CBSE Class 12 Chemistry Biomolecules Important Questions Set 07

Very Short Answer Questions (1 Mark)

Question 1. Deficiency of which vitamin causes beri-beri and pain in joints ?
Answer:
Vitamin B₁.

Teacher's Note:
1. Vitamin B₁ is thiamine; it is found in cereals, yeast and green vegetables.

Question 2. Fresh tomatoes are a better source of vitamin C than those which have been stored for some time. Explain.
Answer:
On prolonged exposure to air, vitamin C present in tomatoes is destroyed due to aerial oxidation.

Teacher's Note:
1. Vitamin C (ascorbic acid) is easily oxidised, and it is also destroyed by cooking; so fresh, raw fruits and vegetables are the best source.

Question 3. Vitamin ‘C’ is a vitamin found in fruits and vegetables. It cannot be stored in our body. Why? (Kerala Board 2012, CBSE 2016)
Answer:
It is soluble in water and hence excess of it is readily excreted in urine.

Teacher's Note:
1. Water-soluble vitamins (B group and C) must be supplied regularly in the diet; fat-soluble ones (A, D, E, K) can be stored.

Question 4. What is hypervitaminoses and avitaminoses ?
Answer:
Excess intake of vitamins A and D causes hypervitaminoses while multiple deficiencies caused by lack of more than one vitamin is called avitaminoses.

Teacher's Note:
1. Hypervitaminosis is possible mainly with fat-soluble vitamins, because they accumulate in the body instead of being excreted.

Question 5. Deficiency of which vitamin causes (i) pernicious anaemia (ii) convulsions ? (CBSE 2010 S)
Answer:
(i) Pernicious anaemia is caused by deficiency of vitamin B₁₂.
(ii) Convulsions are caused by deficiency of vitamin B₆.

Teacher's Note:
1. Vitamin B₆ is pyridoxine and vitamin B₁₂ is cyanocobalamin.

Question 6. B-complex is an often prescribed vitamin. What is complex about it and what is its usefulness ? (CBSE 2007)
Answer:
It is a group of vitamins which contains vitamins B₁, B₂, B₆, B₁₂, biotin, pantothenic acid (vitamin B₅), folic acid and nicotinic acid (vitamin B₃). Since it is not a single vitamin but is a group of vitamins, that is why, it is called vitamin B-complex. It is required to release energy from food and to promote healthy skin and muscles. Its deficiency causes beriberi (vitamin B₁) and pernicious anaemia (Vitamin B₁₂).

Teacher's Note:
1. All B-complex vitamins are water soluble, which is why they are prescribed as regular supplements.

Question 7. Except for vitamin B₁₂, all other vitamins of group B, should be supplied regularly in diet. Why ?
Answer:
Vitamin-B₁₂ does not occur in plants but is exclusively synthesized by the microorganisms present in the body. Therefore, it may not be supplied regularly in diet, while all other vitamins of group B should be supplied regularly in diet.
Moreover, all other vitamins of group B are water soluble and are readily excreted in urine, so they cannot be stored in the body, whereas vitamin B₁₂ is stored in the liver.

Teacher's Note:
1. The key word is storage: vitamin B₁₂ can be stored in the liver for a long time, the other B vitamins cannot.

Question 8. Write down the structure of sugar present in DNA. (Pb. Board 2012)
Answer:
Deoxyribose (2-deoxy-D-ribose). Its open chain structure is
CHO–CH₂–CHOH–CHOH–CH₂OH
In DNA it is present in the five-membered ring (furanose) form, β-D-2-deoxyribose, in which the C-2 carbon carries two H atoms and no –OH group.

Teacher's Note:
1. The only difference from ribose is at C-2: ribose has H and OH there, deoxyribose has two H atoms.

Question 9. Ribose in RNA and Deoxyribose in DNA differ in the structure around which carbon atom ? (CBSE Sample Paper 2018-19)
Answer:
DNA contains 2-deoxyribose while RNA contains ribose. Therefore, RNA and DNA differ in structure around C₂-carbon.

Teacher's Note:
1. “2-Deoxy” in the name itself tells you that the oxygen is missing at C-2.

Question 10. What is a nucleoside ?
Answer:
A nucleoside consists of two components, i.e., a nitrogenous base (purine or pyrimidine) and a five carbon sugar (ribose or deoxyribose). It is obtained when the nitrogenous base is attached to C₁ of sugar by a β-linkage. Thus, in general, a nucleoside may be represented as : Base-Sugar. Examples of nucleosides are adenosine, cytidine, etc.

Teacher's Note:
1. Nucleoside = base + sugar; nucleotide = base + sugar + phosphate. Do not mix them up.

Question 11. What is a nucleotide ? (CBSE 2007; Uttarakhand 2012)
Answer:
A nucleotide consists of all the three basic components of nucleic acids, i.e., a nitrogenous base (purine or pyrimidine), a five carbon sugar (ribose or deoxyribose) and phosphoric acid. A nucleoside is obtained when the nitrogenous base is attached to C₁ of the sugar by a β-linkage and a nucleotide is obtained when C₅′–OH of the sugar is esterified with phosphoric acid. Thus, in general, nucleotides may be represented as : Base-sugar- phosphate.

Teacher's Note:
1. Nucleotides are the monomer units of nucleic acids; they are joined by phosphodiester linkages.

Question 12. Name the base which is present in nucleotide of RNA only. (CBSE 2014)
Answer:
Uracil.

Teacher's Note:
1. Thymine is the base present only in DNA.

Question 13. What type of linkage is present in nucleic acids ? (CBSE 2016)
Answer:
The constituents of nucleic acids (ribose or deoxyribose, sugar, purine or pyrimidine base and phosphoric acid) are joined through covalent linkage, the nucleotides being joined through phosphodiester linkages between C₅′ and C₃′ of the sugar units, whereas DNA consists of two strands, RNA has a single strand. The two strands of DNA are linked together through hydrogen bonds.

Teacher's Note:
1. In nucleic acids the phosphodiester linkage plays the role that the peptide linkage plays in proteins.

Question 14. What type of linkage holds together the monomers of DNA ? (Assam Board 2013)
Answer:
The monomers of DNA are polydeoxyribonucleotides; in each strand they are held together by phosphodiester linkages. The two strands are held together by H-bonds. There are three H-bonds between guanine and cytosine (G ≡ C) and two between adenine and thymine (A = T).

Teacher's Note:
1. Distinguish the two levels: phosphodiester (covalent) bonds within a strand, hydrogen bonds between the paired bases of the two strands.

Question 15. What are the different types of RNA found in a cell ?
or Name the three nucleic acids used in protein synthesis.
or Which nucleic acid is responsible for protein synthesis in all cells.
or Name three types of RNA molecules which perform different functions. (CBSE 2013)
Answer:
mRNA, tRNA, rRNA

Teacher's Note:
1. mRNA carries the message (codons), tRNA brings the amino acids and rRNA forms the ribosomes where proteins are made.

Question 16. What is a Gene ?
Answer:
The segment of a DNA molecule which codes for a specific protein or polypeptide is called a gene.

Teacher's Note:
1. Genes are the units of heredity; each one carries the information for one polypeptide chain.

Question 17. Which of the following base pairs are found in DNA and which of them in RNA ?
(i) A : A, (ii) A : T, (iii) C : G, (iv) G : C, (v) G : A, (vi) A : U, (vii) G : U and (viii) A : C
Answer:
DNA : (ii), (iii), (iv) ; RNA : (vi).

Teacher's Note:
1. A pairs only with T (DNA) or U (RNA); G pairs only with C. The pair A : U is what distinguishes RNA, though G : C pairs also occur in the folded, paired regions of RNA.

Question 18. If a fragment of DNA molecule has the base sequence :
(a) TGATGCCGA (b) ATCTCGTAC
(c) GTAATCCAG (d) AATTCCCGG.
What is the base sequence of the complementary strand ?
Answer:
(a) ACTACGGCT (b) TAGAGCATG
(c) CATTAGGTC (d) TTAAGGGCC.

Teacher's Note:
1. Replace each base by its partner (A ↔ T, G ↔ C) and check that the complementary strand has the same number of letters.

Short Answer Questions (2 Marks)

Question 19. Comment briefly on the chemical nature of insulin and its physiological activity.
Answer:
Chemical nature : Insulin is a polypeptide (peptide) hormone made up of 51 α-amino acids arranged in two polypeptide chains of 21 and 30 amino acids, which are held together by disulphide bridges.
Physiological activity : Insulin is secreted by the β-cells of the pancreas (islets of Langerhans). It controls the metabolism of glucose : when blood glucose level rises, insulin is released and it increases the uptake of glucose by the cells, thus lowering the blood glucose level. Its deficiency causes diabetes mellitus.

Teacher's Note:
1. Insulin works opposite to glucagon; together they keep blood glucose nearly constant.

Question 20. Name the organ of secretion and one principal function of (i) adrenaline (ii) insulin (Assam Board 2012)
Answer:
(i) Adrenaline is secreted by the adrenal glands (adrenal medulla). It increases heart rate, blood pressure and blood glucose level and prepares the body to respond to emergency (stress) conditions.
(ii) Insulin is secreted by the pancreas (β-cells of islets of Langerhans). It controls the metabolism of glucose and lowers the blood glucose level.

Teacher's Note:
1. Name both the gland and one function for full marks.

Question 21. What deficiency diseases are caused due to lack of vitamins A, B, B₆ and K in human diet ?
Answer:
Vitamin A – Xerophthalmia (hardening of cornea of eye) and night blindness.
Vitamin B (B₁) – Beri-beri (loss of appetite, retarded growth).
Vitamin B₆ – Convulsions.
Vitamin K – Increased blood clotting time (haemorrhage).

Teacher's Note:
1. Vitamin K is the “koagulation” vitamin: its lack delays blood clotting.

Question 22. Name the main disease caused due to lack of the vitamin and its source in each of the following : A, B₆ and E.
Answer:
Vitamin A – Night blindness / xerophthalmia ; Source : fish liver oil, carrots, butter, milk.
Vitamin B₆ – Convulsions ; Source : yeast, milk, egg yolk, cereals and grams.
Vitamin E – Increased fragility of RBCs and muscular weakness ; Source : vegetable oils like wheat germ oil, sunflower oil.

Teacher's Note:
1. Vitamin B₆ is pyridoxine and vitamin E is tocopherol; the chemical names are often asked too.

Question 23. What are vitamins ? Which vitamins deficiency cause scurvy and rickets?
Answer:
Vitamins are organic compounds required in the diet in small amounts to perform specific biological functions for normal maintenance of optimum growth and health of the organism. Their deficiency causes specific diseases.
Scurvy is caused by deficiency of vitamin C and rickets by deficiency of vitamin D.

Teacher's Note:
1. Vitamin D can be made in the skin on exposure to sunlight, which is why rickets is linked with lack of sunlight too.

Question 24. What are vitamins ? Deficiency of which vitamin causes (i) Pernicious anaemia, (ii) Convulsions ? (CBSE 2010 S)
Answer:
Vitamins are organic compounds required in the diet in small amounts to perform specific biological functions for normal maintenance of optimum growth and health of the organism.
(i) Pernicious anaemia – vitamin B₁₂.
(ii) Convulsions – vitamin B₆.

Teacher's Note:
1. Pernicious anaemia is a lack of red blood cells (haemoglobin) due to vitamin B₁₂ deficiency.

Question 25. Name two fat soluble vitamins, their sources and the diseases caused due to their deficiency in diet. (CBSE 2009)
Answer:
(i) Vitamin A : Sources – fish liver oil, carrots, butter and milk ; Deficiency disease – xerophthalmia and night blindness.
(ii) Vitamin D : Sources – exposure to sunlight, fish and egg yolk ; Deficiency disease – rickets and osteomalacia.

Teacher's Note:
1. Vitamins E and K are also fat soluble and can be given instead.

Question 26. What are vitamins ? How are they classified? Name the vitamin which is responsible for coagulation of blood. (Uttarakhand Board 2012)
Answer:
Vitamins are organic compounds required in the diet in small amounts to perform specific biological functions for normal maintenance of optimum growth and health of the organism.
They are classified into : (i) fat soluble vitamins – A, D, E and K, and (ii) water soluble vitamins – B group and C.
Vitamin K is responsible for coagulation of blood.

Teacher's Note:
1. Vitamin K deficiency increases the blood clotting time.

Question 27. Name the following : (i) Water soluble vitamins, (ii) fat soluble vitamin and (iii) the disease caused by deficiency of vitamin C. (Jharkhand Board 2012)
Answer:
(i) Vitamins B group (B₁, B₂, B₆, B₁₂) and vitamin C.
(ii) Vitamins A, D, E and K.
(iii) Scurvy (bleeding gums).

Teacher's Note:
1. Vitamin C is the only non-B water soluble vitamin you need to remember.

Question 28. Give deficiency diseases caused by A, D, E, K vitamins. (AP Board 2012)
Answer:
Vitamin A – Xerophthalmia (hardening of cornea of eye) and night blindness.
Vitamin D – Rickets (bone deformities in children) and osteomalacia (soft bones and joint pain in adults).
Vitamin E – Increased fragility of RBCs and muscular weakness.
Vitamin K – Increased blood clotting time.

Teacher's Note:
1. These are the four fat soluble vitamins; excess intake of A and D can also be harmful (hypervitaminosis).

Question 29. Write the chemical name of vitamin B₁ and name the disease caused by its deficiency. (CBSE 2009; UP Board 2012)
Answer:
Chemical name of vitamin B₁ : Thiamine.
Deficiency disease : Beri-beri (loss of appetite, retarded growth).

Teacher's Note:
1. Do not confuse thiamine (B₁) with thymine, the base in DNA.

Question 30. Write the full form of RNA and name different compounds obtained on complete hydrolysis. (Hr. Board 2011)
Answer:
RNA stands for Ribonucleic acid.
On complete hydrolysis, RNA gives (i) a pentose sugar, D-(−)-ribose, (ii) phosphoric acid and (iii) nitrogenous bases : adenine (A), guanine (G), cytosine (C) and uracil (U).

Teacher's Note:
1. DNA would give 2-deoxy-D-ribose and thymine instead of ribose and uracil.

Question 31. Write down the structure of a nucleoside which is present only in RNA.
Answer:
Uridine (uracil + β-D-ribose) is present only in RNA, since uracil is present only in RNA. Uracil is attached through its N-1 to C-1′ of β-D-ribose :

Teacher's Note:
1. Ribose carries an –OH at C-2′; in DNA nucleosides that position has only H.

Short Answer Questions (3 Marks)

Question 32. Explain the terms ; estrogens, androgens and gestogens. Give one example of each type.
Answer:
Estrogens (oestrogens) : Female sex hormones produced mainly in the ovaries which control the development of female secondary sex characteristics and the menstrual cycle, e.g., oestradiol.
Androgens : Male sex hormones produced in the testes which are responsible for the development of male secondary sex characteristics (deep voice, facial hair, general physical constitution), e.g., testosterone.
Gestogens : Female sex hormones which prepare the uterus for implantation of the fertilised egg and maintain pregnancy, e.g., progesterone.

Teacher's Note:
1. All three groups are steroid hormones; progesterone is the “pregnancy hormone”.

Question 33. Explain the term peptide hormones. Give two examples and mention their functions.
Answer:
Hormones which are made up of a number of α-amino acids linked together by peptide bonds (polypeptides) are called peptide hormones.
(i) Insulin (secreted by the pancreas) lowers the blood glucose level by promoting the uptake of glucose by cells; its deficiency causes diabetes mellitus.
(ii) Glucagon (secreted by the pancreas) increases the blood glucose level by converting glycogen into glucose in the liver.
Oxytocin (contraction of uterus during child-birth) is another peptide hormone.

Teacher's Note:
1. Insulin and glucagon have opposite effects; together they keep the blood glucose level within a narrow limit.

Question 34. How are vitamins classified ? Mention the chief sources of vitamins A and C. (CBSE 2008)
Answer:
Vitamins are classified into two groups depending upon their solubility in water or fat :
(i) Fat soluble vitamins : Vitamins A, D, E and K. They are soluble in fat and oils but insoluble in water and are stored in the liver and adipose tissues.
(ii) Water soluble vitamins : B group vitamins and vitamin C. They must be supplied regularly in the diet because they are readily excreted in urine and cannot be stored in the body (except vitamin B₁₂).
Sources : Vitamin A – fish liver oil, carrots, butter and milk. Vitamin C – citrus fruits (amla, orange, lemon), tomatoes and green leafy vegetables.

Teacher's Note:
1. Memory aid: fat soluble vitamins are “A, D, E, K”; all the others are water soluble.

Teacher's Note:
1. A table is the quickest way to present vitamin, source and deficiency disease together.

Question 36. Explain briefly the difference between a nucleoside and a nucleotide. Which nitrogen containing bases are found in nucleotides? (Uttarakhand Board 2012)
Answer:
A nucleoside consists of a nitrogenous base attached to C-1′ of a pentose sugar (ribose or 2-deoxyribose) by a β-linkage : Base–Sugar, e.g., adenosine.
A nucleotide is formed when the C-5′ –OH of the sugar of a nucleoside is esterified with phosphoric acid : Base–Sugar–Phosphate, e.g., adenosine monophosphate (AMP).
The nitrogen containing bases found in nucleotides are the purines adenine (A) and guanine (G), and the pyrimidines cytosine (C), thymine (T, in DNA only) and uracil (U, in RNA only).

Teacher's Note:
1. Nucleotide = nucleoside + phosphate; the phosphate is what links nucleotides together in the nucleic acid chain.

Question 37. Discuss briefly the primary and secondary structures of nucleic acids.
Answer:
Primary structure : Nucleic acids are long chain polymers of nucleotides. The nucleotides are joined together by phosphodiester linkages between C-5′ and C-3′ carbon atoms of the pentose sugars. The sequence of nucleotides (bases) in the chain is called the primary structure of the nucleic acid.
Secondary structure : The secondary structure of DNA was given by James Watson and Francis Crick. According to it, DNA consists of two polynucleotide chains coiled around a common axis to form a right-handed double helix. The two strands are complementary and are held together by hydrogen bonds between specific base pairs : adenine pairs with thymine (two H-bonds) and guanine pairs with cytosine (three H-bonds). RNA molecules are single stranded, though they may fold back on themselves to form double helix regions.

Teacher's Note:
1. Primary structure = order of bases; secondary structure = double helix held by A–T and G–C hydrogen bonds.

Question 38. What are nucleic acids ? Explain their role in replication. (J & K Board 2011)
Answer:
Nucleic acids are long chain polymers of nucleotides found in the nuclei of all living cells. They are of two types : deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). They are responsible for heredity and protein synthesis.
Role in replication : DNA is capable of self-duplication (replication). During cell division, the two strands of the DNA double helix unwind and separate. Each strand acts as a template for the synthesis of a new complementary strand from the nucleotides present in the cell (A pairs with T, G with C). Thus two identical DNA molecules are formed, each having one old and one new strand. These pass on to the daughter cells, so that the genetic information is transmitted from one generation to the next.

Teacher's Note:
1. Because each new DNA keeps one parent strand, replication is called semi-conservative.

Question 39. Discuss briefly the replication of DNA. Can RNA replicate ? Explain.
Answer:
Replication of DNA : The process by which a DNA molecule produces its exact copy is called replication. The two strands of the DNA double helix unwind (the hydrogen bonds break) and separate. Each strand then acts as a template, and nucleotides from the cell pair with the bases of the template (A with T, G with C) and are joined by the enzyme DNA polymerase. Thus, two identical daughter DNA molecules are formed, each containing one original and one new strand.
RNA : RNA normally does not replicate in cells. It is single stranded and is synthesised from DNA by transcription; it then directs protein synthesis. Only in some viruses (RNA viruses) is RNA able to replicate.

Teacher's Note:
1. Replication makes DNA from DNA, transcription makes RNA from DNA and translation makes protein from mRNA.

Question 40. What are the functions of DNA ?
Answer:
(i) DNA is the chemical basis of heredity and is the reserve of genetic information. It is responsible for the transmission of hereditary characters from one generation to the next.
(ii) DNA has the unique property of replication (self-duplication) during cell division, so that identical DNA strands are passed on to the daughter cells.
(iii) DNA directs the synthesis of RNA (transcription), which in turn directs the synthesis of specific proteins (translation). Thus DNA controls protein synthesis.
(iv) DNA fingerprinting is used in forensic science and in determining paternity.

Teacher's Note:
1. DNA → RNA → protein is the central dogma; mention it in any question on functions of nucleic acids.

Teacher's Note:
1. Sugar, base and number of strands are the three structural differences examiners look for first.

Long Answer Questions (5 Marks)

Question 42. What are nucleic acids and how are they classified ? What are their functions ? Name the types of bases present in their nucleotides.
Answer:
Nucleic acids are long chain polymers of nucleotides found in the nuclei of all living cells. Each nucleotide consists of a pentose sugar, a nitrogenous base and a phosphoric acid unit; the nucleotides are joined by phosphodiester linkages between C-5′ and C-3′ of the sugar units.
Classification : Depending upon the sugar present, nucleic acids are of two types :
(i) Deoxyribonucleic acid (DNA), containing 2-deoxy-D-ribose. It has a double helix structure. (ii) Ribonucleic acid (RNA), containing D-ribose. It is single stranded and is of three types : messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA).
Functions : (i) DNA is the chemical basis of heredity; it stores the genetic information and transmits hereditary characters from one generation to the next. (ii) DNA undergoes replication during cell division so that identical DNA is passed on to daughter cells. (iii) Nucleic acids are responsible for the synthesis of proteins : DNA directs the synthesis of RNA (transcription) and RNA directs the synthesis of specific proteins (translation).
Bases : Two types of nitrogenous bases are present : purines – adenine (A) and guanine (G), present in both DNA and RNA; and pyrimidines – cytosine (C), present in both, thymine (T), present only in DNA, and uracil (U), present only in RNA.

Teacher's Note:
1. Each type of nucleic acid contains only four of the five bases: DNA has A, G, C, T; RNA has A, G, C, U.

Question 43. Name the products obtained on complete hydrolysis of DNA. Enumerate the structural difference between DNA and RNA. In what way is a nucleotide different from a nucleoside ? Illustrate with examples.
Answer:
Products of complete hydrolysis of DNA : 2-deoxy-D-ribose (a pentose sugar), phosphoric acid and the nitrogenous bases adenine, guanine, cytosine and thymine.
Structural differences between DNA and RNA :
(i) The sugar in DNA is 2-deoxy-D-ribose while in RNA it is D-ribose.
(ii) DNA contains thymine while RNA contains uracil in its place (A, G and C are common).
(iii) DNA has a double stranded helical structure (two complementary strands held by H-bonds between A–T and G–C), while RNA has a single stranded structure.
(iv) DNA molecules are very large (molecular mass up to 10⁹ u) while RNA molecules are much smaller.
Nucleoside and nucleotide : A nucleoside consists of a nitrogenous base attached to C-1′ of a pentose sugar by a β-linkage (Base–Sugar), e.g., adenosine (adenine + ribose), deoxythymidine (thymine + deoxyribose). A nucleotide is a nucleoside in which the C-5′ –OH of the sugar is esterified with phosphoric acid (Base–Sugar–Phosphate), e.g., adenosine 5′-monophosphate (AMP), deoxyadenosine monophosphate. Thus a nucleotide contains a phosphate group while a nucleoside does not.

Teacher's Note:
1. Name the sugar and the base together for nucleoside examples: adenosine = adenine + ribose.