• Carbohydrates are made up of carbon, hydrogen and oxygen atoms, and are widely distributed in animal and plant tissues.
• Carbohydrates are categorised into 4 major classes, namely monosaccharides (e.g., glucose, ribose), disaccharides (e.g., sucrose, lactose), oligosaccharides (e.g., raffinose) and polysaccharides (e.g., starch, glycogen).
• Monosaccharides can be classified as trioses, tetroses, pentoses, hexoses, etc., depending on the number of carbon atoms in the molecule.
• Pentose sugar such as ribose is an important component of nucleic acids, coenzymes.
• Two monosaccharides linked by glycosidic bond make a disaccharide (e.g., sucrose ).
• Polysaccharides perform various roles as the storage form of energy and also as structural component.
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• Lipids are organic compounds found in living organisms. These are made up of hydrophobic fatty acid chains linked to glycerol via ester bond.
• Fatty acids are long chain hydrocarbon containing carboxylic acid group. Fatty acids may be saturated (having no double bonds) or unsaturated (having one or more double bonds). Nomenclature of fatty acids is based on total number of carbons, total number of double bonds and position of double bonds.
• Lipids are broadly classified into two classes–simple and compound lipids.
• Simple lipids include triacylglycerol and waxes, which are esters of glycerol with fatty acids and esters of high molecular weight alcohol with fatty acids, respectively.
• Compound lipids include membrane lipids which are of two types, glycerophospholipids and sphingolipids. Compound lipids are amphipathic molecules made up of hydrophilic phosphate tails.
• Glycerophospholipid contains glycerol backbone and sphingolipids contain sphingoid base backbone.
• Steroids are another class of compound lipids made up of four-fused ring structure. Cholesterol is the most common steroid found in animals. It is the precursor of all steroid hormones and vitamin D.
• The amino acids are organic compounds containing amine (-NH2) and carboxyl (-COOH) functional groups, along with a side chain (R group) specific to each amino acid.
• There are 20 standard amino acids and some nonstandard amino acids (e.g. 4-hydroxy proline, 5-hydroxy lysine, etc.) and some non-protein amino acids (e.g., L-ornithine, L-citrulline, etc.)
• In a polypeptide chain, the amino acids are linked covalently via peptide bond in a linear fashion to make a protein.
• There are four levels of protein structures, namely primary, secondary, tertiary and quaternary.
• Primary structure of proteins is the linear chain of amino acid sequences linked through peptide bonds.
• Secondary structure of protein is the three dimensional form of polypeptide.
• The two major types of secondary structures are α-helix and β-sheets.
• Tertiary structure is the three dimensional arrangement of protein. • Quaternary structure is the complex arrangement of folded protein subunits, stabilised through hydrogen bonds, electrostatic interactions, etc.
• Nucleic acids are the polymer of nucleotide which contain nitrogenous bases (adenine, guanine, cytosine, thymine and uracil), sugar and phosphate.
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two types of nucleic acids.
• DNA is genetic material of almost all the organisms except some viruses where RNA is the genetic material.
• DNA contains deoxyribose sugar and thymine while RNA contains ribose sugar and uracil in place of thymine.
• In DNA, adenine forms 2 hydrogen bonds with thymine (A=T) and cytosine forms 3 hydrogen bonds with guanine (C≡G).
• J.Watson and F.Crick (1953) described the three dimensional double helical structure of DNA.
• The mRNA, tRNA, rRNA are the major types of RNA.
