Biomolecules — Class 11 Biology

This chapter analyses the chemical composition of living tissues and the major classes of biomolecules. A chemical analysis shows that all the elements found in earth's crust are also present in living matter, but the relative abundance of carbon and hydrogen is higher in organisms, and water is the most abundant compound in a cell. Grinding a tissue in acid and filtering gives an acid-soluble pool of small molecules (18 to 800 daltons) such as amino acids, sugars, fatty acids, glycerol, nucleosides and nucleotides, and an acid-insoluble fraction of macromolecules. There are twenty types of amino acids and five nitrogen bases. Lipids include fatty acids, glycerides, phospholipids and cholesterol. Primary metabolites have known roles, while secondary metabolites such as alkaloids, pigments and antibiotics are typical of plants and microbes. Only proteins, nucleic acids and polysaccharides are true biomacromolecules; lipids separate with them only because of membranes. Proteins are heteropolymers of amino acids with primary, secondary, tertiary and quaternary structure; collagen and RuBisCO are the most abundant proteins. Polysaccharides like cellulose, starch and glycogen are sugar polymers, and nucleic acids (DNA and RNA) are polynucleotides. Enzymes are proteins (some are ribozymes) that lower activation energy; their activity depends on temperature, pH, substrate concentration and cofactors.