Structure of carbohydrates

Glucose is an example of a carbohydrate which is commonly encountered . It is also known as blood sugar, and dextrose.

Its chemical formula is C6H12O6, and this empirical formula is shared by other sugars - called hexoses - 6 carbon sugars. You may wish to know in some detail how these 24 atoms are arranged in the molecule of glucose – the structural formula.

In some books you may see diagrams of the glucose molecule looking like this: This so-called stick diagram really only describes how things are in dry (powder) glucose.

In life - in your blood and inside cells of plants and animals - most of the glucose consists of molecules shaped into a ring (actually a 6-sided figure, a hexagon) which may be drawn with this fairly simple format:

Note that there is an oxygen atom forming part of the ring, and that there are simple lines drawn making up the rest of the ring, and a section sticking out to one side. These lines represent carbon atoms, and -H and -OH groups, most of which have been left out for simplicity. Sometimes the details of just some of these -H and -OH groups are drawn in at one end (or both ends). This is because the orientation of these groups slightly alters the chemistry of the molecule, so the resulting molecules are given different names.

alpha-glucose - showing only -H and -OH at either ends of the moleculebeta-glucose - showing only -H and -OH at either ends of the molecule 
alpha glucose beta glucose

In alpha glucose the -H group of the rightmost Carbon atom (C1) is above the plane of the ring, whereas it projects below the ring in beta glucose.

There are in fact 3 versions of the ring structure called chairs and boat, depending on the bending in or out of the left and right corners, but it is easier to draw it as a flat hexagon. Some of the lines may be thickened or flared out to give an impression of 3D structure on the page

glucose molecule - now with missing H on C5 added

All the other groups can vary in different hexose sugars, which result in slightly different chemical properties.

It is worth knowing the numbers used to describe each of the 6 carbon atoms.

glucose molecule labelled with carbon numbers

This numbering is important when glucose units are joined together.

1-4 linkages (formed by glycosidic bonding) result in a simple linear (end to end) connection, which is found in maltose, a disaccharide:

Repeating the condensation process will result in the polysaccharide starch, which is considered to have 2 components, amylose and amylopectin. The chain of 1-4 linkages so formed can be quite long, and may wind up into a helix. This is the situation in amylose, (a linear part of which is shown here):

4 glucoses joined by 1 alpha 4 bonds as in amylose

On the other hand, 1-6 linkage causes the formation of a branching point which then allows another chain of 1-4 linkages to develop. This 3-D branching also produces other points at which glucose may be added by condensation or removed by hydrolysis. This branching occurs in amylopectin, (a small section of which is shown here):
Amylopectin has long sections of glucoses linked by 1 alpha 4 bonds, with the occasional 1 alpha 6 bond which inserts a branching point

This basically explains the structure of starch and glycogen.



Some computer programs can display molecular structure in a variety of formats. The picture below is a single still image of the glucose molecule from Rasmol, which requires a special plug-in called Chime® in order to work in your browser.

A more convenient alternative is to use the open source Java applet Jmol. Several files on this website have been converted to use this to show features of "simple" biochemical molecules.

Click here to see a file showing the 3-D molecular structure of glucose using Jmol, which opens the opportunity to see and interact with other molecules in 3 dimensions.

If you are very long-suffering, Chime® is available to download free from MDL, after a tedious registration process.

Once you have this plug-in installed you will be able to access a moveable 3-D image when you click on the image below, and you will also be able to alter the display in a number of ways.
Orphan rescue: Click on one of the links below if you are stranded on this page without an escape route:
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