Biomolecules
Book Solutions1
Answer
Monosaccharides are carbohydrates that cannot be hydrolysed further to give simpler units of polyhydroxy aldehyde or ketone. Monosaccharides are classified on the bases of number of carbon atoms and the functional group present in them. Monosaccharides containing an aldehyde group are known as aldoses and those containing a keto group are known as ketoses. Monosaccharides are further classified as trioses, tetroses, pentoses, hexoses, and heptoses according to the number of carbon atoms they contain. For example, a ketose containing 3 carbon atoms is called ketotriose and an aldose containing 3 carbon atoms is called aldotriose.
2
Answer
Those type of carbohydrates which reduces the Fehling’s solution and Tollen’s reagent are termed as reducing sugars.3
Answer
The two main functions of a carbohydrate in a plant are:
(a) Polysaccharides like starch acts as a storage molecules.
(b) Cellulose is used to build the cell wall, and it is a polysaccharide.
4
Categorise the given carbohydrates into monosaccharides and disaccharides.
2-deoxyribose, Ribose, maltose, lactose, galactose and fructose
Answer
Monosaccharides : 2-deoxyribose, galactose, ribose, fructose
Disaccharides : lactose, maltose
5
Answer
Glycosidic linkage refers to the linkage formed between two monosaccharide units through an oxygen atom by the loss of a water molecule. For example, in a sucrose molecule, two monosaccharide units, ∝-glucose and β-fructose, are joined together by a glycosidic linkage.
6
Answer
Glycogen is a carbohydrate (polysaccharide). In animals, carbohydrates are stored as glycogen.
Starch is a carbohydrate consisting of two components − amylose (15 − 20%) and amylopectin (80 − 85%).
However, glycogen consists of only one component whose structure is similar to amylopectin. Also, glycogen is more branched than amylopectin.
7
Answer
Both sucrose and lactose are disaccharides. Sucrose on hydrolysis gives one molecule each of glucose and fructose but lactose on hydrolysis gives one molecule each of glucose and galactose.
8
Answer
Starch consists of amylose and amylopectin. Amylose is a linear polymer of a-D-glucose while cellulose is a linear polymer of (3 -D- glucose. In amylose, C —1 of one glucose unit is connected to C — 4 of the other through a—glycosidic linkage. However in cellulose, C — 1 of one glucose unit is connected to C-4 of the other through 13 — glycosidic linkage. Amylopectin on the other hand has highly branched structure.
9
What happens when D-glucose is treated with the following reagents?
(i) HI (ii) Bromine water (iii) HNO3
Answer
10
Answer
(1) Aldehydes give 2, 4-DNP test, Schiff’s test, and react with NaHSO4 to form the hydrogen sulphite addition product. However, glucose does not undergo these reactions.
(2) The pentaacetate of glucose does not react with hydroxylamine. This indicates that a free −CHO group is absent from glucose.
(3) Glucose exists in two crystalline forms − ∝ andβ. The ∝-form (m.p. = 419 K) crystallises from a concentrated solution of glucose at 303 K and the β-form (m.p = 423 K) crystallises from a hot and saturated aqueous solution at 371 K. This behaviour cannot be explained by the open chain structure of glucose.
11
Answer
Essential amino acids are required by the human body, but they cannot be synthesised in the body. They must be taken through food. For example: valine and leucine.
Non-essential amino acids are also required by the human body, but they can be synthesised in the body. For example: glycine, and alanine.
12
Define the following as related to proteins
(i) Peptide linkage (ii) Primary structure (iii) Denaturation
Answer
(i) Peptide linkage or bond: Proteins are condensation polymers of a-amino acids in which the same or different a-amino acids are joined by peptide bonds. Chemically, a peptide bond is an amide linkage formed between — COOH group of one α-amino acid and —NH2 group of the other α-amino acid by loss of a molecule of water. For example,
(ii) Primary structure: Proteins may contain one or more polypeptide chains. Each polypeptide chain has a large number of a-amino acids which are linked to one another in a specific manner. The specific sequence in which the various amino acids present in a protein linked to one another is called its primary structure. Any change in the sequence of a-amino acids creates a different protein.
(iii) Denaturation:
In a biological system, a protein is found to have a unique 3-dimensional structure and a unique biological activity. In such a situation, the protein is called native protein. However, when the native protein is subjected to physical changes such as change in temperature or chemical changes such as change in pH, its H-bonds are disturbed. This disturbance unfolds the globules and uncoils the helix. As a result, the protein loses its biological activity. This loss of biological activity by the protein is called denaturation. During denaturation, the secondary and the tertiary structures of the protein get destroyed, but the primary structure remains unaltered. One of the examples of denaturation of proteins is the coagulation of egg white when an egg is boiled.
13
Answer
There are two common types of secondary structure of proteins:
(i) ∝-helix structure
(ii) β-pleated sheet structure
∝- Helix structure: In this structure, the −NH group of an amino acid residue forms H-bond with the C=O group of the adjacent turn of the right-handed screw (∝-helix).
β-pleated sheet structure:
This structure is called so because it looks like the pleated folds of drapery. In this structure, all the peptide chains are stretched out to nearly the maximum extension and then laid side by side. These peptide chains are held together by intermolecular hydrogen bonds.
14
What type of bonding helps in stabilising the ∝-helix structure of proteins?
Answer
The α-helix structure of proteins is stabilized by intramolecular H-bonding between C = 0 of one amino acid residue and the N — H of the fourth amino acid residue in the chain. This causes the polypeptide chain to coil up into a spiral structure called right handed a- helix structure.15
Answer
16
How do you explain the amphoteric behaviour of amino acids?
Answer
Amino acids contain an acidic (carboxyl group) and basic (amino group) group in the same molecule. In aqueous solution, they neutralize each other. The carboxyl group loses a proton while the amino group accepts it. As a result, a dipolar or zwitter ion is formed.
17
What are enzymes?
Answer
Enzymes are biological catalyst. Each biological reaction requires a different enzyme. Thus, as compared to conventional catalyst enzymes are very specific and efficient in their action. Each type of enzyme has its own specific optimum conditions of concentration, pH and temperature at which it works best.The enzymes used to catalyse the oxidation of one substrate with the simultaneous reduction of another substrate are named as oxidoreductase enzymes. The name of an enzyme ends with ‘− ase’. For example, the enzyme used to catalyse the hydrolysis of maltose into glucose is named as maltase.18
Answer
As a result of denaturation, globules get unfolded and helixes get uncoiled. Secondary and tertiary structures of protein are destroyed, but the primary structures remain unaltered. It can be said that during denaturation, secondary and tertiary-structured proteins get converted into primary-structured proteins. Also, as the secondary and tertiary structures of a protein are destroyed, the enzyme loses its activity.19
Answer
We can classify vitamins on the basis of solubility in water or fat into two categories.
(a) Water Soluble vitamins: Vitamins which are soluble in water comes in category. For example, B group vitamins (B1, B2, B12, etc) and vitamin C.
(b) Fat soluble vitamins: Those vitamins which are soluble only in fat, not in water come under this group. For example: Vitamins A, D E, and K. However, biotin or vitamin H is neither soluble in water nor in fat. The vitamin which is responsible for coagulation of blood is Vitamin K.
20
Answer
The deficiency of vitamin A leads to xerophthalmia (hardening of the cornea of the eye) and night blindness. The deficiency of vitamin C leads to scurvy (bleeding gums).
The sources of vitamin A are fish liver oil, carrots, butter, and milk. The sources of vitamin C are citrus fruits, amla, and green leafy vegetables.
21
Answer
Nucleic acids are biomolecules found in the nuclei of all living cells, as one of the constituents of chromosomes. There are mainly two types of nucleic acids − deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are also known as polynucleotides as they are long-chain polymers of nucleotides.
Two main functions of nucleic acids are:
(i) DNA is responsible for the transmission of inherent characters from one generation to the next. This process of transmission is called heredity.
(ii) Nucleic acids (both DNA and RNA) are responsible for protein synthesis in a cell. Even though the proteins are actually synthesised by the various RNA molecules in a cell, the message for the synthesis of a particular protein is present in DNA.
22
Answer
A nucleoside contains only two basic components of nucleic acids i.e., a pentose sugar and a nitrogenous base. It is formed when 1-position of pyrimidine (cytosine, thiamine or uracil) or 9-position of purine (guanine or adenine) base is attached to C —1 of sugar (ribose or deoxyribose) by a 13-linkage. Nucleic acids are also called polynucleotides since the repeating structural unit of nucleic acids is a nucleotide.
A nucleotide contains all the three basic components of nucleic acids, i.e., a phosphoric acid group, a pentose sugar and a nitrogenous base. These are obtained by esterification of C5, — OH group of the pentose sugar by phosphoric acid.
23
Answer
In the helical structure of DNA, the two strands are held together by hydrogen bonds between specific pairs of bases. Cytosine forms hydrogen bond with guanine, while adenine forms hydrogen bond with thymine. As a result, the two strands are complementary to each other.24
Answer
25
Answer
(i) Messenger RNA (m-RNA)
(ii) Ribosomal RNA (r-RNA)
(iii) Transfer RNA (t-RNA)