WAEC SSCE - Biology - 2011 (Essay)
Vraag 1 Verslag
(a) Explain the following terms:
(i) putrefaction
(ii) nitrogen fixation
(iii) nitrification
(iv) denitrification
(b) State three differences between nitrogen fixation and denitrification.
(c) Explain how nitrogen from urine gets to the roots of plants. W
(a) Explanation of terms
- (i) Putrefaction: the decomposition (breakdown) of nitrogenous organic matter such as dead plants, animals and their wastes by putrefying bacteria and fungi, releasing ammonia and ammonium compounds.
- (ii) Nitrogen fixation: the conversion of free atmospheric nitrogen gas into nitrogen compounds (ammonia/nitrates) that plants can use, carried out by nitrogen-fixing bacteria (e.g. Rhizobium, Azotobacter) and by lightning.
- (iii) Nitrification: the conversion of ammonia/ammonium compounds first to nitrites (by Nitrosomonas) and then to nitrates (by Nitrobacter) by nitrifying bacteria in the soil.
- (iv) Denitrification: the conversion of soil nitrates back into free nitrogen gas, which escapes into the atmosphere, carried out by denitrifying bacteria (e.g. Pseudomonas).
(b) Three differences between nitrogen fixation and denitrification
| Nitrogen fixation | Denitrification |
|---|---|
| Converts free nitrogen gas into nitrogen compounds (nitrates) | Converts nitrates into free nitrogen gas |
| Adds/increases nitrogen compounds in the soil | Removes/reduces nitrogen compounds from the soil |
| Carried out by nitrogen-fixing bacteria (e.g. Rhizobium) | Carried out by denitrifying bacteria (e.g. Pseudomonas) |
(c) How nitrogen from urine gets to the roots of plants
Urine contains urea, a nitrogenous waste. In the soil, bacteria break down the urea into ammonia/ammonium compounds (ammonification/putrefaction). Nitrifying bacteria then convert the ammonium compounds first into nitrites and then into nitrates (nitrification). The nitrates dissolve in the soil water and are absorbed by the roots of the plant, which use them to make proteins. In this way the nitrogen of urine reaches the roots.
Antwoorddetails
(a) Explanation of terms
- (i) Putrefaction: the decomposition (breakdown) of nitrogenous organic matter such as dead plants, animals and their wastes by putrefying bacteria and fungi, releasing ammonia and ammonium compounds.
- (ii) Nitrogen fixation: the conversion of free atmospheric nitrogen gas into nitrogen compounds (ammonia/nitrates) that plants can use, carried out by nitrogen-fixing bacteria (e.g. Rhizobium, Azotobacter) and by lightning.
- (iii) Nitrification: the conversion of ammonia/ammonium compounds first to nitrites (by Nitrosomonas) and then to nitrates (by Nitrobacter) by nitrifying bacteria in the soil.
- (iv) Denitrification: the conversion of soil nitrates back into free nitrogen gas, which escapes into the atmosphere, carried out by denitrifying bacteria (e.g. Pseudomonas).
(b) Three differences between nitrogen fixation and denitrification
| Nitrogen fixation | Denitrification |
|---|---|
| Converts free nitrogen gas into nitrogen compounds (nitrates) | Converts nitrates into free nitrogen gas |
| Adds/increases nitrogen compounds in the soil | Removes/reduces nitrogen compounds from the soil |
| Carried out by nitrogen-fixing bacteria (e.g. Rhizobium) | Carried out by denitrifying bacteria (e.g. Pseudomonas) |
(c) How nitrogen from urine gets to the roots of plants
Urine contains urea, a nitrogenous waste. In the soil, bacteria break down the urea into ammonia/ammonium compounds (ammonification/putrefaction). Nitrifying bacteria then convert the ammonium compounds first into nitrites and then into nitrates (nitrification). The nitrates dissolve in the soil water and are absorbed by the roots of the plant, which use them to make proteins. In this way the nitrogen of urine reaches the roots.
Vraag 2 Verslag
(a)State the second law of thermodynamics.
(ii) Use the second law of thermodynamics to explain the flow of energy across different trophic levels in a food chain.
(b) (I) State three factors that can likely limit the size of human populations.
(ii) Outline two ways each by which humans can overcome the factors named in (b)(i).
(a)(i) The second law of thermodynamics
The second law of thermodynamics states that whenever energy is converted from one form to another, some of the energy is always lost as heat, so that no energy transfer or transformation is 100% efficient; the entropy (disorder) of the universe always tends to increase.
(a)(ii) Applying it to energy flow in a food chain
Energy enters a food chain when green plants (producers) trap sunlight in photosynthesis. As energy passes from one trophic level to the next (producer to primary consumer to secondary consumer, and so on), only about 10% is passed on and stored in the next level. The other roughly 90% is lost as heat through respiration and in movement, and some is lost in undigested waste and death. Because of this loss at each transfer, less and less energy is available at higher trophic levels. This is why energy flow is one-directional (it does not cycle) and why food chains usually have only four or five links, and why there are fewer organisms at the top than at the bottom (the pyramid of energy).
(b)(i) Three factors that limit the size of human populations
- Food supply (availability of food).
- Disease and epidemics.
- Space / shelter and availability of water (war and natural disasters also accepted).
(b)(ii) Two ways each of overcoming the factors
- Food supply: increase food production through mechanised and improved farming; improve storage, preservation and distribution of food.
- Disease: provide immunisation/vaccination and good medical services; improve sanitation, clean water and public health education.
- Space / water: build more houses and develop new settlements/urban planning; provide dams, boreholes and pipe-borne water and manage resources wisely.
Antwoorddetails
(a)(i) The second law of thermodynamics
The second law of thermodynamics states that whenever energy is converted from one form to another, some of the energy is always lost as heat, so that no energy transfer or transformation is 100% efficient; the entropy (disorder) of the universe always tends to increase.
(a)(ii) Applying it to energy flow in a food chain
Energy enters a food chain when green plants (producers) trap sunlight in photosynthesis. As energy passes from one trophic level to the next (producer to primary consumer to secondary consumer, and so on), only about 10% is passed on and stored in the next level. The other roughly 90% is lost as heat through respiration and in movement, and some is lost in undigested waste and death. Because of this loss at each transfer, less and less energy is available at higher trophic levels. This is why energy flow is one-directional (it does not cycle) and why food chains usually have only four or five links, and why there are fewer organisms at the top than at the bottom (the pyramid of energy).
(b)(i) Three factors that limit the size of human populations
- Food supply (availability of food).
- Disease and epidemics.
- Space / shelter and availability of water (war and natural disasters also accepted).
(b)(ii) Two ways each of overcoming the factors
- Food supply: increase food production through mechanised and improved farming; improve storage, preservation and distribution of food.
- Disease: provide immunisation/vaccination and good medical services; improve sanitation, clean water and public health education.
- Space / water: build more houses and develop new settlements/urban planning; provide dams, boreholes and pipe-borne water and manage resources wisely.
Vraag 3 Verslag
(a) (i) What is the effect of gravity on plant growth?
(ii) Describe an experiment to demonstrate the effect of gravity on the roots of plants.
(b) Explain how bitter taste is detected in humans.
(c) Copy and complete the table below on endocrine glands and their functions.
| S/N | Endocrine glands | Hormone produced | functions |
| 1 | Thyroid | ||
| 2 | Insulin | ||
| 3 | Development of secondary sexual characters of females |
(a)(i) Effect of gravity on plant growth
Gravity determines the direction of growth of plant organs. The root grows downwards towards the pull of gravity (it is positively geotropic), while the shoot grows upwards away from gravity (it is negatively geotropic). This response is called geotropism (gravitropism).
(a)(ii) Experiment to demonstrate the effect of gravity on roots
- Soak and germinate several bean seeds until the radicle (root) appears.
- Pin two or three germinating seeds to a cork so that their roots point in different directions (some horizontal, some upward).
- Place the cork inside a bell jar or wide jar containing moist cotton wool to keep the air humid, and keep it in the dark to remove the effect of light.
- Set up a control in which an identical cork with seeds is fixed to a slowly rotating clinostat (this nullifies the one-sided pull of gravity).
- Leave both set-ups for two to three days and observe.
Observation: in the stationary set-up all the roots curve downwards, whatever their starting direction; on the clinostat the roots grow straight. Conclusion: gravity causes roots to grow downwards (positive geotropism).
(b) How bitter taste is detected in humans
Bitter substances in food dissolve in the saliva and stimulate the taste buds located mainly at the back of the tongue. The chemical stimulates the receptor (gustatory) cells, which generate nerve impulses. The impulses are carried by sensory nerves to the brain (cerebral cortex), where they are interpreted as a bitter taste.
(c) Endocrine glands and their functions
| S/N | Endocrine gland | Hormone produced | Function |
|---|---|---|---|
| 1 | Thyroid | Thyroxine | Controls the rate of metabolism and growth |
| 2 | Pancreas (islets of Langerhans) | Insulin | Lowers and regulates the level of glucose in the blood |
| 3 | Ovary | Oestrogen | Development of secondary sexual characters in females |
Antwoorddetails
(a)(i) Effect of gravity on plant growth
Gravity determines the direction of growth of plant organs. The root grows downwards towards the pull of gravity (it is positively geotropic), while the shoot grows upwards away from gravity (it is negatively geotropic). This response is called geotropism (gravitropism).
(a)(ii) Experiment to demonstrate the effect of gravity on roots
- Soak and germinate several bean seeds until the radicle (root) appears.
- Pin two or three germinating seeds to a cork so that their roots point in different directions (some horizontal, some upward).
- Place the cork inside a bell jar or wide jar containing moist cotton wool to keep the air humid, and keep it in the dark to remove the effect of light.
- Set up a control in which an identical cork with seeds is fixed to a slowly rotating clinostat (this nullifies the one-sided pull of gravity).
- Leave both set-ups for two to three days and observe.
Observation: in the stationary set-up all the roots curve downwards, whatever their starting direction; on the clinostat the roots grow straight. Conclusion: gravity causes roots to grow downwards (positive geotropism).
(b) How bitter taste is detected in humans
Bitter substances in food dissolve in the saliva and stimulate the taste buds located mainly at the back of the tongue. The chemical stimulates the receptor (gustatory) cells, which generate nerve impulses. The impulses are carried by sensory nerves to the brain (cerebral cortex), where they are interpreted as a bitter taste.
(c) Endocrine glands and their functions
| S/N | Endocrine gland | Hormone produced | Function |
|---|---|---|---|
| 1 | Thyroid | Thyroxine | Controls the rate of metabolism and growth |
| 2 | Pancreas (islets of Langerhans) | Insulin | Lowers and regulates the level of glucose in the blood |
| 3 | Ovary | Oestrogen | Development of secondary sexual characters in females |
Vraag 4 Verslag
(a) Make a diagram 8 — 10 cm long to show the relative position of the following organs in the mammalian gut:
(i) stomach
(ii) pancreas
(iii) liver
(iv) gall bladder
(v) duodenum and label fully.
(b) (i) List two diseases of the liver.
(ii) State five effects of the diseases in (b) (i) on the human body.
(c) State the deficiency symptoms of the following vitamins:
(i) vitamin A
(ii) vitamin C
(ii) Vitamin K;
(a) Relative position of organs in the mammalian gut
A large labelled diagram (8 to 10 cm long) should show the stomach on the left, leading into the C-shaped duodenum; the pancreas lying in the loop of the duodenum; the liver above and to the right of the stomach; and the gall bladder tucked under the liver, with its bile duct joining the pancreatic duct to enter the duodenum.
(b)(i) Two diseases of the liver
- Hepatitis (inflammation of the liver)
- Liver cirrhosis (jaundice/liver cancer also accepted)
(b)(ii) Five effects of liver disease on the body
- Jaundice (yellowing of the skin and eyes) from build-up of bile pigment.
- Poor digestion and absorption of fats because bile production falls.
- Accumulation of toxins in the blood, as the liver can no longer detoxify them.
- Failure of blood clotting, leading to easy bleeding (reduced fibrinogen/prothrombin).
- General weakness, loss of appetite and swelling of the abdomen (accumulation of fluid).
(c) Vitamin deficiency symptoms
| Vitamin | Deficiency symptom |
|---|---|
| Vitamin A | Night blindness (poor vision in dim light); dry, rough skin |
| Vitamin C | Scurvy - bleeding, swollen and spongy gums; slow wound healing |
| Vitamin K | Failure of blood to clot (prolonged bleeding) |
Antwoorddetails
(a) Relative position of organs in the mammalian gut
A large labelled diagram (8 to 10 cm long) should show the stomach on the left, leading into the C-shaped duodenum; the pancreas lying in the loop of the duodenum; the liver above and to the right of the stomach; and the gall bladder tucked under the liver, with its bile duct joining the pancreatic duct to enter the duodenum.
(b)(i) Two diseases of the liver
- Hepatitis (inflammation of the liver)
- Liver cirrhosis (jaundice/liver cancer also accepted)
(b)(ii) Five effects of liver disease on the body
- Jaundice (yellowing of the skin and eyes) from build-up of bile pigment.
- Poor digestion and absorption of fats because bile production falls.
- Accumulation of toxins in the blood, as the liver can no longer detoxify them.
- Failure of blood clotting, leading to easy bleeding (reduced fibrinogen/prothrombin).
- General weakness, loss of appetite and swelling of the abdomen (accumulation of fluid).
(c) Vitamin deficiency symptoms
| Vitamin | Deficiency symptom |
|---|---|
| Vitamin A | Night blindness (poor vision in dim light); dry, rough skin |
| Vitamin C | Scurvy - bleeding, swollen and spongy gums; slow wound healing |
| Vitamin K | Failure of blood to clot (prolonged bleeding) |
Vraag 5 Verslag
(a) (i) What is the difference between the genotype and phenotype of an individual?
(ii) State two factors each which determine each of the terms in (a)(i).
(b) In a breeding experiment, a homozygous dog (BB) was crossed with a homozygous white dog (bb) to produce black puppies in the first filial generation (F1) The second filial generation produced a total of two hundred and forty (240) black and white puppies. With the aid of a genetic diagram, determine:
(i) the number of white puppies
(ii) the number of black puppies in the second filial generation.
(a)(i) Difference between genotype and phenotype
The genotype is the genetic constitution of an individual, that is, the actual set of alleles (genes) it carries for a trait, for example BB, Bb or bb. The phenotype is the observable physical or physiological expression of that trait, for example black coat colour or white coat colour, which results from the genotype interacting with the environment.
(a)(ii) Two factors that determine each term
Genotype is determined by:
- the genotypes (alleles) of the parents from which the individual inherits its genes;
- mutation, that is, changes in the genes or chromosomes.
Phenotype is determined by:
- the genotype of the individual;
- the environment (for example diet, temperature and general health).
(b) Breeding experiment: BB (black) × bb (white)
Let B = allele for black coat (dominant) and b = allele for white coat (recessive).
Parental cross (P): a homozygous black dog (BB) crossed with a homozygous white dog (bb). Each parent contributes one gamete type, so every offspring receives one B and one b allele. All the first filial generation (F1) are therefore Bb (black).
F1 × F1 cross: Bb × Bb. The genetic diagram (Punnett square) below shows the F2 offspring.
From the genetic diagram the F2 genotypes are 1 BB : 2 Bb : 1 bb, giving a phenotypic ratio of 3 black : 1 white.
Total number of F2 puppies = 240.
(b)(i) Number of white puppies
White puppies (bb) form \(\tfrac{1}{4}\) of the offspring:
\[ \text{White} = \frac{1}{4} \times 240 = 60 \text{ white puppies} \](b)(ii) Number of black puppies
Black puppies (BB and Bb) form \(\tfrac{3}{4}\) of the offspring:
\[ \text{Black} = \frac{3}{4} \times 240 = 180 \text{ black puppies} \]Answers: 60 white puppies and 180 black puppies in the second filial generation.
Antwoorddetails
(a)(i) Difference between genotype and phenotype
The genotype is the genetic constitution of an individual, that is, the actual set of alleles (genes) it carries for a trait, for example BB, Bb or bb. The phenotype is the observable physical or physiological expression of that trait, for example black coat colour or white coat colour, which results from the genotype interacting with the environment.
(a)(ii) Two factors that determine each term
Genotype is determined by:
- the genotypes (alleles) of the parents from which the individual inherits its genes;
- mutation, that is, changes in the genes or chromosomes.
Phenotype is determined by:
- the genotype of the individual;
- the environment (for example diet, temperature and general health).
(b) Breeding experiment: BB (black) × bb (white)
Let B = allele for black coat (dominant) and b = allele for white coat (recessive).
Parental cross (P): a homozygous black dog (BB) crossed with a homozygous white dog (bb). Each parent contributes one gamete type, so every offspring receives one B and one b allele. All the first filial generation (F1) are therefore Bb (black).
F1 × F1 cross: Bb × Bb. The genetic diagram (Punnett square) below shows the F2 offspring.
From the genetic diagram the F2 genotypes are 1 BB : 2 Bb : 1 bb, giving a phenotypic ratio of 3 black : 1 white.
Total number of F2 puppies = 240.
(b)(i) Number of white puppies
White puppies (bb) form \(\tfrac{1}{4}\) of the offspring:
\[ \text{White} = \frac{1}{4} \times 240 = 60 \text{ white puppies} \](b)(ii) Number of black puppies
Black puppies (BB and Bb) form \(\tfrac{3}{4}\) of the offspring:
\[ \text{Black} = \frac{3}{4} \times 240 = 180 \text{ black puppies} \]Answers: 60 white puppies and 180 black puppies in the second filial generation.
