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This is just a beginning: some information on
glucose, fructose and sucrose. More sugars will be added to this page in
due course.
Structural Formulae:
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. The molecular formula is:
And here is
the structural formula:
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 |
C |
|
O |
are called Pyranose Sugars, e.g. glucose. |
| Monosugars with a
five atom ring |
C |
|
O |
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.
Now try my new Java "Windows
Revision Application":
CARBOHYDRATES
written with Visual J++. |
