This is just a beginning: some information on glucose, fructose and sucrose. More sugars will be added to this page in due course.
Sugars are carbohydrates: this means that they contain the elements Carbon, Hydrogen and Oxygen, and that there is twice as much Hydrogen as there is Oxygen. Hydrogen and Oxygen atoms are in the ratio of two to one as in water molecules. The simplest sugars are called monosugars or monosaccharides. The two shown on this page each have six atoms of Carbon so they are called hexose sugars. Deoxyribose (in DNA) and Ribose (in RNA) only have five atoms of Carbon so they are called pentose sugars.
Glucose is the sugar used in respiration. It circulates around our body dissolved in blood plasma. Glucose can diffuse across cell membranes into our cells where it is used in tissue respiration.
Here is the molecular formula for Glucose:
And here is the structural formula for Glucose:
There are many possible arrangements of the atoms in hexose sugars. Glucose is just one of the possibilities. In the diagram of glucose you can see that there are five atoms of Carbon and one atom of Oxygen in a ring. Sugars like this are called pyranose sugars. Galactose is another pyranose sugar; it has the –OH groups in different positions on the ring.
The “sugar” we put in our tea or coffee (I always have three big spoonfuls) is called sucrose. This sugar is much too big to get into our blood so it has to be digested by an enzyme called sucrase. Note the spelling: sucrose is a sugar, and sucrase is the enzyme which digests it.
Here is the structural formula of Fructose:
Now count up the atoms of Carbon, Hydrogen and Oxygen. You will find that it is just the same as glucose. Glucose and Fructose are structural isomers.
It is possible to join a molecule of glucose and a molecule of fructose together by removing a molecule of water. This will produce a molecule of sucrose. Sucrose is transported by plants in their phloem. It is also stored by some plants such as sugar cane and sugar beet.
Because the atoms of Carbon in a molecule of sucrose are not all directly joined to each other it is not a monosugar but a disugar or disaccharide. I will make a diagram of sucrose soon. In the meantime take it from me, there is an atom of Oxygen holding the two parts of a sucrose molecule together. It is quite easy for the enzyme sucrase to split the molecule in half by adding a molecule of water.
The link between the two parts of the sugar is called a “glucosidic” link. Count up the atoms in the sucrose molecule shown below
Monosugars with a six atom ring C5O are called Pyranose Sugars, e.g. glucose.
Monosugars with a five atom ring C4O are called Furanose Sugars, e.g. fructose.
If you look carefully at the structural formula for sucrose below you will see that the two parts of the sugar are joined together by an atom of Oxygen. There are two parts to the sugar, that is why it is called a disugar.
Here is the structural formula for sucrose:
This page was written in response to a question from Harry in Hawaii.