Structures of carbohydrates, proteins and lipids


Carbohydrates in our diet include sugars and starches.

Diagram showing the structure of carbohydrates

The glucose molecule is small enough to be absorbed directly through the walls of the digestive system.

Starch is a polymer of glucose. It must be broken down into glucose molecules - it is too large to pass through the gut.

Cellulose is also made up of glucose molecules. It makes up plant cell walls. It is therefore a fundamental part of our diet. It cannot be broken down by the digestive system, so is egested from the gut.

Once absorbed by the body, glucose molecules are transported to cells and:

  • used for respiration
  • reassembled into the storage form of carbohydrate in animals - glycogen

Note that in plant metabolism, the glucose produced by photosynthesis is converted into starch for storage, and cellulose, for cell wall synthesis.

In humans and other animals glucose is stored in glycogen. It is not converted into starch.


Proteins are made up of amino acids.

A diagram illustrating amino acids, which are proteins.

Proteins are big molecules that are too large to pass through the gut wall. They must first be broken down into amino acids.

Once inside the body, the amino acids are reassembled into the proteins the individual requires - the process of protein synthesis.

Excess amino acids are broken down in the liver as they cannot be stored.


Lipids are esters of fatty acids and glycerol.

A diagram illustrating how lipids are made up of fatty acids

Lipid molecules are too large to pass through the gut wall and must be digested first.

In the body's cells, they are reassembled into the lipids the cell needs, for instance, for the cell membranes.