Disaccharides

The sugar you put in your tea, sucrose, is a disaccharide. "Di" means two. A disaccharide forms when two monosaccharide molecules join together. As they join, a molecule of water is removed.

2C₆H₁₂O₆ ⇌ C₁₂H₂₂O₁₁ + H₂O

The arrow points both ways because this reaction can go in either direction.

The reaction can also run in the opposite direction. Adding a molecule of water to a disaccharide splits it back into two monosaccharides. This is hydrolysis. (In hydrolysis, a water molecule splits into two parts, H⁺ and OH^-, which attach to either side of the place where the molecule breaks. One large molecule then becomes two smaller ones.) This is how the sugar you eat is broken down in your digestive system.

Shared properties of disaccharides

Like monosaccharides, disaccharides:

exist as crystals.
dissolve in water.
taste sweet.

Comparing the three disaccharides

Type	Where it is found	Facts to know
Maltose	Seeds that are sprouting (germinating)	Glucose + glucose → maltose + water A product formed partway through the hydrolysis of starch.
Sucrose	White and brown sugar, sugarcane and beet, some fruits, the sap that moves through the phloem	Fructose + glucose → sucrose + water This is the ordinary "sugar" we buy from the shop.
Lactose	Milk and milk foods	Galactose + glucose → lactose + water The only sugar that is not found in plants at all. It is less sweet than sucrose. Cow's milk contains between 4% and 6% lactose. Breast milk contains between 6% and 7% lactose.

Spot the pattern

Look closely at the "+" signs in the table above. One part of all three disaccharides is glucose. Only the second part changes — it can be glucose, fructose or galactose.