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Components of Cells
The Macromolecules
Glucose: ring form

Ring Shapes

Glucose molecules form rings. The first carbon atom (C1), which is an aldehyde group (-CHO), creates a hemiacetal with the fifth carbon atom (C5) to make a 6-membered-ring (termed a pyranose). The atoms in this cyclic molecule then arrange themselves in space to minimize the amount of strain on each of the covalent bonds.

The carbon atoms in the glucose ring each have four covalent bonds. The best, or optimum angle, between all these bonds is 109.5o, which results in a perfect tetrahedron. If, for any reason, these bonds are forced into greater, or smaller angles then the molecule will be strained or stressed, and be much less stable.

It follows, therefore, that the glucose molecule will be at its most stable when all the carbon atoms can arrange themselves so that their bond angles are all close to 109.5o.

Cyclohexane

Some idea of how these considerations affect the shape of the molecule in space can be seen by examining the molecule cyclohexane (C6H12), which also forms a simple three-dimensional ring in space. The molecule could be drawn out in several different ways, thus:

The flat or planar version is the most unlikely since, in this arrangement the carbon bond angles would be at least 120o, which is greater than the optimum. Also, in this form, every carbon atom is lined up with every other carbon atom, that forces the hydrogen atoms to also line up, or eclipse one another. This puts the molecule under a lot of strain.

In the boat conformation some of these strains are lessened, and many of the bond angles are much closer to the optimum degree, however two of the hydrogen atoms at the front and back of the "boat" are forced very close to one another (this is called "steric hinderance", and this arrangement is still stressful.

Moving one end of the "boat" downwards produces the chair version of the ring. This shape relieves almost all the stresses and strains, and thus would be the most stable. In this arrangement, six of the hydrogen atoms stick out of the side of the molecule, like oars (termed "equatorial") and the other six stick up or down from the molecule like flags (termed "axial"). This is also a very stable arrangment as no hydrogen exlipses any adjacent hydrogen.

Glucose

Following the example of the cyclohexane molecule, the glucose molecule will also be most stable when it is arranged into a chair shape or conformation.


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© 2004, Professor John Blamire