Lec 5 & 6: General Chemistry - Carbohydrates

Carbohydrates

• Carbohydrates are a group of compounds containing hydroxyl groups or their derivatives, poorly soluble in organic solvents but soluble in water.
• They have several important functions for the organism, including serving as energy stores and metabolic intermediates.

Functions of Carbohydrates

• Ribose and deoxyribose sugars form part of the structural framework of RNA and DNA.
• Carbohydrates are important for tissue formation.
• They form the basis of human blood groups.
• Polysaccharides are structural elements in the cell walls of bacteria and plants.
• Carbohydrates link to many proteins and lipids, mediating interactions among cells and between cells and other elements.

Classification of Carbohydrates

• Monosaccharides: contain 3-9 carbon atoms, mostly 5 or 6, with an aldehyde or ketone group.
• Disaccharides: condensation products of two monosaccharide units, e.g., maltose and sucrose.
• Oligosaccharides: a few monosaccharides (3-10) covalently linked by glycosidic bonds.
• Polysaccharides: polymers consisting of chains of monosaccharide or disaccharide units, e.g., starches and dextrins.

Monosaccharides

• General structure is straight and not ramified.
• Every carbon atom carries a hydroxyl group except one.
• Contain an aldehyde or ketone group.

Nomenclature of Sugars

• Names end in –ose.
• Aldoses have an aldehyde group, while ketoses have a ketone group.
• Number of carbon atoms present in the molecule determines the prefix (e.g., triose, tetrose, pentose, hexose, heptose).

D and L Isomerism

• Isomers are molecules with the same kinds and numbers of atoms joined up in different ways.
• Prefixes D and L designate the absolute configuration of the asymmetric carbon farthest from the aldehyde or ketone group.
• D-isomer has the OH group on the right, while L-isomer has it on the left.

Cyclization of Monosaccharide

• The predominant forms of ribose, glucose, fructose, and many other sugars in solution are not open chains.