Sources of Energy


Carbohydrates such as sugar, starch and glycogen are compounds which are created of carbon, hydrogen and oxygen.

There are three different types of carbohydrates these are:
  • Monosaccharides 
  • Disaccharides
  • Polysaccharide
Monosaccharides are simple sugars. Two examples of these sugars would be the 6-carbon glucose and fructose, their molecular structure is often represented in a simple six-sided unit (shown below). These sugars are soluble in water and are therefore called reducing sugars, this also means that the donate electrons to other substances.

Disaccharides, these are sugars consisting of two monosaccharides molecules joined together. Two examples of these would be maltose and sucrose, once again these are soluble in water. Maltose is described as a reducing sugar since it has properties similar to a monosaccharides and gives a positive result when tested with benedict's solution. Whereas, sucrose is a non-reducing sugar since it does not reduce in water, although if it is boiled in a dilute acid it breaks down into monosaccharides.


A polysaccharide is a carbohydrate composed of many monosaccharide molecules joined together, these are large and insoluble in water.

Starch is a polysaccharide it consists of a long chain of glucose molecules (they contain thousands of monosaccharides). Glycogen is also a polysaccharide.

All of the carbohydrates talked about above are all sources of energy, monosaccharides can be broken down as a respiratory substrate whereas mono and polysaccharides must be broken down in order to be used as a respiratory substrate.
Investigating the identity of an unkown carbohydrate
There are many tests for carbohydrates, these include:
  • The solubility in water test
  • The benedict's test
  • Barfoed's test
  • Clinistix test
  • Iodine
  • Acid Hydrolysis
Solubility in water - this is when a small sample of the carbohydrate is added to water, if it dissolves it is either a mono or disaccharide sugar. Whereas, if it turns cloudy it is a polysaccharide.
The benedict's test - This is when a small amount of benedict's agent is added to the carbohydrate and brought to the boil in a water bath. If it turns orange it means that a reducing sugar is present.
Barfoed's test - This is very similar to the benedict's test the only thing that changes is that barfoed's reagent is added.
Clinistix test - This involves a clinistix reagent strip being dipped into a carbohydrate solution, if it turns purple glucose is present.
Iodine test - A few drops of an iodine solution is added to a carbohydrate, if it turns blue/black then starch is present whereas, when purple/red appears then glycogen is present.
Acid hydrolosis - This is when hydrolochloric acid is added to the carbohydrate, Benedict's reagent is then added and follows the same procedure as the Benedict's test.