Chemistry higher tier
Carbohydrates are probably familiar to you; they are one of the main classes of foodstuffs in our diet.
Carbohydrates are found in foods such as pasta, bread, potatoes and many plant based foods.
They act as an energy source in our diets. Carbohydrates are molecules which contain only the
elements carbon, hydrogen and oxygen.
There are two main types of carbohydrates; these are simple
sugars such as glucose or sucrose and polymers such as starch and cellulose.
The monomer needed to make the polymers starch and cellulose is the simple sugar glucose. Glucose is made in the
leaves of plants by photosynthesis. The glucose is used as a source of energy by the plant but
some of this glucose will be converted starch. The starch is stored in many parts of plants and
it will be hydrolysed (broken down) back into glucose sugar and used as an energy source when required by
the plant cells.
The image below shows a model of the sugar glucose; perhaps the first thing to note is that it has a ring structure. The ring consists of 5 atoms of carbon and 1 atom of oxygen. If you count the atoms present you will see that there are 6 atoms of carbon (black balls), 6 atoms of oxygen (red balls) and 12 atoms of hydrogen (white balls). This means glucose has the formula C6H12O6. It is worth mentioning that the hydrogen and oxygen are present in the ratio of 2;1; exactly as in a molecule of water. You will find this often in the formula of many sugar molecules.
In fact you may even see the formula of many sugars written as Cx(H2O)y.
For glucose x and y would be 6; to give C6(H2O)6 which is just a simple way to of writing
C6H12O6. Sucrose, the sugar we add to sweeten tea and coffee
has the formula C12H24O12; here again the hydrogen and oxygen are present in the ratio of 2:1. We
can simplify its formula to give C12(H2O)12.
Drawing out a molecule of glucose as shown in the image above would take too long; the drawing
opposite is a quick short-hand way to draw the glucose structure.
Starch is a polymer and also a carbohydrate made by joining together hundreds and in some cases thousands of glucose molecules to produce a large straight chain polymer; part of the starch polymer chain is shown below, here two glucose molecules have linked together. A water molecule is lost as the two glucose molecules join in this condensation reaction.
To produce starch instead of 2 glucose molecules combining we need hundreds of them to condense.
Note
Cellulose is another carbohydrate polymer found in the cell walls of plants. It provides rigidity and strength to the plant. Its structure is very similar to that of starch in that it is a polymer made from the monomer glucose. However the glucose molecules are arranged slightly different in cellulose than in starch. The diagram below outlines the structure of these two carbohydrate polymers.
