Chapter 9: Biomolecules | Summary

Compounds in the acid-soluble pool have molecular weights ranging from 18 to about 800 daltons and are called micromolecules or simply biomolecules. The acid-insoluble fraction contains four types of organic compounds: proteins, nucleic acids, polysaccharides, and lipids. With the exception of lipids, these have molecular weights of ten thousand daltons or more and are called macromolecules or biomacromolecules.

Lipids, although of small molecular weight (not exceeding 800 Da), appear in the acid-insoluble fraction because they form cell membranes and other membrane structures. When tissue is ground, these membranes break into vesicles that are not water-soluble and thus co-separate with the macromolecular fraction. The acid-soluble pool represents the cytoplasmic composition, while the macromolecular fraction comes from cytoplasm and organelles. Together, they represent the total chemical composition of living tissues. Water is the most abundant chemical in living organisms, comprising 70–90% of cellular mass.

Proteins are polypeptides — linear chains of amino acids linked by peptide bonds. Since there are 20 types of amino acids, each protein is a heteropolymer (not a homopolymer). Amino acids can be essential (must be obtained through diet) or non-essential (synthesized by the body). Proteins perform a wide variety of functions: collagen provides structural support, trypsin acts as an enzyme, insulin functions as a hormone, antibodies fight infections, receptors mediate sensory signals, and GLUT-4 enables glucose transport into cells.

Collagen is the most abundant protein in the animal world. Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO) is the most abundant protein in the entire biosphere. The diversity of protein function arises from the diversity of their amino acid sequences and the complex three-dimensional structures they adopt.

Polysaccharides are long-chain polymers of monosaccharides and form another major class of biomacromolecules. Cellulose is a homopolymer of glucose and forms the cell walls of plants; it is also the basis of paper and cotton fibre. Starch is another glucose polymer that serves as an energy store in plant tissues. Glycogen is the animal storage polysaccharide. Inulin is a polymer of fructose. Starch forms helical secondary structures that can hold iodine (I2) molecules, giving a blue colour, whereas cellulose lacks such complex helices.

More complex polysaccharides exist in nature, built from amino sugars and chemically modified sugars such as glucosamine and N-acetyl galactosamine. For example, chitin is a complex polysaccharide that forms the exoskeleton of arthropods. These complex polysaccharides are mostly homopolymers. Polysaccharides thus serve dual roles — as structural components (cellulose, chitin) and as energy reserves (starch, glycogen).

Nucleic acids are polynucleotides and represent the third major class of true macromolecules. The building block of a nucleic acid is a nucleotide, which consists of three chemically distinct components: a nitrogenous base (heterocyclic compound), a monosaccharide sugar, and a phosphoric acid group. The nitrogenous bases in nucleic acids are adenine, guanine, uracil, cytosine, and thymine. Adenine and guanine are purines, while cytosine, uracil, and thymine are pyrimidines.

When a nitrogenous base is attached to a sugar, it forms a nucleoside (e.g., adenosine, uridine). When a phosphate group is also esterified to the sugar, it forms a nucleotide (e.g., adenylic acid, thymidylic acid). The sugar in polynucleotides is either ribose (in RNA) or 2-deoxyribose (in DNA). DNA and RNA function as the genetic material of organisms, carrying hereditary information from parent to progeny.

Protein structure is described at four hierarchical levels. The primary structure refers to the linear sequence of amino acids in the polypeptide chain, with the first (N-terminal) and last (C-terminal) amino acids defining the two ends. The secondary structure arises when the polypeptide thread folds into regular patterns such as the alpha-helix (right-handed, like a revolving staircase) or the beta-pleated sheet, stabilized by hydrogen bonds.

The tertiary structure is the overall three-dimensional folding of the entire polypeptide chain upon itself, resembling a hollow woolen ball. It is stabilized by hydrogen bonds, disulfide bonds, and other interactions. Tertiary structure is absolutely essential for the biological activity of proteins. The quaternary structure describes the arrangement of two or more polypeptide subunits relative to each other. Adult human haemoglobin is a classic example, comprising two alpha and two beta subunits.

Enzymes are mostly proteins (though some nucleic acids called ribozymes also have catalytic activity) that catalyse biochemical reactions by lowering activation energy. The active site is a crevice or pocket in the enzyme's tertiary structure into which the substrate fits. During catalysis, the substrate binds to the active site forming an enzyme-substrate (ES) complex, which transitions through a high-energy transition state before releasing the product and regenerating the free enzyme. Enzymes are highly specific, each catalysing a unique reaction. They function optimally at specific temperature and pH; high temperatures denature enzymes. Enzyme activity can be inhibited by competitive inhibitors that structurally resemble the substrate.

Enzymes are classified into six classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. Many enzymes require non-protein cofactors for activity. These include prosthetic groups (tightly bound organic compounds, e.g., haem in catalase), co-enzymes (loosely associated organic compounds, often vitamins, e.g., NAD), and metal ions (e.g., zinc in carboxypeptidase). The protein part of an enzyme requiring a cofactor is called the apoenzyme. Removal of the cofactor destroys catalytic activity.

Chapter 9 establishes that despite the diversity of life, all living organisms share a remarkably similar chemical composition. The key insight is the distinction between micromolecules (acid-soluble, MW less than 1000 Da) and macromolecules (acid-insoluble, MW above 10,000 Da). The macromolecular fraction — proteins, nucleic acids, and polysaccharides — is responsible for the structural integrity and functional complexity of cells. Proteins, with their four levels of structure, perform the most diverse range of functions. Nucleic acids serve as genetic material. Enzymes, as biological catalysts, are central to all metabolic activities, lowering activation energy and enabling life-sustaining reactions to occur at biological temperatures.

For CBSE board examinations, this chapter is highly important. Students should master the distinction between primary and secondary metabolites, the structure and classification of biomolecules (amino acids, lipids, carbohydrates, nucleotides), the four levels of protein structure with examples, the mechanism and factors affecting enzyme action, and the six classes of enzymes with examples. Questions on RuBisCO, collagen, haemoglobin subunits, competitive inhibition, cofactors, and the carbonic anhydrase example frequently appear in board exams. A solid understanding of this chapter also forms the foundation for later chapters on cell biology, respiration, and photosynthesis.