A level Biology exam revision resources written by A level Examiners


These are the smallest units of carbohydrates. They are used to build up larger carbohydrate molecules, but also have specific biological functions of their own. The number of carbon atoms in a monosaccharide is small (between 3-7). Monosaccharides all have the following features: They are all

  • soluble in water
  • sweet tasting
  • reducing sugars

The most common and important monosaccharide is glucose, which is a six-carbon sugar, so its formula is C6H12O6. Its structure shown in figure 1 below

Figure 1

Figure 1 shows the ring structure of glucose but is very time consuming to use so it can be simplified to figure 2.

Figure 2
Biochemistry   Exam Advice

Other diagrams similar to figure 2 are used by different specifications. Examiners will not penalise any diagram that is technically correct.

The carbon atoms are often numbered in sequence to enable bonds formed between them to be more easily identified. This is shown in figure 3 below

Figure 3

The glucose molecules in the diagrams are labelled Biochemistry-glucose. This refers to the particular isomer that they are. Isomers are different structural forms of molecules, which have the same formula. Figure 4, below, shows the two main isomers of glucose.

Figure 4

The difference between the structures of the two molecules of glucose is the position of the hydroxyl (OH) group on the first carbon atom on the right hand side. The significance of this difference will become clear when the structures of the polysaccharides starch, glycogen and cellulose are examined.   Exam Advice

Glucose is important as the monomer that forms larger molecules. It is the monomer of the disaccharide maltose and the polysaccharides starch, glycogen and cellulose. Glucose also has biological functions as a monosaccharide: It is

  • the main respiratory substrate (that is the molecule used primarily as fuel to generate adenosine triphosphate (ATP) in living organisms.
  • the main transport sugar in the blood of mammals

Many other monosaccharides exist. One of the other monosaccharides which are important in A level biology is ribose. This is a five-carbon sugar (C5H10O5) that is found in the structure of nucleic acids and ATP.

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