IV/IV - 2 (of 4)
The monosaccharides and disaccharides together make up the group of carbohydrates known as the
“simple sugars”. They are called “simple” because they are readily broken down and absorbed by the
body, due to their simple structure, and therefore give an immediate energy supply.
During digestion, these sugar molecules are broken down by the bodies digestive enzymes. Some
people do not produce the enzyme lactase that is responsible for digesting lactose, commonly found in
milk. These people suffer from the disease lactose intolerance. They cannot digest lactose, or dairy
products containing lactose.
Solubility in water
Simple sugars have many groups composed of one hydrogen atom linked to one oxygen atom, this
oxygen atom being attached to the molecule. These -OH groups make sugars soluble into water,
because they can interact with water molecules. In the absence of water, these groups will be slightly
attracted to each other and will cause the sugar particles to line up in a regular pattern. This structure
is called a crystal structure. It is because they form these crystals that we are able to see sugars. A
single disaccharide is far too small to see with the naked eye, but when millions of disaccharides join
up to form a crystal the sugar is visible. These crystals can greatly vary in size – for example in caster
sugar, the crystals are fairly large and obvious, but in icing sugar, the crystals are much smaller.
The presence of these OH groups also makes the simple sugars readily soluble in water. In the
presence of water, their OH groups will preferentially bind to water molecules than to each other,
because these OH groups are more strongly attracted to the water molecules, so the sugar crystal will
break up and the sugar units will evenly place themselves throughout the water. The sugar is said to
have “dissolved”. The solution will thicken as the presence of the sugar molecules reduces the ability
of the water molecules to move around each other freely.
Fructose is the most soluble of the simple sugars (it can dissolve in one quarter of its volume of water),