WAEC SSCE - Biology - 2012 (Essay)

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(a)(i) Epigeal germination

Epigeal germination is a type of seed germination in which the cotyledons are carried above the soil surface. This is brought about by rapid elongation of the hypocotyl (the part of the axis below the cotyledons), which arches and pulls the cotyledons out of the soil. The cotyledons then turn green, spread out and function as the first leaves (foliage leaves) carrying out photosynthesis until the true leaves develop. Examples include cowpea, groundnut and castor oil.

(a)(ii) Three differences between epigeal and hypogeal germination

(Example of hypogeal: maize, oil palm.)

(b)(i) Seed dormancy

Seed dormancy is a resting condition in which a viable (living) seed fails to germinate even when the external conditions (water, oxygen and suitable temperature) are favourable, until certain internal or external requirements are met.

(b)(ii) Three ways of breaking seed dormancy

• Scarifying or cracking the hard seed coat (mechanically or by heat/fire) to allow water and oxygen in.
• Soaking the seeds in water or in dilute acid to soften or wear away the seed coat.
• Exposing the seeds to a period of low temperature (chilling) or removing germination inhibitors, or treating with growth hormones such as gibberellin.

(c) Six advantages of using contraceptives in human populations

• They help to control and reduce the rate of population growth (check overpopulation).
• They enable proper child-spacing, improving the health of the mother.
• They reduce maternal and infant mortality.
• They allow parents to have only the number of children they can adequately care for, improving family welfare.
• They help to prevent unwanted pregnancies.
• Some (for example condoms) help to prevent the spread of sexually transmitted infections such as HIV/AIDS.

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(a) Role of the stomach in digestion

The stomach is a muscular, J-shaped organ that stores food temporarily and continues digestion. Its walls contain gastric glands that secrete gastric juice. When food arrives, the muscular walls churn and mix it with the gastric juice to form a semi-liquid mass called chyme. The gastric juice contains:

• Hydrochloric acid, which provides the acidic medium needed for the enzymes to act, kills bacteria in the food, and activates pepsin.
• Pepsin, an enzyme that begins the digestion of proteins, breaking them down into peptones and polypeptides.
• Rennin (in young mammals), which curdles the soluble milk protein caseinogen into insoluble casein so it can be acted upon by pepsin.

The stomach also absorbs a little water, alcohol and simple substances, and releases the chyme gradually into the duodenum. Mucus secreted by the wall protects the stomach lining from being digested by its own acid and enzyme.

(b)(i) Three parts of plants in which food can be stored, with examples

(Leaves, as in onion bulb scales, are another storage part.)

(c) Regulation of blood sugar level in mammals

The level of glucose in the blood is kept fairly constant (about 90 mg per 100 cm³) by two hormones from the islets of Langerhans in the pancreas, working with the liver.

• When blood sugar rises (for example after a meal), the pancreas secretes insulin. Insulin causes the liver and muscles to convert the excess glucose into glycogen for storage, and increases the oxidation of glucose by cells. This lowers the blood sugar back to normal.
• When blood sugar falls (for example during fasting or exercise), the pancreas secretes glucagon. Glucagon causes the liver to convert stored glycogen back into glucose which is released into the blood, raising the level to normal. (The hormone adrenaline also raises blood sugar in the same way.)

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(a)(i) What is a gene?

A gene is a unit of heredity; it is a small segment of DNA on a chromosome that carries the coded information for a particular characteristic (trait) and is passed on from parents to offspring.

(a)(ii) Difference between genotype and phenotype

The genotype is the genetic make-up of an organism, that is, the actual combination of genes (alleles) it possesses for a trait (for example TT, Tt or tt). The phenotype is the observable or physical expression of that trait (for example tall or short), resulting from the interaction of the genotype with the environment.

(b) Explanation of terms

• (i) Hybrid - An offspring that is heterozygous for a trait, resulting from a cross between two genetically different (pure-breeding) parents (for example Tt).
• (ii) Pure breeding - An organism that is homozygous for a trait and always produces offspring identical to itself for that trait when self-crossed (for example TT or tt).
• (iii) Nucleotide - The basic structural unit (building block) of a nucleic acid, made up of a nitrogenous base, a pentose sugar and a phosphate group.

(c) Genetic diagram

Let smooth seed coat be S (dominant) and rough seed coat be s (recessive).

A homozygous rough pea is ss. The smooth plant whose parents were both rough (ss) must have received an s allele from each rough parent, so it is heterozygous smooth, Ss.

Cross: ss (rough) \( \times \) Ss (smooth)

Gametes: ss produces all s; Ss produces S and s.

Result: Offspring genotypes are Ss : ss in the ratio 1 : 1. Phenotypes are smooth : rough = 1 : 1 (that is, 50% smooth-seeded and 50% rough-seeded).

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(a) Four factors that affect the diffusion of substances

• Concentration gradient - the steeper the difference in concentration between two regions, the faster the rate of diffusion.
• Temperature - a rise in temperature increases the kinetic energy of the particles, so they move and diffuse faster.
• Surface area - the larger the surface area over which diffusion occurs, the faster the rate.
• Distance / thickness of the membrane - the shorter the distance (thinner the membrane), the faster the diffusion (the size and molecular weight of the diffusing particles also matter: small, light particles diffuse faster).

(b) Explanation of terms

• (i) Active transport - The movement of substances across a cell membrane against a concentration gradient (from a region of lower to higher concentration) using energy released from respiration (ATP).
• (ii) Transpiration - The loss of water in the form of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves.

(c) Four ways plants reduce a high rate of transpiration

• Reducing the number of stomata, and having sunken stomata in pits.
• Having a thick waxy cuticle on the leaf surface.
• Reducing the leaf surface area or reducing leaves to spines/scales.
• Closing the stomata (especially during the hottest part of the day) and shedding leaves in the dry season; presence of hairs on the leaf surface also helps.

(d) Features of red blood cells and how they adapt the cell to its functions

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(a) Explanation of terms

• (i) Conservation - Conservation is the careful management, protection and wise use of natural resources (such as forests, wildlife, water and soil) so that they are not exhausted or destroyed, and are preserved for the benefit of present and future generations.
• (ii) Endangered species - An endangered species is a species of organism whose population has fallen so low that it is in danger of becoming extinct (dying out completely) unless steps are taken to protect it.

(b)(i) Five reasons why conservation of forests is important

• Forests provide timber, wood, fuel, fruits, herbs and other useful products.
• They serve as a habitat and source of food for wildlife, preserving biodiversity.
• They prevent soil erosion and help to maintain soil fertility.
• They help to regulate climate and the water cycle, and reduce flooding.
• They purify the air by absorbing carbon dioxide and releasing oxygen (and serve as sites for recreation, tourism and research).

(b)(ii) Four ways forest reserves can be conserved

• Controlled (selective) felling of trees and enforcing laws against indiscriminate cutting.
• Reafforestation - replanting trees to replace those that are cut.
• Preventing and controlling bush burning.
• Creating forest reserves and game reserves, and educating the public on the value of forests.

(c)(i) Importance of decomposers in the ecosystem

Decomposers break down the dead bodies and waste products of plants and animals, releasing simple inorganic nutrients (such as nitrates, carbon dioxide and mineral salts) back into the soil and air. This recycles nutrients so that they can be re-used by green plants, and keeps the environment clean by removing dead organic matter.

(c)(ii) One plant and one animal decomposer

• Plant decomposer: a fungus such as Mucor (mushroom / mould), or bacteria.
• Animal decomposer: earthworm (or termite).

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(a) Explanation of terms

• (i) Disease - A disease is any disorder or departure from the normal state of health of the body, in which the normal functioning of a part or the whole of the body is disturbed, usually accompanied by characteristic signs and symptoms.
• (ii) Symptoms of disease - Symptoms are the observable or felt signs and abnormal changes in the body that indicate the presence of a disease (for example fever, headache, weakness, vomiting or rashes).

(b)(i) Two physical and two chemical barriers

Physical barriers: the skin (intact epidermis) which keeps out pathogens; hairs and mucus / cilia lining the respiratory tract that trap microbes.

Chemical barriers: hydrochloric acid in the stomach which kills swallowed bacteria; lysozyme enzyme in tears, saliva and sweat which destroys bacteria (sebum on the skin is also chemical).

(b)(ii) How vaccination protects the body from infectious diseases

A vaccine contains dead, weakened (attenuated) or modified pathogens, or their harmless toxins (antigens). When introduced into the body, these antigens stimulate the white blood cells (lymphocytes) to produce specific antibodies against that pathogen without causing the disease. The body also produces memory cells. If the real pathogen later enters the body, the memory cells enable the body to produce the correct antibodies rapidly and in large amounts, destroying the pathogen before it can cause disease. This gives the body active, long-lasting immunity.

(c) Difference between an antibody and an antigen

An antigen is a foreign substance (often a protein on the surface of a pathogen) that, when it enters the body, stimulates the production of antibodies. An antibody is a protein produced by the white blood cells (lymphocytes) in response to an antigen, which combines with and neutralises or destroys that specific antigen.

(d) Causative agents

• (i) Malaria - a protozoan, Plasmodium.
• (ii) Cholera - a bacterium, Vibrio cholerae.
• (iii) AIDS - a virus, the Human Immunodeficiency Virus (HIV).