Carbohydrates

Carbohydrates

• Maltose yields two molecules of glucose on hydrolysis.
• Lactose (milk sugar) is found in milk, occurs as an equilibrium mixture of α and β forms at body temperature, and is not very soluble or sweet.
• Lactose is dextrorotatory and yields one molecule of D-Glucose and one molecule of D-Galactose on hydrolysis by the enzyme lactase.
• Lactose has reducing properties and can form α and β forms, exhibiting mutarotation.

Sucrose

• Sucrose is ordinary table sugar, also known as 'Cane sugar', obtained from sugarcane, sugar beet, and sugar maple.
• Sucrose is very soluble and sweet, and yields one molecule of D-Glucose and one molecule of D-Fructose on hydrolysis by the enzyme sucrase.
• Sucrose does not have reducing properties due to its linked aldehyde and ketone groups.
• Sucrose is dextrorotatory, but its hydrolytic products (glucose and fructose) are levorotatory, resulting in 'Invert Sugar' and the process of 'Inversion'.

Polysaccharides

• Polysaccharides are complex substances, polymerized products of many monosaccharide units.
• Examples include amylopectin, a polymer of D-Glucose units, with glucose units joined by α1 → 4 glucosidic linkages and branching at branch points by α1 → 6 glucosidic linkage.
• Inulin is a polymer of D-fructose, has a low molecular weight, and is found in the bulbs of onion and garlic.
• Inulin is levorotatory, gives no color with iodine, and is hydrolysed by acids or the enzyme inulinase.
• Cellulose is a polymer of glucose, found in the cell walls of plants, and is not of nutritional value in humans.
• Dextrins are products of lower molecular weight formed by the partial hydrolysis of starch by acids or enzymes.
• Dextran is a polymer of D-Glucose, made up of units of D-Glucose molecules, and is used as a plasma expander.
• Agar is a homopolysaccharide made up of repeated units of galactose, which is sulphated, and is present in seaweed.

Optical Activity

• Optical activity is the ability of a substance to rotate the plane of polarization of a beam of light passed through it.
• The presence of asymmetric carbon atoms confers optical activity on a compound.
• Dextrorotatory compounds rotate the plane of polarization to the right, while laevorotatory compounds rotate it to the left.
• A racemic mixture is a mixture of equal amounts of dextrorotatory and laevorotatory isomers, which has no optical activity.
• Resolution is the separation of optically active isomers from a racemic mixture.

Cyclic Structures

• The open-chain form of D-Glucose can be condensed to form two different forms of glucose, α-D-Glucose and β-D-Glucose, through cyclization.
• Carbon-1 becomes asymmetric after cyclization, with four different groups attached to it.