WAEC SSCE - Biology - 2013 (Essay)
Question 1 Rapport
(a)(i) What is excretion?
(ii) List the excretory organs in humans and name one waste product excreted by each organ
(B) Name the excretory organs in:
(i) insects
(ii) earthworms.
(a)(i) Excretion
Excretion is the process by which living organisms remove the waste products of metabolism (for example carbon dioxide, urea, excess water and mineral salts) from their bodies. If these wastes were allowed to accumulate they would be poisonous to the organism.
(a)(ii) Excretory organs in humans and the waste product excreted by each
| Excretory organ | Waste product excreted |
|---|---|
| Kidneys | Urea, excess water and mineral salts (urine) |
| Lungs | Carbon dioxide and water vapour |
| Skin | Excess water, mineral salts and some urea (sweat) |
| Liver | Bile pigments (from breakdown of haemoglobin); also deaminates excess amino acids |
(b) Excretory organs in other animals
- (i) Insects: Malpighian tubules (they excrete uric acid).
- (ii) Earthworms: nephridia.
Détails de la réponse
(a)(i) Excretion
Excretion is the process by which living organisms remove the waste products of metabolism (for example carbon dioxide, urea, excess water and mineral salts) from their bodies. If these wastes were allowed to accumulate they would be poisonous to the organism.
(a)(ii) Excretory organs in humans and the waste product excreted by each
| Excretory organ | Waste product excreted |
|---|---|
| Kidneys | Urea, excess water and mineral salts (urine) |
| Lungs | Carbon dioxide and water vapour |
| Skin | Excess water, mineral salts and some urea (sweat) |
| Liver | Bile pigments (from breakdown of haemoglobin); also deaminates excess amino acids |
(b) Excretory organs in other animals
- (i) Insects: Malpighian tubules (they excrete uric acid).
- (ii) Earthworms: nephridia.
Question 2 Rapport
(a) Describe the carbon cycle.
(b) State the functions of:
(i) nitrogen fixing bacteria
(ii) nitrifying bacteria
(iii) denitrifying bacteria; in nature.
(a) The carbon cycle
The carbon cycle is the continuous circulation of carbon between the atmosphere, living organisms and the non-living environment.
- Carbon exists in the air as carbon dioxide. Green plants take in this carbon dioxide and, by photosynthesis, fix it into organic food (carbohydrates), so carbon becomes part of the plant body.
- Animals feed on the plants (and on other animals), so the carbon passes along food chains into animal bodies (feeding/nutrition).
- Plants and animals return carbon dioxide to the air through respiration.
- When plants and animals die, decomposers (bacteria and fungi) break down their bodies, releasing carbon dioxide back into the air during decay.
- Combustion (burning) of wood and of fossil fuels (coal, petroleum, natural gas), which were formed from the buried remains of organisms, also returns carbon dioxide to the atmosphere.
In this way the amount of carbon in circulation is kept roughly in balance.
(b) Functions of bacteria in nature
- (i) Nitrogen-fixing bacteria: convert free atmospheric nitrogen into nitrogen compounds (ammonia/nitrates) that plants can absorb and use; for example Rhizobium in the root nodules of legumes and free-living Azotobacter.
- (ii) Nitrifying bacteria: convert ammonium compounds to nitrites (Nitrosomonas) and nitrites to nitrates (Nitrobacter), making nitrate available to plants (nitrification).
- (iii) Denitrifying bacteria: break down nitrates in the soil into free gaseous nitrogen, which is released back into the atmosphere (denitrification).
Détails de la réponse
(a) The carbon cycle
The carbon cycle is the continuous circulation of carbon between the atmosphere, living organisms and the non-living environment.
- Carbon exists in the air as carbon dioxide. Green plants take in this carbon dioxide and, by photosynthesis, fix it into organic food (carbohydrates), so carbon becomes part of the plant body.
- Animals feed on the plants (and on other animals), so the carbon passes along food chains into animal bodies (feeding/nutrition).
- Plants and animals return carbon dioxide to the air through respiration.
- When plants and animals die, decomposers (bacteria and fungi) break down their bodies, releasing carbon dioxide back into the air during decay.
- Combustion (burning) of wood and of fossil fuels (coal, petroleum, natural gas), which were formed from the buried remains of organisms, also returns carbon dioxide to the atmosphere.
In this way the amount of carbon in circulation is kept roughly in balance.
(b) Functions of bacteria in nature
- (i) Nitrogen-fixing bacteria: convert free atmospheric nitrogen into nitrogen compounds (ammonia/nitrates) that plants can absorb and use; for example Rhizobium in the root nodules of legumes and free-living Azotobacter.
- (ii) Nitrifying bacteria: convert ammonium compounds to nitrites (Nitrosomonas) and nitrites to nitrates (Nitrobacter), making nitrate available to plants (nitrification).
- (iii) Denitrifying bacteria: break down nitrates in the soil into free gaseous nitrogen, which is released back into the atmosphere (denitrification).
Question 3 Rapport
(a) What is photosynthesis?
(b) List: (i) four external factors
(ii) two internal factors; that affect the rate of photosynthesis.
(c)(i)List the major products of the light dependent stage of photosynthesis.
(ii)State the importance of each of the products listed in (c)(i) above.
(d)(i) Explain why there are no green plants at the lower depths of some lakes.
(ii) State why decomposers are important to flowering plants.
(a) Photosynthesis
Photosynthesis is the process by which green plants manufacture their own food (glucose) from carbon dioxide and water, using light energy trapped by chlorophyll, and releasing oxygen as a by-product. It is summarised as:
\[ 6CO_2 + 6H_2O \xrightarrow[\text{chlorophyll}]{\text{light}} C_6H_{12}O_6 + 6O_2 \](b) Factors affecting the rate of photosynthesis
- (i) Four external factors: light intensity, concentration of carbon dioxide, temperature, and availability of water.
- (ii) Two internal factors: amount of chlorophyll present, and the number/distribution of stomata (also acceptable: accumulation of end products, leaf structure).
(c)(i) Major products of the light-dependent stage
- Oxygen.
- ATP (energy).
- Reduced hydrogen carrier (NADPH2), formed from the splitting of water (photolysis).
(c)(ii) Importance of each product
- Oxygen: released to the atmosphere and used by living organisms for respiration.
- ATP: supplies the energy used in the light-independent (dark) stage to fix carbon dioxide into glucose.
- Reduced NADP (NADPH2): supplies the hydrogen used to reduce carbon dioxide to carbohydrate in the dark stage.
(d)(i) Why there are no green plants at the lower depths of some lakes
Light does not penetrate to the great depths of the water; without light the green plants cannot carry out photosynthesis to make food, so they cannot survive there.
(d)(ii) Why decomposers are important to flowering plants
Decomposers break down dead organisms and wastes, releasing mineral nutrients (for example nitrates, phosphates, carbon dioxide) back into the soil and air, which the flowering plants absorb and use for growth and photosynthesis.
Détails de la réponse
(a) Photosynthesis
Photosynthesis is the process by which green plants manufacture their own food (glucose) from carbon dioxide and water, using light energy trapped by chlorophyll, and releasing oxygen as a by-product. It is summarised as:
\[ 6CO_2 + 6H_2O \xrightarrow[\text{chlorophyll}]{\text{light}} C_6H_{12}O_6 + 6O_2 \](b) Factors affecting the rate of photosynthesis
- (i) Four external factors: light intensity, concentration of carbon dioxide, temperature, and availability of water.
- (ii) Two internal factors: amount of chlorophyll present, and the number/distribution of stomata (also acceptable: accumulation of end products, leaf structure).
(c)(i) Major products of the light-dependent stage
- Oxygen.
- ATP (energy).
- Reduced hydrogen carrier (NADPH2), formed from the splitting of water (photolysis).
(c)(ii) Importance of each product
- Oxygen: released to the atmosphere and used by living organisms for respiration.
- ATP: supplies the energy used in the light-independent (dark) stage to fix carbon dioxide into glucose.
- Reduced NADP (NADPH2): supplies the hydrogen used to reduce carbon dioxide to carbohydrate in the dark stage.
(d)(i) Why there are no green plants at the lower depths of some lakes
Light does not penetrate to the great depths of the water; without light the green plants cannot carry out photosynthesis to make food, so they cannot survive there.
(d)(ii) Why decomposers are important to flowering plants
Decomposers break down dead organisms and wastes, releasing mineral nutrients (for example nitrates, phosphates, carbon dioxide) back into the soil and air, which the flowering plants absorb and use for growth and photosynthesis.
Question 4 Rapport
(a)(i) What is ecological succession?
(ii) Describe the process of ecological succession on a bare plot of land.
(b) Explain the following terms:
(i) biosphere
(ii)population
(iii)community
(iv)ecosystem.
(c) Name one instrument each used for measuring:
(i) relative humidity
(ii) wind speed
(a)(i) Ecological succession
Ecological succession is the gradual and orderly replacement of one community of organisms by another in an area over time, until a stable climax community in equilibrium with its environment is established.
(a)(ii) Ecological succession on a bare plot of land
The succession may be represented as follows. The rising curve shows the gradual increase in vegetation biomass, soil depth and complexity of the community from the pioneer stage to the climax stage.
- A bare plot is first colonised by pioneer organisms such as algae, lichens and mosses which can tolerate harsh conditions.
- As the pioneers grow and die, their remains decay and add humus and mineral nutrients to the surface. Their activities also help to break down the surface and form a thin soil layer.
- The improved soil retains more water and supports grasses, herbs, ferns and small animals such as insects and worms.
- Continued death and decay of organisms make the soil deeper and more fertile. Perennial grasses are then replaced by shrubs and small trees.
- Eventually, large trees and a greater variety of animals, including birds and mammals, become established.
- A stable climax community, such as a forest where climatic conditions permit, is finally formed. It remains in dynamic equilibrium with the environment.
(b) Explanation of terms
(i) Biosphere: the parts of the earth, including land, water and air, in which life exists.
(ii) Population: all the organisms of the same species living in a particular area at a given time.
(iii) Community: all the populations of different species living and interacting in the same habitat.
(iv) Ecosystem: a functional natural unit made up of living organisms and their non-living environment interacting with one another.
(c) Instruments
| Quantity measured | Instrument |
|---|---|
| (i) Relative humidity | Hygrometer, or wet-and-dry bulb thermometer |
| (ii) Wind speed | Anemometer |
Détails de la réponse
(a)(i) Ecological succession
Ecological succession is the gradual and orderly replacement of one community of organisms by another in an area over time, until a stable climax community in equilibrium with its environment is established.
(a)(ii) Ecological succession on a bare plot of land
The succession may be represented as follows. The rising curve shows the gradual increase in vegetation biomass, soil depth and complexity of the community from the pioneer stage to the climax stage.
- A bare plot is first colonised by pioneer organisms such as algae, lichens and mosses which can tolerate harsh conditions.
- As the pioneers grow and die, their remains decay and add humus and mineral nutrients to the surface. Their activities also help to break down the surface and form a thin soil layer.
- The improved soil retains more water and supports grasses, herbs, ferns and small animals such as insects and worms.
- Continued death and decay of organisms make the soil deeper and more fertile. Perennial grasses are then replaced by shrubs and small trees.
- Eventually, large trees and a greater variety of animals, including birds and mammals, become established.
- A stable climax community, such as a forest where climatic conditions permit, is finally formed. It remains in dynamic equilibrium with the environment.
(b) Explanation of terms
(i) Biosphere: the parts of the earth, including land, water and air, in which life exists.
(ii) Population: all the organisms of the same species living in a particular area at a given time.
(iii) Community: all the populations of different species living and interacting in the same habitat.
(iv) Ecosystem: a functional natural unit made up of living organisms and their non-living environment interacting with one another.
(c) Instruments
| Quantity measured | Instrument |
|---|---|
| (i) Relative humidity | Hygrometer, or wet-and-dry bulb thermometer |
| (ii) Wind speed | Anemometer |
Question 5 Rapport
(a)(i) What is conservation?
(ii) State six factors responsible for the decline of the abundance and variety of wildlife.
(b) Outline six ways in which the government can improve the situation in (a)(ii) above.
(c)(i) What is Eutrophication?
(ii) State two causes of eutrophication.
(a)(i) Conservation
Conservation is the wise use, protection and careful management of natural resources (wildlife, forests, soil, water) so that they are not destroyed or wasted but are preserved and made available for present and future generations.
(a)(ii) Six factors responsible for the decline in abundance and variety of wildlife
- Indiscriminate hunting and poaching of wild animals.
- Destruction of habitats through deforestation and clearing of land for farming and settlement.
- Bush burning, which destroys animals and their habitats.
- Pollution of air, water and soil.
- Overgrazing and overexploitation of resources.
- Introduction of pests, diseases and predators; and use of pesticides.
(b) Six ways the government can improve the situation
- Establish and protect game reserves, national parks and forest reserves.
- Enact and enforce laws against poaching, indiscriminate hunting and bush burning.
- Ban or license the hunting of endangered species and set closed seasons.
- Practise afforestation and reforestation to restore habitats.
- Educate the public on the importance of wildlife conservation.
- Control pollution and the misuse of pesticides; and set up breeding/restocking programmes.
(c)(i) Eutrophication
Eutrophication is the excessive enrichment of a body of water with mineral nutrients (especially nitrates and phosphates), which causes a rapid overgrowth of algae and water plants; when these die and decay, the dissolved oxygen is used up and aquatic animals die.
(c)(ii) Two causes of eutrophication
- Run-off of nitrate and phosphate fertilizers from farmland into water.
- Discharge of sewage and detergents (phosphates) into water bodies.
Détails de la réponse
(a)(i) Conservation
Conservation is the wise use, protection and careful management of natural resources (wildlife, forests, soil, water) so that they are not destroyed or wasted but are preserved and made available for present and future generations.
(a)(ii) Six factors responsible for the decline in abundance and variety of wildlife
- Indiscriminate hunting and poaching of wild animals.
- Destruction of habitats through deforestation and clearing of land for farming and settlement.
- Bush burning, which destroys animals and their habitats.
- Pollution of air, water and soil.
- Overgrazing and overexploitation of resources.
- Introduction of pests, diseases and predators; and use of pesticides.
(b) Six ways the government can improve the situation
- Establish and protect game reserves, national parks and forest reserves.
- Enact and enforce laws against poaching, indiscriminate hunting and bush burning.
- Ban or license the hunting of endangered species and set closed seasons.
- Practise afforestation and reforestation to restore habitats.
- Educate the public on the importance of wildlife conservation.
- Control pollution and the misuse of pesticides; and set up breeding/restocking programmes.
(c)(i) Eutrophication
Eutrophication is the excessive enrichment of a body of water with mineral nutrients (especially nitrates and phosphates), which causes a rapid overgrowth of algae and water plants; when these die and decay, the dissolved oxygen is used up and aquatic animals die.
(c)(ii) Two causes of eutrophication
- Run-off of nitrate and phosphate fertilizers from farmland into water.
- Discharge of sewage and detergents (phosphates) into water bodies.
Question 6 Rapport
(a) (i) What is respiration?
(ii) In a tabular form, state four differences between gaseous exchange and aerobic respiration
(b)(i) Explain the term residual air.
(ii) What is the importance of residula air to mammals?
(c) State four characteristic features associated with respiratory structures.
(d)(i) What is oxygen debt?
(ii) Outline three activities that can result in oxygen debt.
(a)(i) Respiration
Respiration is the process by which food substances (mainly glucose) are oxidised within living cells to release energy (stored as ATP) needed for the activities of the organism, usually with the uptake of oxygen and release of carbon dioxide and water.
(a)(ii) Four differences between gaseous exchange and aerobic respiration
| Gaseous exchange | Aerobic respiration |
|---|---|
| A physical process (diffusion of gases) | A chemical/biochemical process |
| Takes place at respiratory surfaces (for example lungs, gills) | Takes place inside cells (mitochondria) |
| Involves only the exchange (in and out) of oxygen and carbon dioxide | Involves oxidation/breakdown of glucose |
| No energy (ATP) is released | Energy (ATP) is released |
(b)(i) Residual air
Residual air is the volume of air that remains in the lungs and cannot be breathed out even after the most forceful expiration.
(b)(ii) Importance of residual air
It keeps the alveoli (air sacs) inflated so they do not collapse, and it allows continuous gaseous exchange between breaths, so oxygen is still absorbed into the blood even during expiration.
(c) Four characteristic features of respiratory structures
- They have a large surface area for maximum gaseous exchange.
- They are thin (one or few cells thick) for rapid diffusion of gases.
- They are moist so that gases dissolve before diffusing.
- They are well supplied with blood vessels (highly vascular) to maintain a steep diffusion gradient.
(d)(i) Oxygen debt
Oxygen debt is the extra amount of oxygen that the body needs to take in after vigorous exercise, to oxidise the lactic acid that accumulated in the muscles during anaerobic respiration.
(d)(ii) Three activities that can result in oxygen debt
- Sprinting / fast running (a race).
- Weight lifting / heavy physical work.
- Vigorous swimming or fast cycling.
Détails de la réponse
(a)(i) Respiration
Respiration is the process by which food substances (mainly glucose) are oxidised within living cells to release energy (stored as ATP) needed for the activities of the organism, usually with the uptake of oxygen and release of carbon dioxide and water.
(a)(ii) Four differences between gaseous exchange and aerobic respiration
| Gaseous exchange | Aerobic respiration |
|---|---|
| A physical process (diffusion of gases) | A chemical/biochemical process |
| Takes place at respiratory surfaces (for example lungs, gills) | Takes place inside cells (mitochondria) |
| Involves only the exchange (in and out) of oxygen and carbon dioxide | Involves oxidation/breakdown of glucose |
| No energy (ATP) is released | Energy (ATP) is released |
(b)(i) Residual air
Residual air is the volume of air that remains in the lungs and cannot be breathed out even after the most forceful expiration.
(b)(ii) Importance of residual air
It keeps the alveoli (air sacs) inflated so they do not collapse, and it allows continuous gaseous exchange between breaths, so oxygen is still absorbed into the blood even during expiration.
(c) Four characteristic features of respiratory structures
- They have a large surface area for maximum gaseous exchange.
- They are thin (one or few cells thick) for rapid diffusion of gases.
- They are moist so that gases dissolve before diffusing.
- They are well supplied with blood vessels (highly vascular) to maintain a steep diffusion gradient.
(d)(i) Oxygen debt
Oxygen debt is the extra amount of oxygen that the body needs to take in after vigorous exercise, to oxidise the lactic acid that accumulated in the muscles during anaerobic respiration.
(d)(ii) Three activities that can result in oxygen debt
- Sprinting / fast running (a race).
- Weight lifting / heavy physical work.
- Vigorous swimming or fast cycling.
