Page "Carbohydrate" Paragraph 27

Monosaccharides are the major source of fuel for metabolism, being used both as an energy source ( glucose being the most important in nature ) and in biosynthesis.

When monosaccharides are not immediately needed by many cells they are often converted to more space-efficient forms, often polysaccharides.

In many animals, including humans, this storage form is glycogen, especially in liver and muscle cells.

In plants, starch is used for the same purpose.