WAEC SSCE - Biology - 2020 (Essay)
Vraag 1 Verslag
(a) Explain briefly how the human ear carries out its function of balancing.
(b) (i) make a drawing 8 cm - 10 cm long of the structure of the human ear and label fully.
(ii) Describe briefly the structure of the middle ear in humans.
(a) How the human ear carries out its function of balancing
Balance is controlled by the inner ear, using the three semicircular canals together with the utricle and saccule.
- The three semicircular canals are set at right angles to one another, so they detect movement of the head in all three planes.
- Each canal is filled with fluid (endolymph) and has a swollen base, the ampulla, which contains sensory hair cells.
- When the head moves or turns in any direction, the endolymph moves and bends the sensory hair cells in the ampulla, setting up nerve impulses.
- The utricle and saccule contain tiny chalk grains (otoliths) that respond to the position of the head with respect to gravity.
- The impulses travel along the auditory (vestibular) nerve to the brain (cerebellum), which interprets them and relays impulses to the body muscles so that posture is adjusted and balance is maintained.
(b)(i) Drawing of the structure of the human ear (8 cm to 10 cm long, fully labelled)
(b)(ii) Structure of the middle ear in humans
The middle ear is an air-filled chamber in the skull.
- It is separated from the outer ear by the ear drum (tympanic membrane / tympanum).
- It contains three small bones, the ear ossicles: the malleus (hammer), the incus (anvil) and the stapes (stirrup), linked in a chain.
- The malleus is attached to the tympanic membrane, each bone is in direct contact with the next, and the stapes fits into the oval window (fenestra ovalis) of the inner ear. In this way the ossicles transmit and amplify the sound vibrations from the ear drum to the oval window.
- The Eustachian tube arises at the base of the middle ear and links it to the back of the throat (pharynx), equalising the air pressure on both sides of the ear drum.
Antwoorddetails
(a) How the human ear carries out its function of balancing
Balance is controlled by the inner ear, using the three semicircular canals together with the utricle and saccule.
- The three semicircular canals are set at right angles to one another, so they detect movement of the head in all three planes.
- Each canal is filled with fluid (endolymph) and has a swollen base, the ampulla, which contains sensory hair cells.
- When the head moves or turns in any direction, the endolymph moves and bends the sensory hair cells in the ampulla, setting up nerve impulses.
- The utricle and saccule contain tiny chalk grains (otoliths) that respond to the position of the head with respect to gravity.
- The impulses travel along the auditory (vestibular) nerve to the brain (cerebellum), which interprets them and relays impulses to the body muscles so that posture is adjusted and balance is maintained.
(b)(i) Drawing of the structure of the human ear (8 cm to 10 cm long, fully labelled)
(b)(ii) Structure of the middle ear in humans
The middle ear is an air-filled chamber in the skull.
- It is separated from the outer ear by the ear drum (tympanic membrane / tympanum).
- It contains three small bones, the ear ossicles: the malleus (hammer), the incus (anvil) and the stapes (stirrup), linked in a chain.
- The malleus is attached to the tympanic membrane, each bone is in direct contact with the next, and the stapes fits into the oval window (fenestra ovalis) of the inner ear. In this way the ossicles transmit and amplify the sound vibrations from the ear drum to the oval window.
- The Eustachian tube arises at the base of the middle ear and links it to the back of the throat (pharynx), equalising the air pressure on both sides of the ear drum.
Vraag 2 Verslag
(a) Explain briefly the following types of fertilization in animals; (i) external fertilization (ii) internal fertilization
(b) Name two groups of animal each that exhibit the types of fertilization in (a)
(c)(i) If the placenta in a pregnant woman is detached from the uterine wall, give three effect this will have on the foetus
(ii) Name three other features in the uterus of a pregnant woman that are useful for the development of the foetus
(d) Explain briefly how the activities of organisms bring bout dynamic equilibrium in the habitat.
(e) State four problems that organisms in the intertidal zone of a marine habitat could encounter.
(f) Explain briefly the reasons that the following factors are necessary for germination; (i) moisture (ii) viable seeds
(g) Explain briefly the reason light energy is considered a limiting factor in the production of foods by autotrophs.
(a) Types of fertilisation
- (i) External fertilisation: the fusion of the male and female gametes takes place outside the body of the female, usually in water. The female lays her eggs and the male sheds sperm over them in the surrounding water.
- (ii) Internal fertilisation: the fusion of the male and female gametes takes place inside the body of the female, the sperm being deposited into her reproductive tract during mating.
(b) Two groups of animals for each type
- External fertilisation: bony fishes and amphibians (frogs/toads).
- Internal fertilisation: mammals and birds (also reptiles).
(c)(i) Three effects on the foetus if the placenta detaches from the uterine wall
- The foetus is cut off from its supply of food and oxygen, so it may suffocate and die.
- Waste products (carbon dioxide, urea) cannot be removed from the foetus, so they accumulate and poison it.
- It causes bleeding (haemorrhage) and may lead to miscarriage or premature birth of the foetus.
(c)(ii) Three other features of the uterus useful for foetal development
- A thick, muscular wall that protects the foetus and contracts to expel it at birth.
- A rich supply of blood vessels in the uterine lining that supplies food and oxygen and removes wastes.
- The amnion containing amniotic fluid that surrounds and cushions the foetus against shock (and the umbilical cord linking foetus to placenta).
(d) How the activities of organisms bring about dynamic equilibrium in a habitat
In a stable habitat, producers, consumers and decomposers interact so that materials and numbers stay roughly balanced. Green plants make food and release oxygen while animals consume food and release carbon dioxide, so the gases are kept balanced through photosynthesis and respiration. Predators feed on prey and so control the number of prey, while the food supply controls the number of predators, keeping populations steady. Decomposers break down dead matter and return mineral nutrients to the soil for plants to reuse. These continuous, opposing activities balance one another, so the habitat stays in a steady state called dynamic equilibrium.
(e) Four problems of organisms in the intertidal zone of a marine habitat
- Danger of drying out (desiccation) when the tide goes out and they are exposed to air and sun.
- Strong wave action that can wash them away or break them off the rocks.
- Wide changes in temperature and salinity as pools warm up, cool down or are diluted by rain.
- Exposure to predators (birds at low tide, fish at high tide) and changing oxygen supply.
(f) Why the following are necessary for germination
- (i) Moisture (water): water is absorbed to soften the seed coat, activate the enzymes, and dissolve and transport the stored food to the growing embryo.
- (ii) Viable seeds: the seed must be alive with a healthy, living embryo; a dead (non-viable) embryo cannot divide and grow, so no germination can occur.
(g) Why light energy is a limiting factor in food production by autotrophs
Autotrophs (green plants) make food by photosynthesis, and light provides the energy for the process. When light is dim or absent, the rate of photosynthesis is low no matter how much carbon dioxide and water are available, so the amount of food made is restricted. Because a shortage of light holds back (limits) the rate of food production, light energy is described as a limiting factor.
Antwoorddetails
(a) Types of fertilisation
- (i) External fertilisation: the fusion of the male and female gametes takes place outside the body of the female, usually in water. The female lays her eggs and the male sheds sperm over them in the surrounding water.
- (ii) Internal fertilisation: the fusion of the male and female gametes takes place inside the body of the female, the sperm being deposited into her reproductive tract during mating.
(b) Two groups of animals for each type
- External fertilisation: bony fishes and amphibians (frogs/toads).
- Internal fertilisation: mammals and birds (also reptiles).
(c)(i) Three effects on the foetus if the placenta detaches from the uterine wall
- The foetus is cut off from its supply of food and oxygen, so it may suffocate and die.
- Waste products (carbon dioxide, urea) cannot be removed from the foetus, so they accumulate and poison it.
- It causes bleeding (haemorrhage) and may lead to miscarriage or premature birth of the foetus.
(c)(ii) Three other features of the uterus useful for foetal development
- A thick, muscular wall that protects the foetus and contracts to expel it at birth.
- A rich supply of blood vessels in the uterine lining that supplies food and oxygen and removes wastes.
- The amnion containing amniotic fluid that surrounds and cushions the foetus against shock (and the umbilical cord linking foetus to placenta).
(d) How the activities of organisms bring about dynamic equilibrium in a habitat
In a stable habitat, producers, consumers and decomposers interact so that materials and numbers stay roughly balanced. Green plants make food and release oxygen while animals consume food and release carbon dioxide, so the gases are kept balanced through photosynthesis and respiration. Predators feed on prey and so control the number of prey, while the food supply controls the number of predators, keeping populations steady. Decomposers break down dead matter and return mineral nutrients to the soil for plants to reuse. These continuous, opposing activities balance one another, so the habitat stays in a steady state called dynamic equilibrium.
(e) Four problems of organisms in the intertidal zone of a marine habitat
- Danger of drying out (desiccation) when the tide goes out and they are exposed to air and sun.
- Strong wave action that can wash them away or break them off the rocks.
- Wide changes in temperature and salinity as pools warm up, cool down or are diluted by rain.
- Exposure to predators (birds at low tide, fish at high tide) and changing oxygen supply.
(f) Why the following are necessary for germination
- (i) Moisture (water): water is absorbed to soften the seed coat, activate the enzymes, and dissolve and transport the stored food to the growing embryo.
- (ii) Viable seeds: the seed must be alive with a healthy, living embryo; a dead (non-viable) embryo cannot divide and grow, so no germination can occur.
(g) Why light energy is a limiting factor in food production by autotrophs
Autotrophs (green plants) make food by photosynthesis, and light provides the energy for the process. When light is dim or absent, the rate of photosynthesis is low no matter how much carbon dioxide and water are available, so the amount of food made is restricted. Because a shortage of light holds back (limits) the rate of food production, light energy is described as a limiting factor.
Vraag 3 Verslag
(a)(i) What is a deficiency disease?
(ii) Complete the table below by naming five nutrient deficiency diseases in humans and stating one remedy for each of the diseases.
(b) Outline a chemical test for (i) Starch in a tuber of yam (ii) Glucose in orange fruit
| Nutrient deficiency disease | Remedy |
(a)(i) What is a deficiency disease?
A deficiency disease is a disease that results from the lack or inadequate supply of a particular nutrient (such as a vitamin or a mineral element) in the diet over a period of time.
(a)(ii) Five nutrient deficiency diseases and their remedies
| Nutrient deficiency disease | Remedy |
|---|---|
| Scurvy (lack of vitamin C) | Eat fresh citrus fruits and vegetables (oranges, guava, tomatoes) |
| Rickets (lack of vitamin D) | Take vitamin D, milk and eggs; expose the body to sunlight |
| Kwashiorkor (lack of protein) | Eat protein-rich foods (beans, fish, meat, milk, eggs) |
| Anaemia (lack of iron) | Eat iron-rich foods (liver, green leafy vegetables); take iron supplements |
| Goitre (lack of iodine) | Use iodised table salt; eat sea foods |
(Night blindness from lack of vitamin A, corrected by eating carrots, red palm oil and green vegetables, is an acceptable alternative.)
(b) Chemical tests
(i) Test for starch in a tuber of yam
- Place a small piece of the yam on a white tile.
- Add two or three drops of iodine solution to it.
- Observation: the yam turns blue-black.
- Inference: starch is present.
(ii) Test for glucose in orange fruit
- Put some orange juice into a test tube.
- Add an equal volume of Benedict's (or Fehling's) solution.
- Heat the mixture in a water bath (boil).
- Observation: the blue colour changes through green and yellow to a brick-red precipitate.
- Inference: a reducing sugar (glucose) is present.
Antwoorddetails
(a)(i) What is a deficiency disease?
A deficiency disease is a disease that results from the lack or inadequate supply of a particular nutrient (such as a vitamin or a mineral element) in the diet over a period of time.
(a)(ii) Five nutrient deficiency diseases and their remedies
| Nutrient deficiency disease | Remedy |
|---|---|
| Scurvy (lack of vitamin C) | Eat fresh citrus fruits and vegetables (oranges, guava, tomatoes) |
| Rickets (lack of vitamin D) | Take vitamin D, milk and eggs; expose the body to sunlight |
| Kwashiorkor (lack of protein) | Eat protein-rich foods (beans, fish, meat, milk, eggs) |
| Anaemia (lack of iron) | Eat iron-rich foods (liver, green leafy vegetables); take iron supplements |
| Goitre (lack of iodine) | Use iodised table salt; eat sea foods |
(Night blindness from lack of vitamin A, corrected by eating carrots, red palm oil and green vegetables, is an acceptable alternative.)
(b) Chemical tests
(i) Test for starch in a tuber of yam
- Place a small piece of the yam on a white tile.
- Add two or three drops of iodine solution to it.
- Observation: the yam turns blue-black.
- Inference: starch is present.
(ii) Test for glucose in orange fruit
- Put some orange juice into a test tube.
- Add an equal volume of Benedict's (or Fehling's) solution.
- Heat the mixture in a water bath (boil).
- Observation: the blue colour changes through green and yellow to a brick-red precipitate.
- Inference: a reducing sugar (glucose) is present.
Vraag 4 Verslag
(a) Explain briefly; (i) independent assortment of genes (ii) the reason blood group O in humans can only exist in homozygous form while blood groups A and B can exist both in homozygous and heterozygous forms.
(b) Complete the table below by naming the classes of vertebrates in their evolutionary trend and giving one example each of the classes.
| Classes of vertebrate | One example |
(a)(i) Independent assortment of genes
Independent assortment is the principle that during the formation of gametes (in meiosis), the two alleles of one gene separate into gametes independently of the alleles of another gene carried on a different pair of chromosomes. As a result, the members of one pair of contrasting characters are distributed to the gametes randomly and independently of the members of another pair, producing new combinations of characters.
(a)(ii) Why blood group O exists only in the homozygous form
Blood group O is produced by the allele i, which is recessive to both IA and IB. A recessive character can only be expressed when the allele is present in a double (homozygous) dose. Therefore group O can only appear as ii. Blood groups A and B, however, are produced by the dominant alleles IA and IB. A dominant allele shows its effect whether it is paired with an identical allele or with the recessive i. Hence group A can be IAIA (homozygous) or IAi (heterozygous), and group B can be IBIB (homozygous) or IBi (heterozygous).
(b) Classes of vertebrates in evolutionary order with examples
| Class of vertebrate | One example |
|---|---|
| Pisces (fishes) | Tilapia |
| Amphibia | Toad |
| Reptilia | Lizard |
| Aves (birds) | Domestic fowl |
| Mammalia | Man (human being) |
Antwoorddetails
(a)(i) Independent assortment of genes
Independent assortment is the principle that during the formation of gametes (in meiosis), the two alleles of one gene separate into gametes independently of the alleles of another gene carried on a different pair of chromosomes. As a result, the members of one pair of contrasting characters are distributed to the gametes randomly and independently of the members of another pair, producing new combinations of characters.
(a)(ii) Why blood group O exists only in the homozygous form
Blood group O is produced by the allele i, which is recessive to both IA and IB. A recessive character can only be expressed when the allele is present in a double (homozygous) dose. Therefore group O can only appear as ii. Blood groups A and B, however, are produced by the dominant alleles IA and IB. A dominant allele shows its effect whether it is paired with an identical allele or with the recessive i. Hence group A can be IAIA (homozygous) or IAi (heterozygous), and group B can be IBIB (homozygous) or IBi (heterozygous).
(b) Classes of vertebrates in evolutionary order with examples
| Class of vertebrate | One example |
|---|---|
| Pisces (fishes) | Tilapia |
| Amphibia | Toad |
| Reptilia | Lizard |
| Aves (birds) | Domestic fowl |
| Mammalia | Man (human being) |
Vraag 5 Verslag
(a) Complete the table below;
| Disease | Causative organism | Vector | Control |
| Cholera | |||
| Trypanosome | |||
| Female Anopheles mosquito | |||
| Ringworm | None |
(b) Explain briefly the biological basis of preserving foods using each of the following methods;
(i) salting (ii) irradiation (iii) smoking
(a) Completed table
| Disease | Causative organism | Vector | Control |
|---|---|---|---|
| Cholera | Vibrio cholerae (a bacterium) | Housefly / contaminated water | Provide clean water and good sanitation; proper sewage disposal; vaccination |
| Sleeping sickness (trypanosomiasis) | Trypanosome (Trypanosoma species) | Tsetse fly | Clear bushes; destroy tsetse fly breeding sites; treat patients with drugs |
| Malaria | Plasmodium (a protozoan) | Female Anopheles mosquito | Destroy mosquito breeding sites; use insecticide-treated nets; take antimalarial drugs |
| Ringworm | Fungus (Tinea / Microsporum) | None (spread by contact) | Maintain personal hygiene; apply antifungal ointment; avoid sharing clothing |
(b) Biological basis of food preservation
(i) Salting
Adding salt creates a very concentrated (hypertonic) medium around the food. Water is drawn out of the micro-organisms by osmosis, dehydrating them so they cannot grow, multiply or spoil the food. The dry, salty condition also removes the water that microbes need.
(ii) Irradiation
Food is exposed to ionising radiation (such as gamma rays). The radiation destroys or damages the cells and DNA of bacteria, fungi and other micro-organisms (and inactivates enzymes), killing them so they can no longer cause spoilage.
(iii) Smoking
Smoking dries the food by heat, removing water that micro-organisms need. In addition, the smoke deposits antimicrobial chemicals (such as phenols and creosote) on the food surface which inhibit the growth of bacteria and fungi.
Antwoorddetails
(a) Completed table
| Disease | Causative organism | Vector | Control |
|---|---|---|---|
| Cholera | Vibrio cholerae (a bacterium) | Housefly / contaminated water | Provide clean water and good sanitation; proper sewage disposal; vaccination |
| Sleeping sickness (trypanosomiasis) | Trypanosome (Trypanosoma species) | Tsetse fly | Clear bushes; destroy tsetse fly breeding sites; treat patients with drugs |
| Malaria | Plasmodium (a protozoan) | Female Anopheles mosquito | Destroy mosquito breeding sites; use insecticide-treated nets; take antimalarial drugs |
| Ringworm | Fungus (Tinea / Microsporum) | None (spread by contact) | Maintain personal hygiene; apply antifungal ointment; avoid sharing clothing |
(b) Biological basis of food preservation
(i) Salting
Adding salt creates a very concentrated (hypertonic) medium around the food. Water is drawn out of the micro-organisms by osmosis, dehydrating them so they cannot grow, multiply or spoil the food. The dry, salty condition also removes the water that microbes need.
(ii) Irradiation
Food is exposed to ionising radiation (such as gamma rays). The radiation destroys or damages the cells and DNA of bacteria, fungi and other micro-organisms (and inactivates enzymes), killing them so they can no longer cause spoilage.
(iii) Smoking
Smoking dries the food by heat, removing water that micro-organisms need. In addition, the smoke deposits antimicrobial chemicals (such as phenols and creosote) on the food surface which inhibit the growth of bacteria and fungi.
