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Monosaccharides and disaccharides

Monosaccharides are classed by how many carbon atoms they have.

Pentose sugars have 5 carbons and hexose sugars have 6 carbons. Both are usually ring shaped.

The carbons are numbered so that we can consistently refer to the sugar structure.

DNA strands are called 3' and 5', referring to the position of the exposed carbon in the sugar backbone.

We number the carbon atoms clockwise from the oxygen. Deoxyribose is a pentose sugar. Glucose is a hexose sugar.
We number the carbon atoms clockwise from the oxygen. Deoxyribose is a pentose sugar. Glucose is a hexose sugar.

Disaccharides are two monosaccharides joined together. All the monosaccharides listed in the table (glucose, fructose and galactose) are hexose sugars.

Disaccharide Monosaccharides Occurrence
Sucrose Glucose and fructose Fruits
Lactose Glucose and galactose Milk
Maltose Two $$\alpha$$-glucoses In germinating seeds