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Carbohydrates
The Structure and Function of Carbohydrates
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Ah, the sweet taste of sugar. Most of us love and adore its taste. Sugar and its "derivatives" are called carbohydrates. They can be simple sugars, also known as monosaccharides, double sugars, also known as disaccharides, as well as polysaccharides which are composed of many sugars.

Monosaccharides

Generally speaking, a monosaccharide (mono-, sacchar-) has a formula that is some multiple of CH2O. For instance, glucose has a formula of C6H12O6. Glucose is the most common monosaccharide.

Image credit: Steven Berg


Glucose is typical of the structure of monosaccharides. Hydroxyl groups (-OH) are attached to all carbons except one. The carbon without an attached hydroxyl group is double-bonded to an oxygen to form what is known as a carbonyl group. The location of this group determines whether or not a sugar is known as a ketone or an aldehyde sugar. If the group is not terminal then the sugar is known as a ketone. If the group is at the end, it is known as an aldehyde.

Disaccharides

Two monosaccharides joined together by a glycosidic linkage is called a double sugar or disaccharide. The most common disaccharide is sucrose. It is composed of glucose and fructose. Sucrose is commonly used by plants to transport sugar from one part of the plant to another.

Sucrose with glycosidic linkage in yellow.
Image credit: Steven Berg


Polysaccharides

Polysaccharides are composed of hundreds to thousands of monosaccharides combined together. These monosaccharides are joined together through dehydration synthesis.

Some examples of polysaccharides include starch, cellulose and glycogen. Polysaccharides have several functions including structural support and storage.

Image credit: G. R. Frank



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