CBSE Biology Past year paper Delhi (Set - 2) Solutions - 2013, Class 12 | Additional Study Material for NEET PDF Download

 Page 1


  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
CBSE 
Class XII Biology (Theory) 
Board Paper 2013 - (Set 2) 
SOLUTION 
Time: 3 hrs Total Marks: 70 
 
SECTION A 
1. Ans. The equation of net primary productivity of an ecosystem is 
NPP = GPP - R 
       where NPP = Net primary productivity 
GPP = Gross primary productivity 
R = Respiratory rate 
 
2. Ans.  
(a) 50,000 different strains of rice: Genetic diversity 
(b) Estuarine and alpine meadows: Ecological diversity 
 
3. Ans. Cellulase enzyme is used for isolating genetic material from plant cells 
and not from animal cells because it breaks down the plant cell wall made of 
cellulose. Animal cells do not have a cell wall. 
 
4. Ans. MOET stands for multiple ovulation embryo transfer technology which is 
a programme for herd improvement. The importance of MOET is to increase 
herd size in a short time.  
 
5. Ans. Thorns in Bougainvillea and tendrils of Cucurbita are the homologous 
organs as both arise in the axillary position. 
 
6. Ans. DNA-dependent DNA polymerase is the enzyme which polymerises the 
DNA strand in the 5' to 3' direction resulting in continuous and discontinuous 
replication. 
 
7. Ans. Sharing of injection needles between two individuals is not 
recommended because it causes fatal and dangerous diseases such as AIDS 
and hepatitis which are incurable. 
 
8. Ans. An anther with a malfunctioning tapetum often fails to produce viable 
gametophytes because the tapetum provides nutritive materials to the 
dividing microsporocytes. 
 
 
Page 2


  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
CBSE 
Class XII Biology (Theory) 
Board Paper 2013 - (Set 2) 
SOLUTION 
Time: 3 hrs Total Marks: 70 
 
SECTION A 
1. Ans. The equation of net primary productivity of an ecosystem is 
NPP = GPP - R 
       where NPP = Net primary productivity 
GPP = Gross primary productivity 
R = Respiratory rate 
 
2. Ans.  
(a) 50,000 different strains of rice: Genetic diversity 
(b) Estuarine and alpine meadows: Ecological diversity 
 
3. Ans. Cellulase enzyme is used for isolating genetic material from plant cells 
and not from animal cells because it breaks down the plant cell wall made of 
cellulose. Animal cells do not have a cell wall. 
 
4. Ans. MOET stands for multiple ovulation embryo transfer technology which is 
a programme for herd improvement. The importance of MOET is to increase 
herd size in a short time.  
 
5. Ans. Thorns in Bougainvillea and tendrils of Cucurbita are the homologous 
organs as both arise in the axillary position. 
 
6. Ans. DNA-dependent DNA polymerase is the enzyme which polymerises the 
DNA strand in the 5' to 3' direction resulting in continuous and discontinuous 
replication. 
 
7. Ans. Sharing of injection needles between two individuals is not 
recommended because it causes fatal and dangerous diseases such as AIDS 
and hepatitis which are incurable. 
 
8. Ans. An anther with a malfunctioning tapetum often fails to produce viable 
gametophytes because the tapetum provides nutritive materials to the 
dividing microsporocytes. 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
SECTION B 
 
9. Ans. Gene therapy of ADA deficiency: 
(i) The patient lacks functional T-lymphocytes and therefore fails to fight the 
infecting pathogens.  
(ii) Lymphocytes are extracted from the patient’s bone marrow, and a normal 
functional copy of the gene coding for ADA is introduced into these 
lymphocytes with the help of a retrovirus.  
(iii) The cells so treated are introduced into the patient’s bone marrow.  
(iv) The lymphocytes produced by these cells contain a functional ADA gene 
and reactivate the victim’s immune system for life. 
  
10.  Ans. 
(i) PCR: Polymerase chain reaction  
Application of PCR: It is useful to detect genetic disease in the foetus 
before birth. 
(ii) ELISA: Enzyme-linked immunosorbent assay  
Application of ELISA: It is useful in the early diagnosis of diseases using 
antigen–antibody interactions.  
OR 
(a) Exonucleases remove nucleotides from the ends of the DNA, whereas 
endonucleases make cuts at specific positions within the DNA. 
(b) Each restriction endonuclease inspects the DNA molecule in search of a 
specific recognition sequence. When it gets its specific recognition 
sequence, it binds to the site and cuts each of the two strands of the double 
helix at specific points by hydrolysing the phosphodiester backbones.   
 
11. Ans.  
Two sources of e-wastes: 
(i) Parts of computers and television sets 
(ii) Smart phones, parts of air conditioners and refrigerators 
        Two ways for disposal of e-wastes:  
(i) Recycling of e-wastes 
(ii) Incineration of e-wastes, i.e. burning e-wastes completely into ashes 
 
12. Ans. The pyramid of energy is a graphical representation of the amount of 
accumulated energy per unit area in different trophic levels of a food chain. 
An energy pyramid is always upright because there is a gradual decrease in 
the energy at successive trophic levels. This happens because according to the 
10% law of energy transfer, only 10% of the total energy is transferred from 
one trophic level to another. 
 
 
Page 3


  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
CBSE 
Class XII Biology (Theory) 
Board Paper 2013 - (Set 2) 
SOLUTION 
Time: 3 hrs Total Marks: 70 
 
SECTION A 
1. Ans. The equation of net primary productivity of an ecosystem is 
NPP = GPP - R 
       where NPP = Net primary productivity 
GPP = Gross primary productivity 
R = Respiratory rate 
 
2. Ans.  
(a) 50,000 different strains of rice: Genetic diversity 
(b) Estuarine and alpine meadows: Ecological diversity 
 
3. Ans. Cellulase enzyme is used for isolating genetic material from plant cells 
and not from animal cells because it breaks down the plant cell wall made of 
cellulose. Animal cells do not have a cell wall. 
 
4. Ans. MOET stands for multiple ovulation embryo transfer technology which is 
a programme for herd improvement. The importance of MOET is to increase 
herd size in a short time.  
 
5. Ans. Thorns in Bougainvillea and tendrils of Cucurbita are the homologous 
organs as both arise in the axillary position. 
 
6. Ans. DNA-dependent DNA polymerase is the enzyme which polymerises the 
DNA strand in the 5' to 3' direction resulting in continuous and discontinuous 
replication. 
 
7. Ans. Sharing of injection needles between two individuals is not 
recommended because it causes fatal and dangerous diseases such as AIDS 
and hepatitis which are incurable. 
 
8. Ans. An anther with a malfunctioning tapetum often fails to produce viable 
gametophytes because the tapetum provides nutritive materials to the 
dividing microsporocytes. 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
SECTION B 
 
9. Ans. Gene therapy of ADA deficiency: 
(i) The patient lacks functional T-lymphocytes and therefore fails to fight the 
infecting pathogens.  
(ii) Lymphocytes are extracted from the patient’s bone marrow, and a normal 
functional copy of the gene coding for ADA is introduced into these 
lymphocytes with the help of a retrovirus.  
(iii) The cells so treated are introduced into the patient’s bone marrow.  
(iv) The lymphocytes produced by these cells contain a functional ADA gene 
and reactivate the victim’s immune system for life. 
  
10.  Ans. 
(i) PCR: Polymerase chain reaction  
Application of PCR: It is useful to detect genetic disease in the foetus 
before birth. 
(ii) ELISA: Enzyme-linked immunosorbent assay  
Application of ELISA: It is useful in the early diagnosis of diseases using 
antigen–antibody interactions.  
OR 
(a) Exonucleases remove nucleotides from the ends of the DNA, whereas 
endonucleases make cuts at specific positions within the DNA. 
(b) Each restriction endonuclease inspects the DNA molecule in search of a 
specific recognition sequence. When it gets its specific recognition 
sequence, it binds to the site and cuts each of the two strands of the double 
helix at specific points by hydrolysing the phosphodiester backbones.   
 
11. Ans.  
Two sources of e-wastes: 
(i) Parts of computers and television sets 
(ii) Smart phones, parts of air conditioners and refrigerators 
        Two ways for disposal of e-wastes:  
(i) Recycling of e-wastes 
(ii) Incineration of e-wastes, i.e. burning e-wastes completely into ashes 
 
12. Ans. The pyramid of energy is a graphical representation of the amount of 
accumulated energy per unit area in different trophic levels of a food chain. 
An energy pyramid is always upright because there is a gradual decrease in 
the energy at successive trophic levels. This happens because according to the 
10% law of energy transfer, only 10% of the total energy is transferred from 
one trophic level to another. 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
13.  Ans. Small animals are cold-blooded organisms. They do not have constant 
body temperature and need to spend energy to generate heat through 
metabolism. So, they are rarely found in the polar regions. 
 
14.  Ans.  
(a) Cleistogamous flowers are bisexual flowers which remain closed, and 
pollen grains may only pollinate the stigma of the same flower. Example: 
Lathyrus. Thus, in such flowers, cross-pollination cannot occur and only 
autogamy occurs. Therefore, cleistogamy ensures autogamy. 
(b) Advantage of cleistogamy: It maintains the purity of the race and avoids 
mixing. 
Disadvantage of cleistogamy: Less chances of the production of new 
species and varieties. 
 
15. Ans. Chorionic villi are finger-like projections which arise from the 
trophoblast layer and develop in the zygote after it has undergone 
implantation. 
Functions of chorionic villi: 
(a) They take part in the formation of the placenta, which is the connecting 
link between the mother and the foetus. 
(b) They supply oxygen and nutrients to the growing embryo. 
 
16. Ans. In a cross between two tall pea plants, some offspring produced were 
dwarf which ensures that both parents are heterozygous (Tt). 
 
 
Phenotypic ratio: 3:1 
Genotypic ratio: 1:2:1 
 
 
 
 
 
 
 
Page 4


  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
CBSE 
Class XII Biology (Theory) 
Board Paper 2013 - (Set 2) 
SOLUTION 
Time: 3 hrs Total Marks: 70 
 
SECTION A 
1. Ans. The equation of net primary productivity of an ecosystem is 
NPP = GPP - R 
       where NPP = Net primary productivity 
GPP = Gross primary productivity 
R = Respiratory rate 
 
2. Ans.  
(a) 50,000 different strains of rice: Genetic diversity 
(b) Estuarine and alpine meadows: Ecological diversity 
 
3. Ans. Cellulase enzyme is used for isolating genetic material from plant cells 
and not from animal cells because it breaks down the plant cell wall made of 
cellulose. Animal cells do not have a cell wall. 
 
4. Ans. MOET stands for multiple ovulation embryo transfer technology which is 
a programme for herd improvement. The importance of MOET is to increase 
herd size in a short time.  
 
5. Ans. Thorns in Bougainvillea and tendrils of Cucurbita are the homologous 
organs as both arise in the axillary position. 
 
6. Ans. DNA-dependent DNA polymerase is the enzyme which polymerises the 
DNA strand in the 5' to 3' direction resulting in continuous and discontinuous 
replication. 
 
7. Ans. Sharing of injection needles between two individuals is not 
recommended because it causes fatal and dangerous diseases such as AIDS 
and hepatitis which are incurable. 
 
8. Ans. An anther with a malfunctioning tapetum often fails to produce viable 
gametophytes because the tapetum provides nutritive materials to the 
dividing microsporocytes. 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
SECTION B 
 
9. Ans. Gene therapy of ADA deficiency: 
(i) The patient lacks functional T-lymphocytes and therefore fails to fight the 
infecting pathogens.  
(ii) Lymphocytes are extracted from the patient’s bone marrow, and a normal 
functional copy of the gene coding for ADA is introduced into these 
lymphocytes with the help of a retrovirus.  
(iii) The cells so treated are introduced into the patient’s bone marrow.  
(iv) The lymphocytes produced by these cells contain a functional ADA gene 
and reactivate the victim’s immune system for life. 
  
10.  Ans. 
(i) PCR: Polymerase chain reaction  
Application of PCR: It is useful to detect genetic disease in the foetus 
before birth. 
(ii) ELISA: Enzyme-linked immunosorbent assay  
Application of ELISA: It is useful in the early diagnosis of diseases using 
antigen–antibody interactions.  
OR 
(a) Exonucleases remove nucleotides from the ends of the DNA, whereas 
endonucleases make cuts at specific positions within the DNA. 
(b) Each restriction endonuclease inspects the DNA molecule in search of a 
specific recognition sequence. When it gets its specific recognition 
sequence, it binds to the site and cuts each of the two strands of the double 
helix at specific points by hydrolysing the phosphodiester backbones.   
 
11. Ans.  
Two sources of e-wastes: 
(i) Parts of computers and television sets 
(ii) Smart phones, parts of air conditioners and refrigerators 
        Two ways for disposal of e-wastes:  
(i) Recycling of e-wastes 
(ii) Incineration of e-wastes, i.e. burning e-wastes completely into ashes 
 
12. Ans. The pyramid of energy is a graphical representation of the amount of 
accumulated energy per unit area in different trophic levels of a food chain. 
An energy pyramid is always upright because there is a gradual decrease in 
the energy at successive trophic levels. This happens because according to the 
10% law of energy transfer, only 10% of the total energy is transferred from 
one trophic level to another. 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
13.  Ans. Small animals are cold-blooded organisms. They do not have constant 
body temperature and need to spend energy to generate heat through 
metabolism. So, they are rarely found in the polar regions. 
 
14.  Ans.  
(a) Cleistogamous flowers are bisexual flowers which remain closed, and 
pollen grains may only pollinate the stigma of the same flower. Example: 
Lathyrus. Thus, in such flowers, cross-pollination cannot occur and only 
autogamy occurs. Therefore, cleistogamy ensures autogamy. 
(b) Advantage of cleistogamy: It maintains the purity of the race and avoids 
mixing. 
Disadvantage of cleistogamy: Less chances of the production of new 
species and varieties. 
 
15. Ans. Chorionic villi are finger-like projections which arise from the 
trophoblast layer and develop in the zygote after it has undergone 
implantation. 
Functions of chorionic villi: 
(a) They take part in the formation of the placenta, which is the connecting 
link between the mother and the foetus. 
(b) They supply oxygen and nutrients to the growing embryo. 
 
16. Ans. In a cross between two tall pea plants, some offspring produced were 
dwarf which ensures that both parents are heterozygous (Tt). 
 
 
Phenotypic ratio: 3:1 
Genotypic ratio: 1:2:1 
 
 
 
 
 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
17.  Ans. 
(a) IgE antibodies are produced in response to the entry of an allergen such as 
dust, pollen and animal dander. The allergens then combine with the mast 
cells and cause their bursting. A sufficient quantity of histamine and 
serotonin is released with the mast cells resulting in an inflammatory 
response. 
(b) The use of drugs such as anti-histamine, adrenalin and steroids quickly 
reduce the symptoms of allergy. 
 
18. Ans.  
(a) An E. coli bacterium has transformed or not when a recombinant DNA 
bearing ampicillin-resistant gene is transferred into it is analysed by 
growing E. coli cells on a culture medium containing ampicillin. The genes 
encoding the resistance to antibiotics such as ampicillin and tetracycline 
act as selectable markers for E. coli, and only the transformed cells will 
survive as they carry recombinant DNA with ampicillin-resistant gene. The 
normal cells do not possess the ability of resistance against any of these 
antibiotics and will die as they are ampicillin sensitive. 
(b) Ampicillin-resistant genes (amp
R
) serve as a selectable marker. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 5


  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
CBSE 
Class XII Biology (Theory) 
Board Paper 2013 - (Set 2) 
SOLUTION 
Time: 3 hrs Total Marks: 70 
 
SECTION A 
1. Ans. The equation of net primary productivity of an ecosystem is 
NPP = GPP - R 
       where NPP = Net primary productivity 
GPP = Gross primary productivity 
R = Respiratory rate 
 
2. Ans.  
(a) 50,000 different strains of rice: Genetic diversity 
(b) Estuarine and alpine meadows: Ecological diversity 
 
3. Ans. Cellulase enzyme is used for isolating genetic material from plant cells 
and not from animal cells because it breaks down the plant cell wall made of 
cellulose. Animal cells do not have a cell wall. 
 
4. Ans. MOET stands for multiple ovulation embryo transfer technology which is 
a programme for herd improvement. The importance of MOET is to increase 
herd size in a short time.  
 
5. Ans. Thorns in Bougainvillea and tendrils of Cucurbita are the homologous 
organs as both arise in the axillary position. 
 
6. Ans. DNA-dependent DNA polymerase is the enzyme which polymerises the 
DNA strand in the 5' to 3' direction resulting in continuous and discontinuous 
replication. 
 
7. Ans. Sharing of injection needles between two individuals is not 
recommended because it causes fatal and dangerous diseases such as AIDS 
and hepatitis which are incurable. 
 
8. Ans. An anther with a malfunctioning tapetum often fails to produce viable 
gametophytes because the tapetum provides nutritive materials to the 
dividing microsporocytes. 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
SECTION B 
 
9. Ans. Gene therapy of ADA deficiency: 
(i) The patient lacks functional T-lymphocytes and therefore fails to fight the 
infecting pathogens.  
(ii) Lymphocytes are extracted from the patient’s bone marrow, and a normal 
functional copy of the gene coding for ADA is introduced into these 
lymphocytes with the help of a retrovirus.  
(iii) The cells so treated are introduced into the patient’s bone marrow.  
(iv) The lymphocytes produced by these cells contain a functional ADA gene 
and reactivate the victim’s immune system for life. 
  
10.  Ans. 
(i) PCR: Polymerase chain reaction  
Application of PCR: It is useful to detect genetic disease in the foetus 
before birth. 
(ii) ELISA: Enzyme-linked immunosorbent assay  
Application of ELISA: It is useful in the early diagnosis of diseases using 
antigen–antibody interactions.  
OR 
(a) Exonucleases remove nucleotides from the ends of the DNA, whereas 
endonucleases make cuts at specific positions within the DNA. 
(b) Each restriction endonuclease inspects the DNA molecule in search of a 
specific recognition sequence. When it gets its specific recognition 
sequence, it binds to the site and cuts each of the two strands of the double 
helix at specific points by hydrolysing the phosphodiester backbones.   
 
11. Ans.  
Two sources of e-wastes: 
(i) Parts of computers and television sets 
(ii) Smart phones, parts of air conditioners and refrigerators 
        Two ways for disposal of e-wastes:  
(i) Recycling of e-wastes 
(ii) Incineration of e-wastes, i.e. burning e-wastes completely into ashes 
 
12. Ans. The pyramid of energy is a graphical representation of the amount of 
accumulated energy per unit area in different trophic levels of a food chain. 
An energy pyramid is always upright because there is a gradual decrease in 
the energy at successive trophic levels. This happens because according to the 
10% law of energy transfer, only 10% of the total energy is transferred from 
one trophic level to another. 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
13.  Ans. Small animals are cold-blooded organisms. They do not have constant 
body temperature and need to spend energy to generate heat through 
metabolism. So, they are rarely found in the polar regions. 
 
14.  Ans.  
(a) Cleistogamous flowers are bisexual flowers which remain closed, and 
pollen grains may only pollinate the stigma of the same flower. Example: 
Lathyrus. Thus, in such flowers, cross-pollination cannot occur and only 
autogamy occurs. Therefore, cleistogamy ensures autogamy. 
(b) Advantage of cleistogamy: It maintains the purity of the race and avoids 
mixing. 
Disadvantage of cleistogamy: Less chances of the production of new 
species and varieties. 
 
15. Ans. Chorionic villi are finger-like projections which arise from the 
trophoblast layer and develop in the zygote after it has undergone 
implantation. 
Functions of chorionic villi: 
(a) They take part in the formation of the placenta, which is the connecting 
link between the mother and the foetus. 
(b) They supply oxygen and nutrients to the growing embryo. 
 
16. Ans. In a cross between two tall pea plants, some offspring produced were 
dwarf which ensures that both parents are heterozygous (Tt). 
 
 
Phenotypic ratio: 3:1 
Genotypic ratio: 1:2:1 
 
 
 
 
 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
17.  Ans. 
(a) IgE antibodies are produced in response to the entry of an allergen such as 
dust, pollen and animal dander. The allergens then combine with the mast 
cells and cause their bursting. A sufficient quantity of histamine and 
serotonin is released with the mast cells resulting in an inflammatory 
response. 
(b) The use of drugs such as anti-histamine, adrenalin and steroids quickly 
reduce the symptoms of allergy. 
 
18. Ans.  
(a) An E. coli bacterium has transformed or not when a recombinant DNA 
bearing ampicillin-resistant gene is transferred into it is analysed by 
growing E. coli cells on a culture medium containing ampicillin. The genes 
encoding the resistance to antibiotics such as ampicillin and tetracycline 
act as selectable markers for E. coli, and only the transformed cells will 
survive as they carry recombinant DNA with ampicillin-resistant gene. The 
normal cells do not possess the ability of resistance against any of these 
antibiotics and will die as they are ampicillin sensitive. 
(b) Ampicillin-resistant genes (amp
R
) serve as a selectable marker. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 
CBSE XII  |  Biology 
Board Paper 2013 – Delhi – Set 2 (Solution) 
 
     
SECTION C 
19. Ans. 
(a) The fruit juices bought from the market are clearer as compared to those 
made at home because of the addition of proteases and pectinases in them. 
(b) The bioactive molecules produced by 
(i) Trichoderma polysporum is cyclosporin A (immunosupressor). 
(ii) Monascus purpureus is statins (lowers blood cholesterol level). 
 
20. Ans. 
(a) Transgenic animals are so called because they contain a foreign or 
transgene and have been modified by insertion of recombinant DNA. 
Positive traits have been inserted in them to produce products which are 
beneficial to humans. 
(b) The role of transgenic animals in 
(i) Vaccine safety: The transgenic animals are predominantly used for testing 
of vaccines before they are used on human beings. Example: Transgenic 
mice are used to test the safety of the polio vaccine.  
(ii) Biological products: Many human diseases are controlled by biological 
products. The transgenic animals which produce these products are 
introduced with DNA which codes for a particular product such as human 
protein (a-I-antitrypsin) for treating emphysema. In 1997, the first 
transgenic cow (Rosie) was produced. She was capable of secreting human 
protein-enriched milk. The milk contained human alpha-lactalbumin and 
was nutritionally a more balanced product for human babies than cow 
milk. 
 
21. Ans. Causes of desertification: 
(i) Improper farming practices: Farming practices are improper when crops 
are continually grown, harvested and the soil is not given enough time to 
replace its nutrients. This leads to loss of fertility of soil. 
(ii) Soil erosion: Excessive ploughing of land may also cause soil erosion. 
Animal grazing may also cause lack of natural vegetation (forest) and 
hence lack of plant roots to bind the soil particles together, causing soil 
erosion.  
(iii) Deforestation: Human beings cut down trees to serve their own purpose 
such as construction of houses and roads. Increase in industrialisation 
also increases deforestation. 
(iv) Mining: Mining activities and leaching of minerals destroy soil quality and 
make it infertile. 
OR 
   An algal bloom destroys the quality of a freshwater body in the following  
ways: