Carbohydrates

Study Notes

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 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 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 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.

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.

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