Structures of carbohydrates, proteins and lipids


Carbohydrates in our diet include sugars and starch.

Diagram showing the structure of carbohydrates

The glucose molecule is small enough to be absorbed directly through the walls of the digestive system but starch is a polymer of glucose. Starch must be broken down first by carbohydrase enzymes called amylase into maltose and then into glucose molecules by another carbohydrase.


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 by protease enzymes. These amino acids can then be used to make proteins in the body.


Lipids are made of fatty acids and glycerol.

A diagram illustrating how lipids are made up of fatty acids

In a lipid, there may be 1, 2 or 3 fatty acids combined with glycerol - fatty acid 1, 2 and 3 may be the same or different.

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

Digestive enzymes are used to break down food in the gut into small, soluble molecules that can be absorbed through the gut wall.