Lactose is the chemical name for what is more commonly called milk sugar. One of the ways of classifying certain sugars has to do with indicating the number of carbons present; a pentose would be a five-carbon sugar. Lactose, however, is not a pentose nor does it contain any pentose sugar units.
Lactose
Lactose, or milk sugar, is a carbohydrate, and consists of the elements carbon, hydrogen and oxygen. It's classified as a disaccharide, meaning it consists of two smaller sugar units, called monosaccharides, that are chemically bonded together, explain Reginald Garrett and Charles Grisham in their book Biochemistry. The chemical formula of lactose is C12H22O11, meaning that it's not a pentose, as it contains far more than five carbons.
Monosaccharides
Not only is lactose not a pentose, the monosaccharides that make it up, called glucose and galactose, aren't pentoses either. Both glucose and galactose have the chemical formula C6H12O6, though despite their identical formulas, they have different structures. As such, these monosaccharide units are both hexoses, or six-carbon sugars. In fact, all the common dietary sugars, including fructose -- fruit sugar -- are either hexoses or are made up of hexoses bonded together.
Pentoses
The most common place to find pentoses in your body is in your genetic material, including DNA. Your DNA -- deoxyribonucleic acid -- is the genetic material in the nuclei of your cells that stores instructions on how the cells should make proteins. The structure of DNA is ladder-like, and is made up of repeating units called nucleotides. The nucleotides include deoxyribose sugars, which are pentoses, explain Mary Campbell and Shawn Farrell in their book, Biochemistry.
RNA
Another kind of pentose in your body is ribose, which makes up the nucleotides that compose RNA, or ribonucleic acid. One common kind of RNA acts as a working copy of your genetic information. Your DNA can't leave the nucleus of the cell, but the machinery in the cell that takes the information from DNA and makes protein is outside the nucleus. The RNA copies the information from the DNA and carries it outside the cell.
References
- Biochemistry; Reginald Garrett and Charles Grisham; 2007
- Biochemistry; Mary Campbell and Shawn Farrell; 2005
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