

A population is the total number of organisms of the same species living together in a given area at a particular time.

(c)(ii) Five effects of overpopulation on a community

• Increased competition for food, water, space, light and other resources.
• Shortage of food leading to famine, malnutrition and starvation.
• Rapid spread of diseases and epidemics due to overcrowding.
• Depletion or exhaustion of natural resources and increased environmental pollution.
• Higher death rate, unemployment, poor sanitation and pressure on social amenities (housing, schools, hospitals).

Answer Details

(a) Chemical equations

(i) Aerobic respiration

\[ C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6CO_{2} + 6H_{2}O + \text{energy} \]

(glucose + oxygen \( \rightarrow \) carbon dioxide + water + energy)

(ii) Anaerobic respiration in plants (alcoholic fermentation)

\[ C_{6}H_{12}O_{6} \rightarrow 2C_{2}H_{5}OH + 2CO_{2} + \text{energy} \]

(glucose \( \rightarrow \) ethanol + carbon dioxide + energy)

(iii) Anaerobic respiration in animals (in muscle)

\[ C_{6}H_{12}O_{6} \rightarrow 2C_{3}H_{6}O_{3} + \text{energy} \]

(glucose \( \rightarrow \) lactic acid + energy)

(b) Four liver functions a patient with liver cancer cannot perform

• Deamination of excess amino acids and formation of urea.
• Regulation of blood sugar (converting glucose to glycogen and back).
• Detoxification of poisonous substances and drugs.
• Production of bile for the emulsification of fats (also storage of vitamins and iron, and manufacture of plasma proteins).

(c) Diagram of a flame cell

Make a labelled drawing 8 - 10 cm long of a flame cell showing: the cup-shaped cell body with its nucleus, a bunch of beating cilia (the flame) projecting into a cavity, the hollow tube (excretory duct/tubule) leading away, and the connection to the branching excretory tubule network.

(d) One appropriate plant hormone for each condition

• (i) Ripening of fruits - ethylene (ethene).
• (ii) Breaking dormancy in seeds - gibberellin (gibberellic acid).
• (iii) Weed control - auxin (synthetic auxin such as 2,4-D used as a selective weedkiller).
• (iv) Leaf fall (abscission) - abscisic acid.

Answer Details

(a) Short notes

• (i) Ecological niche - An ecological niche is the particular role or function of an organism in its habitat, that is, the total way of life of the organism including where it lives, what it feeds on, how it obtains its food, its relationships with other organisms, and the conditions it needs to survive and reproduce. No two species can occupy exactly the same niche in the same habitat for long.
• (ii) Population density - Population density is the number of individuals of a species per unit area (or per unit volume) of a habitat. It is calculated as \[ \text{Population density} = \frac{\text{Total number of individuals}}{\text{Total area (or volume) of the habitat}} \]
• (iii) Climax community - A climax community is the final, stable and self-perpetuating community of organisms that is established at the end of a natural succession, and which remains more or less unchanged as long as the environmental conditions (especially climate) remain constant.

(b) Interactions between the abiotic and biotic components of an ecosystem

An ecosystem is made up of biotic components (living things: producers, consumers and decomposers) and abiotic components (non-living factors such as sunlight, temperature, water, air, soil and mineral salts). These two interact continuously.

• Green plants (producers) use abiotic factors such as sunlight, carbon dioxide, water and mineral salts to manufacture food by photosynthesis; the food and oxygen produced support the animals.
• Consumers (animals) feed on plants and on one another, obtaining energy and nutrients; they release carbon dioxide into the air by respiration, which plants re-use.
• Decomposers break down dead organisms and waste, returning mineral nutrients such as nitrates and carbon dioxide to the soil and air (nutrient recycling) for re-use by plants.
• Abiotic factors such as temperature, rainfall and light determine the type, number and distribution of organisms that can live in the habitat, while the organisms in turn modify the abiotic environment (for example plants provide shade and add humus to the soil).

In this way there is a continuous flow of energy and cycling of materials between the living and non-living parts of the ecosystem.

Answer Details

(a) Four characteristics that can be genetically transmitted in humans

• Blood group (ABO / Rhesus).
• Skin (and eye) colour.
• Ability to roll the tongue.
• Height (and shape of the nose, hair type, colour blindness, sickle-cell trait).

(b)(i) Two sites each where meiosis occurs

In plants: the anther (in the pollen sacs) and the ovule (embryo sac) of the flower.

In animals: the testes (in males) and the ovaries (in females).

(b)(ii) Two differences between meiosis and mitosis

(c) Blood-group paternity test

The ABO blood groups are controlled by three alleles: I^A and I^B (co-dominant) and i (recessive). Group O is ii.

Genotypes given:

• Woman: blood group A, heterozygous = I^Ai
• Son: blood group O = ii
• Mr James: blood group A, homozygous = I^AI^A

For the son to be group O (ii), he must receive an i allele from each parent. The mother (I^Ai) can supply an i allele. The alleged father must therefore also supply an i allele.

Cross: mother I^Ai \( \times \) Mr James I^AI^A

(i) Conclusion: All possible children of this cross are blood group A (I^AI^A or I^Ai). None can be group O. Therefore the woman's claim is disproved: Mr James cannot be the father of the group O son.

(ii) Reason: Mr James, being homozygous group A (I^AI^A), can only pass on an I^A allele. A group O child must receive an i allele from both parents, so a homozygous group A man cannot father a group O child.

Answer Details

(a)(i) Two types of germination of seeds, with examples

• Epigeal germination - the cotyledons are carried above the soil surface. Example: cowpea (or groundnut).
• Hypogeal germination - the cotyledons remain below the soil surface. Example: maize (or oil palm).

(a)(ii) Experiment to show that oxygen, water and warmth are necessary for germination

Aim: To demonstrate that seeds need oxygen, water and warmth to germinate.

Set up four boiling tubes (labelled A, B, C, D), each containing a few viable bean seeds resting on cotton wool:

The boiled, cooled water in C removes dissolved oxygen and the oil layer prevents air from re-entering. Leave the tubes for several days (about a week).

Observation: Only the seeds in tube A (the control), which has water, air/oxygen and warmth, germinate. The seeds in B (no water), C (no oxygen) and D (no warmth) fail to germinate.

Conclusion: Since germination occurs only when water, oxygen and warmth are all present, these three factors are necessary for the germination of seeds.

(b) Distinguishing features

(i) Two features of epiphytes

• They grow on (are attached to) other plants for support but are not parasitic.
• They possess special aerial (clinging) roots and manufacture their own food, absorbing moisture from the air/rain.

(ii) Two features of parasites

• They live on or in a host from which they obtain their food (they possess haustoria that penetrate the host tissues).
• They usually lack chlorophyll or have it reduced, and they harm the host.

(c) Two examples of parasitic plants

Cassytha (dodder / love-vine) and mistletoe (Loranthus / Tapinanthus); Striga is another.