1. Biomolecules: Macromolecules which are naturally occurring in biological systems are called biomolecules. Examples: polysaccharides (starch, cellulose, etc.), proteins, enzymes, vitamins, hormones, etc.
2. Carbohydrates: These are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis, e.g., glucose, sucrose, cellulose, starch, etc.
3. Classification of carbohydrates:
(a) Monosaccharides: The simple carbohydrates that cannot be broken further into smaller units on hydrolysis, e.g., glucose and fructose, ribose, etc.
(b) Oligosaccharides: These are the carbohydrates which on hydrolysis give two to ten units of monosaccharides, e.g., sucrose, maltose, raffinose, stachyose, etc.
(c) Polysaccharides: These are the carbohydrates which produce a large number of monosaccharide units on hydrolysis, e.g., starch, cellulose, etc.
Importance of carbohydrates:
(i) Carbohydrates act as biofuel to provide energy for functioning of living systems.
C6H12O6 + 6O2 → 6CO2 + H2O + 2832kJ
(ii) Carbohydrates are used as storage molecules as starch in plants and glycogen in animals.
(iii) D-Ribose and 2-Deoxy-D-ribose are present in RNA and DNA, respectively.
(iv) Cellulose acts as structural material of cell walls of bacteria and plants.
(v) Carbohydrates provide raw material for many important industries like textiles, paper, lacquers and breweries.
4. (i) Reducing sugars: Those carbohydrates which contain free aldehydic or ketonic group and reduce Fehling’s solution and Tollens’ reagent are called reducing sugars, e.g., all monosaccharides, maltose and lactose.
(ii) Non-reducing sugars: Those sugars which do not have free aldehydic or ketonic group and do not reduce Fehling’s solution or Tollens’ reagent are called non-reducing sugars, e.g., sucrose.
5. Preparation of Glucose
(a) From sucrose:
(b) From starch: Commercially, glucose is obtained by hydrolysis of starch by boiling it with dil. H2SO4 at 393 K under pressure.
6. (a) Structure of Glucose: Glucose is a six carbon straight chain aldose which has one aldehydic group (—CHO), one primary hydroxyl group (—CH2OH) and four secondary hydroxyl groups (—CHOH).
If the —OH group attached to C-5 is on the right side, the glucose is assigned D–configuration; if the —OH group attached to C-5 is on the left side, it is assigned L–configuration. The (+) and (–) signs represent the optical rotation as dextro and laevo, respectively and have no relationship with D and L configuration.
(b) Cyclic structure of glucose: The glucose has been shown to possess cyclic structure represented as follows:
Haworth Structures
7. Reactions of Glucose
8. Structure of Fructose: Fructose is a ketohexose and has the molecular formula C6H12O6. It belongs to D-series and is a laevorotatory compound.
The cyclic structures of two anomers of fructose are represented by Haworth structures as given below.
9. Disaccharides: The sugar which on hydrolysis gives two units of monosaccharides is called disaccharide. Disaccharides are crystalline solids and are soluble in water. Sucrose, maltose and lactose are disaccharides.
Hydrolysis of sucrose is called inversion of cane sugar. Sucrose is a disaccharide because on hydrolysis, it produces two monosaccharides namely D-(+)-glucose and D-(–)-fructose.
(a) Ring structure of a sucrose molecule: A sucrose molecule is composed of a-glucose and b-fructose units.
(b) Ring structure of a maltose molecule: A maltose molecule is composed of two a-D-glucose units in which C-1 of one glucose (I) is linked to C-4 of another glucose unit (II).
(c) Ring structure of a lactose molecule: A lactose molecule is composed of b-D-galactose and b-D-glucose units.
10. Polysaccharides: Polysaccharides are the carbohydrates which yield a large number of monosaccharide molecules upon hydrolysis. Starch, cellulose and glycogen are examples of polysaccharides.
(a) Starch: The fundamental unit of starch is a-D-glucose.
Structure of starch: Starch is a polymer of a-glucose and consists of two components—amylose and amylopectin. Amylose is a long unbranched chain with 200–1000 a-D-(+)-glucose units held by C1–C4 glycosidic linkage.
Amylopectin is a branched chain polymer of a-D-glucose units in which the chain is formed by C1–C4 glycosidic linkage, whereas branching occurs by C1–C6 glycosidic linkage.
(b) Cellulose: Cellulose is a polysaccharide. The fundamental structural unit of cellulose is b-D-glucose.
Structure of cellulose: Cellulose is a linear polymer of b-D-glucose which are joined by glycosidic linkage between C1 of one glucose unit and C4 of the next glucose unit.
11. Amino acids: Those compounds, whose molecule contains both the carboxylic acid group and the amino group are called amino acids. There are twenty amino acids which form protein. The amino acids which are synthesised in body are known as non-essential amino acids, e.g., glycine, alanine. Those amino acids which cannot be synthesised in body and must be obtained through diet are known as essential amino acids, e.g., valine, lysine.
Amino acids have also been classified as neutral, acidic and basic amino acids. Amino acids like glycine, valine, etc. which contain one —NH2 and one —COOH group are called neutral amino acids. Those amino acids such as aspartic acid, glutamic acid, etc. which contain one —NH2 group and two —COOH groups are called acidic amino acids and amino acids such as lysine, histidine, etc., which contain two —NH2 groups and one —COOH group are called basic amino acids.
12. Proteins are complex nitrogenous organic molecules which are essential for growth and maintenance of body. Chemically, proteins are the polymers of a-amino acids which are linked by peptide bonds
(a) Types of proteins based on molecular shape:
(i) Fibrous proteins: They have thread-like molecules which tend to lie side by side to form fibres, e.g., keratin, collagen, myosin, fibroin, etc. In such proteins, the molecules are held together by hydrogen and disulphide bonds. They are insoluble in water. They are the chief structural materials of animal tissues.
(ii) Globular proteins: They have molecules which are folded into compact units that often form spheroidal shapes. The area of contact between molecules are small and inter-molecular forces are comparatively weak, e.g., insulin, thyroglobulin, albumin, haemoglobin and fibrinogen. In clotting of blood, fibrinogen gets converted into fibrous protein, fibrin.
(b) Structure of Proteins: There are four levels at which the structure of proteins are studied. These are primary, secondary, tertiary and quarternary levels.
(i) Primary structure of proteins: The sequence in which various amino acids are arranged in a protein is called its primary structure. Any change in the sequence of amino acids creates different protein which alters biological functions.
(ii) Secondary structure of proteins: It refers to shape in which a long polypeptide chain exists. A protein may assume a-helix structure or b-pleated sheet structure. The a-helix structure results due to regular coiling of polypeptide chain which is stabilised by intramolecular hydrogen bonding. Keratin in hair, nails, wool and myosin in nucleus have a-helix structure. In b-pleated sheet structure, all peptide chains are stretched to nearly maximum extension and then arranged side by side and held together by intermolecular hydrogen bonding. Silk has b-pleated sheet structure.
(iii) Tertiary structure of proteins: The tertiary structure of proteins represents 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 2° and 3° structures of proteins are hydrogen bonds, disulphide linkages, van der Waals forces and electrostatic force of attraction.
(iv) Quaternary structure: Some of the proteins are composed of two or more polypeptide chains referred to as sub-units. The spatial arrangement of these subunits with respect to each other is known as quaternary structure.
(c) Denaturation of Proteins: When a protein in its native form is subjected to a change, such as change in temperature or change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of protein. During denaturation, 2° and 3° structures are destroyed but 1° structure remains intact, e.g., coagulation of egg while on boiling, curdling of milk, etc.
13. (a) Enzymes: Enzymes are essential biological catalysts which are required to catalyse biological reactions, e.g., maltose, lactose, invertase, etc. Almost all the enzymes are globular proteins.
(b) Oxidoreductase enzymes: Enzymes which catalyse the oxidation of one substrate with simultaneous reduction of another substrate.
(c) Phenylketonuria: Disease caused by deficiency of the enzyme phenylalanine hydroxylase.
(d) Albinism: Disease caused due to deficiency of an enzyme tyrosinase.
(e) Streptokinase: Enzyme which dissolves the blood clot formed in coronary artery which leads to heart trouble.
14. Nucleic Acids: Nucleic acids are long chain polymers of nucleotides. They play an important role in transmission of hereditary characteristics and biosynthesis of proteins.
Types of nucleic acids: There are two types of nucleic acids. These are DNA and RNA.
(a) Constituents of nucleic acids:
(i) Pentose sugar
(ii) Phosphoric acid
(iii) Nitrogenous bases.
In DNA, b-D-2-deoxyribose sugar is present while in RNA b-D-ribose sugar is present.
Nitrogen containing bases: There are two types of nitrogen containing bases found in nucleic acids. These are pyrimidines and purines.
Pyrimidines: There are three bases derived from pyrimidines. These are cytosine (C), thymine (T) and uracil (U). In DNA, T is present but in RNA, U is present.
Purines: There are two bases derived from purine. These are adenine (A) and guanine (G).
Nucleoside: A unit formed by the attachment of a base to 1′-position of sugar is known as nucleoside.
Nucleotide: When nucleoside is linked to phosphoric acid at 5′-position of sugar moiety, the unit obtained is called nucleotide.
Nucleotides are joined together by phosphodiester linkage between 5′ and 3′ carbon atoms of the pentose sugar.
(b) Deoxyribonucleic acid (DNA): It contains a pentose sugar deoxyribose, and adenine, guanine, thymine and cytosine bases. A phosphate group is present at C-5 of the sugar unit. The repeating units, deoxyribonucleotides, are linked by phosphate group. Thus, they are the biopolymers of deoxyribonucleotides and have double helix structure of polynucleotides. The two strands of DNA are said to be complementary to each other. Adenine forms hydrogen bonds with thymine whereas cytosine forms hydrogen bonds with guanine. They are responsible for genetic characteristics and for sending information and instruction in the cell for the synthesis of specific protein.
(c) Ribonucleic acid (RNA): It contains ribose sugar, bases from pyrimidine bases—uracil and cytosine, and two bases from purine base—adenine and guanine. A phosphate group is present at C-5 of the sugar unit. The repeating units, ribonucleotides, are linked by phosphate group. They are the polymers of ribonucleotide and have a single helix structure. RNA is associated with the process of learning and memory storage, and helps in biosynthesis of protein.
15. Functions of Nucleic Acids: Two main functions of nucleic acids are:
(a) Replication or heredity transfer: The double helix of DNA is the storehouse of the genetic information of the organism which is contained in the sequence of bases A, T, C, G on the strands of DNA. The process by which a DNA molecule produces two identical molecules of itself in the nucleus of the cell is called replication.
(b) Protein synthesis: This is brought about in two steps:
(i) Transcription: Copying of sequence of bases from the DNA strand onto the RNA molecule is called transcription. During transcription, the double helix of the DNA partially unwinds and one of the two DNA strands serves as a template for the synthesis of RNA strand called messenger RNA (mRNA) which is complementary to a segment of the DNA chain.
(ii) Translation: This is the process in which mRNA directs protein synthesis in the cytoplasm of cell with involvement of transfer RNA (tRNA) and ribosomal particles (rRNA protein complexes).
16. (a) Codon: The sequence of nucleotides in mRNA molecules are read in a serial order in sets of three (triplet) at a time. Each triplet is called a codon. It specifies one amino acid. The mRNA codon recognises the amino acids through tRNAs which carry specific amino acids.
(b) Gene: The sequence of bases or nucleotides in the DNA molecule which regulates the synthesis of a specific protein is called a gene. Every protein in the cell has a corresponding gene. The relationship between a nucleotide triplet (codon) and the amino acid is called genetic code.
(c) Mutation: The chemical change in the sequence of bases in the DNA molecule can lead to synthesis of protein with an altered amino acid sequence is called mutation. This is brought about spontaneously by exposure to UV-rays, X-rays and chemicals.
17. Vitamins: Vitamins are generally regarded as 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.
Vitamins are classified into two groups depending upon their solubility in fat or water:
(i) Fat-soluble vitamins: Vitamins A, D, E and K are soluble in fat and oils but insoluble in water. They are stored in liver and adipose tissues.
(ii) Water-soluble vitamins: Vitamins belonging to group B and vitamin C are soluble in water. They must be supplied regularly in diet because they are readily excreted in urine and cannot be stored (except vitamin B12) in our body.
Table 14.1: Some Important Vitamins, their Sources and their Deficiency Diseases
S. No. | Name of Vitamins | Sources | Deficiency Diseases |
1. | Vitamin A | Fish liver oil, carrots, butter and milk | Xerophthalmia (hardening of cornea of eye), night blindness |
2. | Vitamin B1 (Thiamine) | Yeast, milk, green vegetables and cereals | Beri-beri (loss of appetite, retarded growth) |
3. | Vitamin B2 (Riboflavin) | Milk, egg white, liver, kidney | Cheilosis (fissuring at corners of mouth and lips), digestive disorders and burning sensation of the skin. |
4. | Vitamin B6 (Pyridoxine) | Yeast, milk, egg yolk, cereals and grams | Convulsions |
5. | Vitamin B12 | Meat, fish, egg and curd | Pernicious anaemia (RBC deficient in haemoglobin) |
6. | Vitamin C (Ascorbic acid) | Citrus fruits, amla and green leafy vegetables. | Scurvy (bleeding gums) |
7. | Vitamin D | Exposure to sunlight, fish and egg yolk | Rickets (bone deformities in children) and osteomalacia (soft bones and joint pain in adults) |
8. | Vitamin E | Vegetable oils like wheat germ oil, sunflower oil, etc. | Increased fragility of RBCs and muscular weakness |
9. | Vitamin K | Green leafy vegetables | Increased blood clotting time |
18. Hormones: Hormones are molecules that act as intercellular messengers. These are produced by endocrine glands in the body and are released directly in the blood stream. From here these are transported to the site of their action.
Functions of hormones:
(i) They help to maintain the balance of biological activities in the body. For example, insulin keeps the blood glucose level within the range, epinephrine and norepinephrine mediate response to external stimuli, growth hormones and sex hormones play role in growth and development.
(ii) The hormones released by gonads are responsible for development of secondary sexual characters.
(iii) Adrenal cortex release glucocorticoids and mineralocorticoids. The glucocorticoids control the carbohydrate metabolism, modulate inflammatory reactions and are involved in reactions to stress. The mineralocorticoids control the level of excretion of water and salt by the kidney.
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