Carbohydrates: Definition and Classification

Study Notes

Carbohydrates are aldehyde or ketone derivatives of polyhydric alcohols, or compounds that yield these derivatives upon hydrolysis.
Monosaccharides are simple sugars containing one sugar unit and cannot be further hydrolyzed. Examples include glucose and fructose.
Oligosaccharides contain 2-10 sugar units and yield monosaccharides upon hydrolysis. Examples include lactose and maltose.
Polysaccharides contain more than 10 sugar units and yield monosaccharides upon acid hydrolysis. Examples include starch and glycogen.

Classified by the number of carbon atoms: Bioses (2C), Trioses (3C), Tetroses (4C), Pentoses (5C), and Hexoses (6C)
Further subdivided by functional group:
- Aldoses: Begin with a formyl group (H(C=O))
- Ketoses: Contain a keto group (C=O) between two carbons

Ribose: Structural component in nucleic acids (DNA, RNA) and free nucleotides (ATP).
Glucose (grape sugar, blood sugar, dextrose): Produced by hydrolysis of starch, glycogen, sucrose, maltose, and lactose. It is fermentable, reducing, and the main sugar utilized by the body.
- Excess glucose in urine (glucosuria) is a sign of diabetes mellitus.
Mannose: Subunit in glycoproteins and glycolipids, obtained by hydrolysis of plant gums.
Galactose: Subunit of milk sugar (lactose), component of glycolipids, glycoproteins, and mucopolysaccharides.
Fructose (fruit sugar): Main sugar in honey and fruits, the sweetest sugar known.
- Converted to glucose in the body, but failure of its metabolism leads to accumulation in cells and blood (hereditary fructose intolerance).
- It is a reducing and fermentable sugar, also known as the "semen sugar."

Optical activity is the ability of sugars to rotate plane-polarized light.
Dextrorotatory (d or +): Rotate light to the right, such as glucose, galactose, and starch.
Levorotatory (l or -): Rotate light to the left, such as fructose and invert sugar.
Optical activity is due to the presence of asymmetric carbon atoms, carbon atoms attached to four different groups or atoms.
A racemic mixture is a mixture of equal amounts of dextrorotatory and levorotatory isomers, resulting in no optical activity.