20.8: Simple Sugars
- Page ID
- 49612
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. Sugars are thus all aldehydes and ketones and are usually referred to as aldoses or ketoses. A molecule like glucose is called an aldehexose since it contains six carbon atoms. The difference between different sugars can be very subtle. Note, for instance, that mannose and glucose differ only in the geometrical arrangement of the OH group on one carbon atom. Subtle as this difference is, molecules in a living cell can tell the difference.
A careful consideration of the geometry of this structure reveals that not one but two cyclic structures are possible. These are called α- and β-glucose and are shown in Figure \(\PageIndex{2}\). In the β form the C—O bond on carbon atom 1 (shown in dark color) is parallel to the C—O bond on carbon atom 4, while in the α form these two bonds are at an angle of 180° – 109.5° = 70.5°. This geometric difference may seem relatively trivial, but it turns out to be important when glucose molecules are used as building blocks to form larger entities.
