Key Points

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that produce these units on hydrolysis. Many have a general formula of $C_x(H_2O)_y$.

Carbohydrates are classified based on their behavior on hydrolysis into monosaccharides (e.g., glucose), oligosaccharides (2-10 units, e.g., sucrose), and polysaccharides (many units, e.g., starch).

Reducing sugars reduce Fehling's solution and Tollens' reagent due to a free aldehyde or ketonic group (e.g., maltose, lactose). Non-reducing sugars do not have this free group (e.g., sucrose).

A glycosidic linkage is an oxide bond formed between two monosaccharide units with the loss of a water molecule. This linkage connects monosaccharides to form disaccharides and polysaccharides.

Starch is a polymer of $\alpha$-glucose and consists of two components: Amylose, a linear chain with C1-C4 linkage, and Amylopectin, a branched chain with C1-C4 and C1-C6 linkages.

Cellulose is a straight-chain polysaccharide composed only of $\beta$-D-glucose units. The units are joined by glycosidic linkages between C1 of one glucose and C4 of the next.

Proteins are polymers of $\alpha$-amino acids, which contain both an amino ($-NH_2$) and a carboxyl ($-COOH$) group. In aqueous solution, they exist as dipolar ions called zwitterions.

A peptide bond is an amide linkage ($-CO-NH-$) that connects two amino acids. It is formed when the carboxyl group of one amino acid reacts with the amino group of another, eliminating a water molecule.

Proteins have four levels of structure: primary (sequence of amino acids), secondary ($\alpha$-helix and $\beta$-pleated sheet), tertiary (overall 3D folding), and quaternary (arrangement of multiple polypeptide chains).

Denaturation is the loss of a protein's biological activity due to the disruption of its secondary and tertiary structures by physical or chemical changes, like heat or pH. The primary structure remains intact.

Enzymes are globular proteins that act as biological catalysts. They are highly specific for a particular reaction and substrate, and they work by lowering the activation energy.

Vitamins are classified based on their solubility. Fat-soluble vitamins are A, D, E, and K, stored in fat tissues. Water-soluble vitamins are B group and C, which must be supplied regularly in the diet.

Nucleic acids (DNA and RNA) are polymers of nucleotides. Each nucleotide is composed of a pentose sugar, a nitrogen-containing heterocyclic base, and a phosphate group.

DNA contains $\beta$-D-2-deoxyribose sugar and the bases A, G, C, T, and is double-stranded. RNA contains $\beta$-D-ribose sugar and the bases A, G, C, U, and is single-stranded.

A nucleoside is formed by the attachment of a base to the 1' position of a sugar. A nucleotide is formed when a nucleoside is linked to phosphoric acid at the 5' position of the sugar moiety.

DNA has a double helix structure where two strands are held together by hydrogen bonds between specific base pairs. Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G).