Biomolecules are the organic molecules that form the basis of life, including carbohydrates, proteins, lipids, nucleic acids, and vitamins. These molecules are constructed from simple building blocks and perform the essential functions of energy storage, catalysis, heredity, and structural support. The chapter provides a systematic study of carbohydrates, proteins, enzymes, vitamins, and nucleic acids, linking their chemical structures to their biological roles.
Carbohydrates, the most abundant biomolecules, are polyhydroxy aldehydes or ketones that serve as the primary energy source. They are classified as monosaccharides, oligosaccharides, and polysaccharides depending on the number of sugar units they contain. The chapter describes the structures of glucose and fructose, their ring forms, and the reactions that define their chemistry, including the formation of glycosidic bonds.
Proteins are polymers of amino acids linked by peptide bonds and are responsible for catalysing reactions as enzymes, transporting oxygen as haemoglobin, and providing structure as collagen and keratin. The chapter covers the classification and structure of amino acids, the four levels of protein structure, and the biological role of enzymes. Nucleic acids, DNA and RNA, carry the genetic information and direct protein synthesis. The chapter concludes with a survey of vitamins, which are essential micronutrients required in small amounts for normal physiological function.
Carbohydrates are optically active polyhydroxy aldehydes or ketones with the general formula Cx(H2O)y. They are classified on the basis of their behaviour towards hydrolysis:
Glucose (C6H12O6) is an aldohexose found in grapes and honey. It is a white crystalline solid with a sweet taste. Its open-chain structure has an aldehyde group and five hydroxyl groups, and it shows optical activity with the dextrorotatory form called D-glucose.
On the basis of reactions: - Glucose reacts with acetic anhydride to give a pentaacetate, proving the presence of five -OH groups. - It reacts with hydroxylamine to form an oxime, proving the presence of a carbonyl group. - Its reaction with HCN followed by hydrolysis gives a heptanoic acid derivative, confirming it is an open chain of six carbons. - Reaction with HI gives n-hexane, showing the straight chain structure.
Glucose exists predominantly in cyclic form. In aqueous solution, the open-chain form is in equilibrium with the pyranose (six-membered ring) and furanose (five-membered ring) forms. The anomeric carbon is the hemiacetal carbon, and the alpha and beta forms differ in the orientation of the hydroxyl group at this carbon. In solution, alpha-glucose and beta-glucose interconvert through the open-chain form, a process called mutarotation.
Fructose is a ketohexose, isomeric with glucose. It is a ketone and is found in honey and fruits. Fructose is the sweetest of all sugars. Its structure is a ketose with the ketone group on the second carbon.
Proteins are polymers of amino acids connected by peptide bonds. Amino acids contain both an amino group and a carboxyl group. The general structure is H2N-CH(R)-COOH, where R is the side chain. In aqueous solution at a particular pH called the isoelectric point, amino acids exist as zwitterions, carrying both positive and negative charges.
Amino acids link through the condensation of the amino group of one with the carboxyl group of another, forming a peptide bond: $$-CO-NH-$$
Oligopeptides contain a few amino acids, while proteins contain 50 or more. Proteins are classified as fibrous (long, insoluble, structural, e.g., keratin, collagen) and globular (spherical, soluble, functional, e.g., albumin, haemoglobin, insulin).
Denaturation is the loss of biological activity of a protein caused by a change in pH, temperature, or the action of certain chemicals. During denaturation, the secondary and tertiary structures are destroyed but the primary structure remains intact. Boiling an egg coagulates the albumin protein by denaturation.
Enzymes are proteins that catalyse biochemical reactions. They are highly specific, acting on a particular substrate, and their activity depends on temperature, pH, and substrate concentration. Enzymes work by lowering the activation energy of the reaction. The mechanism involves the formation of an enzyme-substrate complex at the active site, following the lock and key model.
Each enzyme has an optimum temperature and pH at which its activity is maximum. Human enzymes typically work best at body temperature (about 310 K) and near neutral pH. Certain factors such as the presence of inhibitors can reduce enzyme activity, and some enzymes require cofactors such as metal ions or coenzymes for activity.
Vitamins are essential organic micronutrients that the body cannot synthesise in sufficient amounts and must be obtained from the diet. They are classified as:
Important vitamins and their deficiency diseases include: - Vitamin A: Night blindness and xerophthalmia. - Vitamin B1 (thiamine): Beriberi. - Vitamin B2 (riboflavin): Skin and mouth disorders. - Vitamin B6: Anaemia and nervous disorders. - Vitamin B12: Pernicious anaemia. - Vitamin C (ascorbic acid): Scurvy, characterised by bleeding gums and poor wound healing. - Vitamin D: Rickets in children and osteomalacia in adults. - Vitamin E: Infertility and muscle degeneration. - Vitamin K: Delayed blood clotting.
Nucleic acids are macromolecules that carry genetic information. They are polymers of nucleotides. Each nucleotide consists of three components: 1. A nitrogenous base (purine: adenine, guanine; or pyrimidine: cytosine, thymine, uracil). 2. A pentose sugar (ribose in RNA, deoxyribose in DNA). 3. A phosphate group.
DNA (deoxyribonucleic acid) is a double helix in which two polynucleotide chains run antiparallel and are held together by hydrogen bonds between complementary bases: adenine pairs with thymine (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen bonds). The structure is often described as a double helix, and the ratio of bases obeys Chargaff's rule: A = T and G = C.
RNA (ribonucleic acid) is a single-stranded molecule that contains uracil instead of thymine and ribose instead of deoxyribose. It exists in three forms: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). RNA transmits genetic information from DNA to the ribosomes and directs protein synthesis.
| Type | Number of Units | Examples | Reducing or Non-reducing |
|---|---|---|---|
| Monosaccharide | 1 | Glucose, fructose | Reducing |
| Disaccharide | 2 | Sucrose, maltose, lactose | Sucrose non-reducing |
| Polysaccharide | Many | Starch, cellulose, glycogen | Non-reducing |
| Vitamin | Solubility | Deficiency Disease |
|---|---|---|
| A | Fat-soluble | Night blindness |
| B1 | Water-soluble | Beriberi |
| B12 | Water-soluble | Pernicious anaemia |
| C | Water-soluble | Scurvy |
| D | Fat-soluble | Rickets |
| K | Fat-soluble | Delayed clotting |
Biomolecules unite organic chemistry with biology, showing how a limited set of building blocks gives rise to the extraordinary complexity of living systems. Carbohydrates provide energy and structural support, proteins perform catalysis and transport, nucleic acids store and transmit genetic information, and vitamins regulate essential metabolic processes. The structures of glucose, the peptide bond, and the DNA double helix are among the most important concepts in all of chemistry. Understanding the four levels of protein structure and the mechanisms of enzyme action provides insight into how life processes are regulated. This chapter prepares students for advanced study in biochemistry and molecular biology and highlights the relevance of chemistry to medicine, nutrition, and biotechnology.