2 Sugars

The main points to understand and remember about sugars are threefold:

1. They are important metabolically. Sugars are the major energy storage molecules for living organisms. Their carbon rings contain large amounts of energy. For example, the complete metabolism of glucose:

   releases 686 kcal/mol.

2. There are two ways for sugars to polymerize, through alpha and beta linkages:

   Alpha Linkages

   

   

   Beta Linkages

   

   

   To distinguish between alpha and beta linkages, examine the position of the hydrogen on the first carbon molecule. In an alpha linkage, the hydrogen is pointing up, and in a beta linkage it is pointing down.

   Sucrose and lactose are composed of two sugars and are therefore termed disaccharides.

   Amylose and cellulose, composed of long chains of glucose molecules, are examples of polysaccharides. The only structural difference between them is the linkage between the glucose molecules, yet the compounds have very different properties. Cellulose is a tough material found in plant cell walls, whereas amylose (a form of starch) is water soluble and used by plants as a carbon storage compound.

3. There are four basic sugars that we will deal with: