WAEC SSCE - Biology - 2012 (Essay)
Question 1 Report
(a) Explain briefly the following terms:
(i) conservation
(ii) endangered species.
(b) State: (i) five reasons why conservation of forests is important;
(ii) four ways by which forest reserves can be conserved.
(c)(i) What is the importance of decomposers in the ecosystem?
(ii) Name one plant and one animal decomposer.
(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).
Answer Details
(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).
Question 2 Report
(a) (i) What is a gene?
(ii) Differentiate between the terms genotype and phenotype
(b) Explain the following terms:
(i) hybrid
(ii) pure breeding
(iii) nucleotide.
(c) In garden pea seeds, smooth seed coat is dominant over rough seed coat. With the aid of a genetic diagram, determine the result expected if a homozygous rough pea is crossed with a smooth seed coat plant whose parents were rough coated.
(a)(i) What is a gene?
A gene is a unit of heredity located at a specific position (locus) on a chromosome, made up of a segment of DNA, which carries the coded information for a particular protein and is responsible for the transmission of a particular trait or character from parents to offspring.
(a)(ii) Difference between genotype and phenotype
| Genotype | Phenotype |
|---|---|
| The genetic make-up (the actual combination of alleles/genes) of an organism, e.g. \(SS\), \(Ss\) or \(ss\). | The observable, physical or biochemical characteristics of an organism, e.g. smooth seed coat or rough seed coat. |
| It is inherited and remains constant, independent of the environment. | It is determined jointly by the genotype and the environment. |
(b) Explanation of terms
(i) Hybrid: An offspring produced from a cross between two genetically different parents that possess contrasting forms of a character; such an offspring is heterozygous, e.g. \(Ss\).
(ii) Pure breeding: A condition in which an organism is homozygous for a trait (e.g. \(SS\) or \(ss\)) so that, when self-pollinated or crossed with an identical type, it produces offspring showing only that same trait generation after generation.
(iii) Nucleotide: The basic structural unit (building block) of a nucleic acid, each one consisting of a pentose sugar (deoxyribose in DNA or ribose in RNA), a phosphate group and a nitrogenous base (adenine, guanine, cytosine, and thymine or uracil).
(c) Genetic cross
Let the allele for smooth seed coat be \(S\) (dominant) and the allele for rough seed coat be \(s\) (recessive).
The homozygous rough pea has the genotype \(ss\).
The smooth-seeded plant had rough-coated parents. Since a rough parent can only supply a recessive \(s\) allele, this smooth plant must have received one \(s\) allele; because it is smooth it must also carry one dominant \(S\) allele. It is therefore heterozygous, genotype \(Ss\). (This is a test cross.)
Genetic diagram of the cross
Results:
Genotypic ratio of offspring \(=\;1\;Ss\;:\;1\;ss\).
Phenotypic ratio of offspring \(=\;1\) smooth seed coat \(:\;1\) rough seed coat, i.e. 50% smooth-seeded and 50% rough-seeded plants.
Answer Details
(a)(i) What is a gene?
A gene is a unit of heredity located at a specific position (locus) on a chromosome, made up of a segment of DNA, which carries the coded information for a particular protein and is responsible for the transmission of a particular trait or character from parents to offspring.
(a)(ii) Difference between genotype and phenotype
| Genotype | Phenotype |
|---|---|
| The genetic make-up (the actual combination of alleles/genes) of an organism, e.g. \(SS\), \(Ss\) or \(ss\). | The observable, physical or biochemical characteristics of an organism, e.g. smooth seed coat or rough seed coat. |
| It is inherited and remains constant, independent of the environment. | It is determined jointly by the genotype and the environment. |
(b) Explanation of terms
(i) Hybrid: An offspring produced from a cross between two genetically different parents that possess contrasting forms of a character; such an offspring is heterozygous, e.g. \(Ss\).
(ii) Pure breeding: A condition in which an organism is homozygous for a trait (e.g. \(SS\) or \(ss\)) so that, when self-pollinated or crossed with an identical type, it produces offspring showing only that same trait generation after generation.
(iii) Nucleotide: The basic structural unit (building block) of a nucleic acid, each one consisting of a pentose sugar (deoxyribose in DNA or ribose in RNA), a phosphate group and a nitrogenous base (adenine, guanine, cytosine, and thymine or uracil).
(c) Genetic cross
Let the allele for smooth seed coat be \(S\) (dominant) and the allele for rough seed coat be \(s\) (recessive).
The homozygous rough pea has the genotype \(ss\).
The smooth-seeded plant had rough-coated parents. Since a rough parent can only supply a recessive \(s\) allele, this smooth plant must have received one \(s\) allele; because it is smooth it must also carry one dominant \(S\) allele. It is therefore heterozygous, genotype \(Ss\). (This is a test cross.)
Genetic diagram of the cross
Results:
Genotypic ratio of offspring \(=\;1\;Ss\;:\;1\;ss\).
Phenotypic ratio of offspring \(=\;1\) smooth seed coat \(:\;1\) rough seed coat, i.e. 50% smooth-seeded and 50% rough-seeded plants.
Question 3 Report
(a) Explain briefly four factors that affect the diffusion of substances.
(b) Explain the following terms:
(i) active transport
(ii) transpiration.
(c) State four ways by which plants can reduce high rate of transpiration.
(d) State the features of red blood cells and how these features adapt the cell to perform its functions.
(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
| Feature | How it adapts the cell to its function |
|---|---|
| Biconcave disc shape | Provides a large surface area to volume ratio for rapid uptake and release of oxygen |
| Contains the pigment haemoglobin | Combines readily with oxygen (as oxyhaemoglobin) to transport it round the body |
| Has no nucleus (in mammals) | Leaves more room inside the cell to carry more haemoglobin (and oxygen) |
| Small size and flexible (elastic) membrane | Allows the cell to squeeze through narrow capillaries to deliver oxygen to tissues |
Answer Details
(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
| Feature | How it adapts the cell to its function |
|---|---|
| Biconcave disc shape | Provides a large surface area to volume ratio for rapid uptake and release of oxygen |
| Contains the pigment haemoglobin | Combines readily with oxygen (as oxyhaemoglobin) to transport it round the body |
| Has no nucleus (in mammals) | Leaves more room inside the cell to carry more haemoglobin (and oxygen) |
| Small size and flexible (elastic) membrane | Allows the cell to squeeze through narrow capillaries to deliver oxygen to tissues |
Question 4 Report
(a) Explain the following terms:
(i) disease
(ii) symptoms of diseases.
(b)(i) List two physical and two chemical barriers that prevent pathogens from penetrating the body of an organism.
(ii) Explain how vaccination protects the body from contracting infectious diseases.
(c) Distinguish between an antibody and a antigen.
(d) Name the causative agents of:
(i) Malaria
(ii) Cholera
(iii) AIDS.
(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).
Answer Details
(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).
Question 5 Report
(a) Describe briefly the role of the stomach in digestion.
(b) (i) Name three parts of plants in which food can be stored.
(ii) Give one example in each case.
(c) Explain briefly how the level of sugar in the mammalian blood can be regulated.
(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
| Storage part | Example |
|---|---|
| Root | Cassava (or carrot) |
| Stem | Irish potato / sugar cane |
| Seed / fruit | Maize grain / bean seed |
(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.)
Answer Details
(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
| Storage part | Example |
|---|---|
| Root | Cassava (or carrot) |
| Stem | Irish potato / sugar cane |
| Seed / fruit | Maize grain / bean seed |
(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.)
Question 6 Report
(a) (i) Describe epigeal germination of a seed.
(ii) In a tabular form, state three differences between epige germination and hypogeal germination.
(b)(i) What is seed dormancy?
(ii) State three ways b which dormancy in seeds can be broken.
(c) State six advantages of using contraceptives in puma populations.
(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
| Epigeal germination | Hypogeal germination |
|---|---|
| Cotyledons are carried above the soil | Cotyledons remain below the soil |
| The hypocotyl elongates | The epicotyl elongates |
| Cotyledons turn green and photosynthesise | Cotyledons do not turn green; they remain as food stores |
(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.
Answer Details
(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
| Epigeal germination | Hypogeal germination |
|---|---|
| Cotyledons are carried above the soil | Cotyledons remain below the soil |
| The hypocotyl elongates | The epicotyl elongates |
| Cotyledons turn green and photosynthesise | Cotyledons do not turn green; they remain as food stores |
(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.
