Biomolecules - Carbohydrates (Classification and Structure)

Carbohydrates are defined as optically active polyhydroxy aldehydes or ketones, or compounds which produce such units on hydrolysis. Their general formula is often represented as $C_x(H_2O)_y$.

Classification based on hydrolysis: Monosaccharides (cannot be hydrolyzed further, e.g., $C_6H_{12}O_6$), Oligosaccharides (yield 2-10 units, e.g., Sucrose), and Polysaccharides (yield a large number of units, e.g., Starch, Cellulose).

Reducing sugars contain free aldehydic or ketonic groups and can reduce Fehling's solution and Tollen's reagent. Examples include Glucose, Fructose, and Maltose. Non-reducing sugars like Sucrose lack these free groups.

Glucose ($C_6H_{12}O_6$) is an aldohexose. Its structure was elucidated by reactions: $HI$ reduction to $n$-hexane (proves straight chain), $NH_2OH$ reaction (proves carbonyl group), and $Br_2$ water oxidation to Gluconic acid (proves aldehyde group).

Cyclic structure of Glucose: Glucose exists in two crystalline forms, $\alpha$ and $\beta$, which are anomers. These are cyclic hemiacetals formed by the reaction of the $-OH$ at $C_5$ with the $-CHO$ at $C_1$.

Glycosidic Linkage: The oxide linkage that joins two monosaccharide units through an oxygen atom, involving the loss of a water molecule ($H_2O$).

Sucrose consists of $\alpha$-D-glucose and $\beta$-D-fructose linked by $C_1-C_2$ glycosidic bond. It is a non-reducing sugar and undergoes 'inversion' during hydrolysis.

Starch is a polymer of $\alpha$-D-glucose consisting of two components: Amylose (water-soluble, linear, $C_1-C_4$ linkage) and Amylopectin (water-insoluble, branched, $C_1-C_4$ and $C_1-C_6$ linkages).