CBSE Class 12 Chemistry Biomolecules Important Questions Set 06

Very Short Answer Questions (1 Mark)

Question 1. Write one difference between α-helix and β-pleated structure of proteins. (CBSE 2018)
Answer:
In α-helix structure, intramolecular H-bonding takes place while in β-pleated structure, intermolecular H-bonding takes place.

Teacher's Note:
1. In the β-pleated sheet the chains lie side by side, fully stretched, and are held together by H-bonds between neighbouring chains.

Question 2. Name the central metal atom present in haemoglobin and chlorophyll.
Answer:
Haemoglobin (Fe) and chlorophyll (Mg).

Teacher's Note:
1. Do not confuse with vitamin B₁₂, whose central metal is cobalt.

Question 3. Differentiate between keratin and insulin.
Answer:
Keratin is a fibrous protein while insulin is a globular protein.

Teacher's Note:
1. Fibrous proteins are water-insoluble and structural; globular proteins are water-soluble and usually have a functional (e.g., hormonal) role.

Question 4. What causes the disease sickle cell anaemia ?
Answer:
Defective haemoglobin which is produced due to replacement of one amino acid, i.e., glutamic acid by valine.

Teacher's Note:
1. A change of just one amino acid in the primary structure changes the shape and function of the protein; this is the classic example.

Question 5. Define coenzymes ? Give one example.
Answer:
The non-protein component of an enzyme which is loosely held by the enzyme and which is necessary for its biological activity is called a coenzyme. These are mostly derived from vitamins such as thiamine (B₁), riboflavin (B₂), etc.

Teacher's Note:
1. NAD⁺ and coenzyme A are other common examples; many coenzymes come from the vitamin B group.

Question 6. Name the enzymes present in the saliva of human beings.
Answer:
Amylases first hydrolyse starch to maltose and then to glucose.

Teacher's Note:
1. The amylase of saliva is also called ptyalin; digestion of starch starts in the mouth.

Question 7. The sucrose that we eat in daily life is converted into glucose and fructose. Name the enzyme which facilitates this chemical reaction.
Answer:
Invertase or sucrase :
C₁₂H₂₂O₁₁ (Sucrose) + H₂O \( \xrightarrow{\text{Invertase or Sucrase}} \) C₆H₁₂O₆ (Glucose) + C₆H₁₂O₆ (Fructose)

Teacher's Note:
1. The same hydrolysis is called inversion of cane sugar because the sign of rotation changes from + to −.

Question 8. Name the enzyme which converts glucose into alcohol? (Pb. Board 2012)
Answer:
Zymase ; C₆H₁₂O₆ \( \xrightarrow{\text{Zymase}} \) 2 C₂H₅OH + 2 CO₂.

Teacher's Note:
1. Zymase is present in yeast; in fermentation of sucrose, invertase acts first and zymase then converts the glucose and fructose to ethanol.

Question 9. Certain diseases are caused by enzyme deficiencies. Give examples.
Answer:
Phenylketonuria and albinism.

Teacher's Note:
1. Albinism is due to lack of the enzyme tyrosinase, which is needed to make the pigment melanin.

Question 10. Name the enzyme which is used to dissolve the blood clot.
Answer:
Streptokinase.

Teacher's Note:
1. Streptokinase is given to heart-attack patients to dissolve clots in the coronary arteries.

Question 11. Define hormone. Give one example. (AP Board 2012)
Answer:
Hormones are biomolecules which control various metabolic processes or show physiological activity at a site other than that of their origin. For example, insulin is a peptide hormone. It is produced in the pancreas but controls the metabolism of glucose in the body.

Teacher's Note:
1. Hormones may be steroids (oestrogens, testosterone), polypeptides (insulin) or amino acid derivatives (thyroxine, adrenaline).

Question 12. How many α-amino units are present in a molecule of insulin? (Karnataka Board 2012)
Answer:
51 α-amino acid units present in two chains containing 21 and 30 α-amino acids.

Teacher's Note:
1. The two chains (21 + 30 = 51) are joined by disulphide bridges.

Question 13. Mention the biological importance of insulin. (Karnataka Board 2012)
Answer:
Controls the metabolism of glucose in the body and maintains glucose level in the blood.

Teacher's Note:
1. Insulin lowers blood glucose; glucagon has the opposite effect. Deficiency of insulin causes diabetes mellitus.

Question 14. Deficiency of which vitamin causes (i) night blindness and (ii) poor coagulation of blood ? (CBSE 2014)
Answer:
(i) Vitamin A, (ii) Vitamin K.

Teacher's Note:
1. Both A and K are fat-soluble vitamins and can be stored in the liver and adipose tissue.

Question 15. What is the chemical name of (i) vitamin E and (ii) vitamin C ? (HP Board 2011)
Answer:
(i) Tocopherol (ii) Ascorbic acid.

Teacher's Note:
1. Other names often asked: vitamin A – retinol, B₁ – thiamine, B₂ – riboflavin, B₁₂ – cyanocobalamin.

Question 16. Name the central metal ion present in haemoglobin and vitamin B₁₂. (HP Board 2011)
Answer:
Haemoglobin —Iron (Fe) and vitamin-B₁₂ – Cobalt (Co).

Teacher's Note:
1. Vitamin B₁₂ is the only vitamin that contains a metal (cobalt).

Question 17. Name the vitamins deficiency of which causes.
(i) rickets (CBSE Sample Paper 2017)
(ii) night blindness.
(iii) Scurvy (Bihar Board 2011; Manipur Board 2011; Assam Board 2013)
Answer:
(i) Vitamin D, (ii) Vitamin A, (iii) Vitamin C.

Teacher's Note:
1. Rickets in children and osteomalacia in adults are both due to vitamin D deficiency.

Question 18. Write the name of the vitamin whose deficiency causes bone deformities in children. (CBSE 2015)
Answer:
Vitamin D.

Teacher's Note:
1. The deformity is called rickets; vitamin D is needed for absorption of calcium and phosphorus.

Short Answer Questions (2 Marks)

Question 19. How is a peptide bond formed between two α-amino acids? (Karnataka Board 2012)
Answer:
A peptide bond is formed by condensation between the carboxyl group of one α-amino acid and the amino group of another α-amino acid with the loss of a molecule of water. For example,
H₂N–CH₂–COOH (Glycine) + H₂N–CH(CH₃)–COOH (Alanine) \( \xrightarrow{-\text{H}_2\text{O}} \) H₂N–CH₂–CO–NH–CH(CH₃)–COOH (Glycylalanine, Gly-Ala)

Teacher's Note:
1. Only one molecule of water is lost per peptide bond formed.

Question 20. What are peptides? (Pb. Board 2012)
Answer:
Peptides are amides formed by the condensation of two or more α-amino acids through peptide (–CO–NH–) linkages with the loss of water molecules. Depending upon the number of amino acid units, they are called dipeptides, tripeptides, tetrapeptides, etc. For example, glycylalanine (Gly-Ala) is a dipeptide.

Teacher's Note:
1. A peptide with n amino acid residues has (n − 1) peptide bonds.

Question 21. What are polypeptides ? How are they named ?
Answer:
Peptides containing a large number (generally more than ten) of α-amino acid units joined by peptide linkages are called polypeptides. Polypeptides with more than 100 amino acid residues (molecular mass > 10,000 u) are called proteins.
Naming : The amino acid with the free –NH₂ group (N-terminal) is written on the left and the one with the free –COOH group (C-terminal) on the right. The names of all the amino acids except the C-terminal one are written with the ending ‘-yl’ in place of ‘-ine’ and the C-terminal amino acid is written as such, e.g., glycylalanine (Gly-Ala), alanylglycylvaline (Ala-Gly-Val). Three-letter symbols are commonly used.

Teacher's Note:
1. Gly-Ala and Ala-Gly are different dipeptides; the order of amino acids matters.

Question 22. How are oligopeptides different from polypeptides ?
Answer:
Peptides containing a small number (2 to 10) of α-amino acid units are called oligopeptides (e.g., di-, tri-, tetrapeptides), whereas peptides containing a large number of α-amino acid units (more than ten) are called polypeptides.

Teacher's Note:
1. The prefix oligo- means “few” and poly- means “many”.

Question 23. Distinguish between peptides and proteins.
Answer:
Peptides are formed by the condensation of a small number of α-amino acids and have relatively low molecular masses. Polypeptides containing more than 100 amino acid residues and having molecular mass more than 10,000 u are called proteins. Proteins also have definite three-dimensional (secondary, tertiary, quaternary) structures responsible for their biological activity.

Teacher's Note:
1. The dividing line (100 residues, 10,000 u) is approximate; insulin with 51 residues is usually called a protein hormone.

Question 24. Define denaturation. What is the effect of denaturation on structure of proteins? (Hr. Board 2012; MP Board 2013)
Answer:
When a protein in its native form is subjected to a physical change like change in temperature or a chemical change like change in pH, the hydrogen bonds are disturbed, globules unfold, helices get uncoiled and the protein loses its biological activity. This is called denaturation of protein.
Effect : During denaturation, the secondary and tertiary structures of the protein are destroyed but the primary structure remains intact. For example, coagulation of egg white on boiling.

Teacher's Note:
1. Peptide bonds are not broken in denaturation; only the weaker forces (H-bonds etc.) holding the shape are.

Question 25. Why do proteins form an indispensable part of our food ?
Answer:
Proteins are the most abundant biomolecules of the living system. They are the building blocks of the body : they are needed for growth and repair of tissues (muscles, skin, hair, blood). They also act as enzymes (biocatalysts), hormones (e.g., insulin), carriers (e.g., haemoglobin carries oxygen), antibodies (defence against infection) and contractile proteins (muscle action). Proteins also supply the essential amino acids which our body cannot synthesise. Hence proteins form an indispensable part of our food.

Teacher's Note:
1. Dietary proteins are digested to amino acids, which the body then rebuilds into its own proteins.

Question 26. What are antibodies ? How do they fight bacterium or virus ?
Answer:
Antibodies are proteins (immunoglobulins, a type of globular glycoproteins) produced in the body in response to foreign substances called antigens, such as bacteria, viruses or toxins.
When an antigen enters the body, specific antibodies are produced which combine with the antigen to form an antigen-antibody complex. This neutralises the toxin or makes the foreign particles clump together so that they are destroyed and removed by the white blood cells. Thus antibodies protect the body against infection by bacteria and viruses.

Teacher's Note:
1. Each antibody is specific: it binds only to the antigen that caused its formation. This is the basis of vaccination.

Question 27. What is meant by biocatalysts? (CBSE 2012)
Answer:
Biocatalysts are catalysts produced by living cells which catalyse biochemical reactions in living organisms. Enzymes are biocatalysts. Chemically they are globular proteins. They are highly specific in their action, are effective in very small amounts, work best at body temperature (about 310 K) and optimum pH, and reduce the magnitude of activation energy of the reaction. For example, maltase catalyses the hydrolysis of maltose into glucose.

Teacher's Note:
1. Enzymes speed up reactions by lowering the activation energy, just like ordinary catalysts, but are far more specific.

Question 28. Hormones are chemical messengers. Explain.
Answer:
Hormones are produced by endocrine glands in very small amounts and are released directly into the blood stream. The blood carries them to the target organs where they carry the message from the gland to the organ and control its activity, e.g., insulin produced in the pancreas regulates the metabolism of glucose in the body. Since they transfer information (message) from one group of cells to distant tissues or organs, hormones are called chemical messengers.

Teacher's Note:
1. The key idea is “produced at one place, acts at a distant place”.

Question 29. What are hormones ? Give the name of one male and one female hormone.
Answer:
Hormones are chemical messengers produced by the endocrine glands which are carried by the blood stream to target organs where they regulate various metabolic processes.
Male hormone : Testosterone ; Female hormone : Oestradiol (an oestrogen) or progesterone.

Teacher's Note:
1. Both male and female sex hormones are steroid hormones.

Question 30. What are the functions of steroid and peptide hormones.
Answer:
Steroid hormones : Sex hormones (testosterone, oestrogens, progesterone) control the development of secondary sex characters and the reproductive cycle; glucocorticoids control carbohydrate metabolism, modulate inflammatory reactions and stress; mineralocorticoids control the level of salts and water in the body (excretion of water and salt by the kidney).
Peptide hormones : Insulin lowers and glucagon raises the blood glucose level; oxytocin causes contraction of the uterus and ejection of milk; growth hormones control growth.

Teacher's Note:
1. Steroid hormones are lipid-soluble and enter cells; peptide hormones act on receptors at the cell surface.

Question 31. Give two examples of amine hormones and list their functions.
Answer:
(i) Thyroxine (an iodinated derivative of the amino acid tyrosine, produced in the thyroid gland) controls the rate of metabolism. Its low level causes hypothyroidism (lethargy, obesity) and goitre.
(ii) Adrenaline (epinephrine) (produced in the adrenal medulla) prepares the body for emergency (fight or flight) by increasing the heart rate, blood pressure and blood glucose level.

Teacher's Note:
1. Iodised salt is used to prevent goitre because thyroxine contains iodine.

Short Answer Questions (3 Marks)

Question 32. What are proteins ? How are they classified ? (J & K Board 2012)
Answer:
Proteins are complex nitrogenous polymers of α-amino acids (molecular mass > 10,000 u) linked together by peptide linkages.
On the basis of molecular shape, proteins are classified into two types :
(i) Fibrous proteins : The polypeptide chains run parallel and are held together by hydrogen and disulphide bonds, giving a fibre-like structure. They are generally insoluble in water, e.g., keratin (hair, wool, silk) and myosin (muscles).
(ii) Globular proteins : The polypeptide chains coil around to give a spherical shape. They are usually soluble in water, e.g., insulin and albumins.
On the basis of their hydrolysis products, proteins are also classified as simple proteins (give only α-amino acids, e.g., albumin), conjugated proteins (give α-amino acids and a non-protein prosthetic group, e.g., haemoglobin) and derived proteins (products of partial hydrolysis, e.g., peptones).

Teacher's Note:
1. CBSE answers usually expect the fibrous/globular classification with one example of each.

Question 33. What are proteins ? Describe simple and conjugated proteins with at least one example in each case.
Answer:
Proteins are complex nitrogenous polymers of α-amino acids (molecular mass > 10,000 u) linked together by peptide linkages.
Simple proteins : Proteins which on hydrolysis give only α-amino acids are called simple proteins, e.g., albumin (egg white), globulins, glutelins.
Conjugated proteins : Proteins which on hydrolysis give α-amino acids along with a non-protein part called the prosthetic group are called conjugated proteins, e.g., haemoglobin (prosthetic group haem), casein of milk (phosphoric acid), nucleoproteins (nucleic acid), glycoproteins (carbohydrate).

Teacher's Note:
1. Name the prosthetic group with the example; that makes the distinction clear.

Question 34. Differentiate between the following with suitable examples :
(i) A globular protein and a fibrous protein. (Hr. Board 2011, 2012; CBSE 2010, 2015)
(ii) Primary and secondary structures of proteins. (CBSE 2011)
Answer:
(i) In fibrous proteins, the polypeptide chains run parallel and are held together by hydrogen and disulphide bonds, giving a fibre-like structure; they are insoluble in water, e.g., keratin, myosin. In globular proteins, the polypeptide chains coil around to give a spherical shape; they are usually soluble in water, e.g., insulin, albumins.
(ii) The primary structure of a protein refers to the specific sequence in which the α-amino acids are linked to one another by peptide bonds in a polypeptide chain, e.g., the sequence of 51 amino acids in insulin. The secondary structure refers to the shape in which the polypeptide chain exists as a result of hydrogen bonding between the >C=O and >N–H groups of peptide bonds; it is of two types, α-helix (e.g., α-keratin of hair) and β-pleated sheet (e.g., silk fibroin).

Teacher's Note:
1. Primary structure involves covalent (peptide) bonds; secondary structure is maintained by hydrogen bonds.

Question 35. Name the common types of secondary structures of proteins and give one point of difference.
Answer:
The two common types of secondary structures of proteins are α-helix and β-pleated sheet.
In α-helix, the polypeptide chain is twisted into a right-handed screw (helix) with the >N–H group of each amino acid residue H-bonded to the >C=O group of an adjacent turn of the helix (intramolecular H-bonding), e.g., keratin. In β-pleated sheet, the polypeptide chains are stretched out nearly to maximum extension and laid side by side, held together by intermolecular H-bonds, e.g., silk fibroin.

Teacher's Note:
1. Remember: α-helix = H-bonds within one chain; β-sheet = H-bonds between neighbouring chains.

Question 36. What is a peptide bond ? Explain the tertiary structure of proteins.
Answer:
The amide bond (–CO–NH–) formed between the –COOH group of one α-amino acid and the –NH₂ group of another α-amino acid with the elimination of a molecule of water is called a peptide bond.
Tertiary structure : It represents the overall folding of the polypeptide chain, i.e., further folding of the secondary structure. It gives rise to two major molecular shapes, viz., fibrous and globular. The main forces which stabilise the secondary and tertiary structures of proteins are hydrogen bonds, disulphide linkages, van der Waals forces and electrostatic forces of attraction.

Teacher's Note:
1. Tertiary structure decides whether a protein is fibrous or globular, and hence its biological function.

Question 37. What is denaturation and renaturation of proteins ?
Answer:
Denaturation : When a protein in its native form is subjected to a physical change like change in temperature, or a chemical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helices get uncoiled and the protein loses its biological activity. This is called denaturation of protein. During denaturation, the secondary and tertiary structures are destroyed but the primary structure remains intact, e.g., coagulation of egg white on boiling, curdling of milk.
Renaturation : In some cases, when the denaturing agent is removed or the original conditions of temperature and pH are restored, the protein regains its native structure and biological activity. This process is called renaturation. It is possible only when denaturation is mild (reversible).

Teacher's Note:
1. Boiled egg white cannot be renatured: that denaturation is irreversible.

Question 38. What are enzymes? Give two examples with uses. (MP Board 2013)
Answer:
Enzymes are biocatalysts; chemically they are globular proteins which catalyse specific biochemical reactions in living systems.
Examples :
(i) Invertase catalyses the hydrolysis of sucrose into glucose and fructose :
C₁₂H₂₂O₁₁ + H₂O \( \xrightarrow{\text{Invertase}} \) C₆H₁₂O₆ (Glucose) + C₆H₁₂O₆ (Fructose)
(ii) Zymase converts glucose into ethyl alcohol (used in fermentation) :
C₆H₁₂O₆ \( \xrightarrow{\text{Zymase}} \) 2C₂H₅OH + 2CO₂
Other examples : maltase (maltose → glucose), pepsin (proteins → peptides), streptokinase (used to dissolve blood clots).

Teacher's Note:
1. Enzymes are named by adding the suffix -ase to the name of the substrate or the reaction, e.g., maltase, oxidase.

Question 39. Define enzymes and comment upon the specificity in action of an enzyme. Illustrate with an example.
Answer:
Enzymes are biocatalysts produced by living cells; chemically they are globular proteins which catalyse specific biochemical reactions.
Specificity : Each enzyme catalyses only a particular reaction of a particular substrate. This is because the active site of the enzyme has a specific shape into which only a substrate molecule of complementary shape can fit (lock and key model) to form the enzyme-substrate complex, which then gives the products.
E + S → [ES] → E + P
For example, the enzyme maltase catalyses the hydrolysis of maltose into glucose but has no effect on sucrose; similarly, invertase hydrolyses only sucrose and urease hydrolyses only urea.
C₁₂H₂₂O₁₁ (Maltose) + H₂O \( \xrightarrow{\text{Maltase}} \) 2C₆H₁₂O₆ (Glucose)

Teacher's Note:
1. The lock-and-key idea is the simplest way to explain enzyme specificity in an exam answer.

Question 40. Describe the importance of hormones in living systems.
Answer:
Hormones are chemical messengers produced by endocrine (ductless) glands and carried by the blood stream to target organs. Their importance :
(i) They maintain the balance of biological activities in the body (homeostasis).
(ii) Insulin and glucagon keep the blood glucose level within a narrow limit.
(iii) Thyroxine regulates the metabolic rate; growth hormones control growth and development.
(iv) Sex hormones (oestrogens, androgens, progesterone) control the development of secondary sex characters and the reproductive cycle.
(v) Adrenaline and noradrenaline help the body to respond to external stimuli (“fight or flight”).
(vi) Glucocorticoids control carbohydrate metabolism and mineralocorticoids control the level of salts and water in the body.

Teacher's Note:
1. Hormones act in very small amounts and away from the gland that produces them.

Question 41. What are hormones ? How are they classified ?
Answer:
Hormones are molecules (chemical messengers) produced by the endocrine glands which are carried by the blood stream to target organs where they regulate various metabolic processes.
Chemically, hormones are classified into three types :
(i) Steroid hormones, e.g., oestrogens, androgens (testosterone), progesterone, glucocorticoids.
(ii) Polypeptide (peptide) hormones, e.g., insulin, glucagon, oxytocin.
(iii) Amino acid derivatives (amine hormones), e.g., adrenaline (epinephrine), thyroxine.

Teacher's Note:
1. Learn one example in each class; insulin (peptide), testosterone (steroid) and thyroxine (amino acid derivative) are safest.

Long Answer Questions (5 Marks)

Question 42. What are proteins ? How are they classified ? Discuss briefly their primary, secondary and tertiary structures.
Answer:
Proteins are complex nitrogenous polymers of α-amino acids (molecular mass > 10,000 u) linked together by peptide linkages.
Classification : On the basis of molecular shape, proteins are of two types :
(i) Fibrous proteins : The polypeptide chains run parallel and are held together by hydrogen and disulphide bonds, giving a fibre-like structure. They are generally insoluble in water, e.g., keratin (hair, wool, silk) and myosin (muscles).
(ii) Globular proteins : The polypeptide chains coil around to give a spherical shape. They are usually soluble in water, e.g., insulin and albumins.
Structure of proteins :
(i) Primary structure : The specific sequence in which the α-amino acids are linked to one another by peptide bonds in a polypeptide chain is called the primary structure. Any change in this sequence creates a different protein (e.g., replacement of glutamic acid by valine in haemoglobin causes sickle cell anaemia).
(ii) Secondary structure : It refers to the shape in which a polypeptide chain exists due to hydrogen bonding between >C=O and >N–H groups of the peptide bonds. It is of two types : (a) α-helix, in which the chain is twisted into a right-handed helix stabilised by intramolecular H-bonds, e.g., keratin; (b) β-pleated sheet, in which the chains are stretched out and laid side by side, held together by intermolecular H-bonds, e.g., silk fibroin.
(iii) Tertiary structure : It represents the overall folding of the polypeptide chain, i.e., further folding of the secondary structure, giving rise to fibrous or globular shapes. It is stabilised by hydrogen bonds, disulphide linkages, van der Waals forces and electrostatic forces of attraction.

Teacher's Note:
1. On denaturation the secondary and tertiary structures are lost but the primary structure stays intact; this is a favourite follow-up question.