Digestion of food involves both physical and chemical processes. Through digestion, large food particles are converted into smaller components that can be readily absorbed into the bloodstream.

Mechanical digestion

Food is taken into the mouth where it is broken down into smaller pieces by the teeth. As the teeth grip, cut and chew the food, saliva is released and mixes with the smaller food particles. Saliva moistens and lubricates the food, allowing it to be worked into a small ball called a bolus, which can then be easily swallowed.

This mechanical work on the food not only greatly increases its surface area but also allows it to be mixed with a digestive enzyme present in saliva.

Chemical digestion

Mechanical digestion can only break up the food particles into smaller pieces. A chemical digestion process called enzymatic hydrolysis can break the bonds holding the molecular ‘building blocks’ within the food together. For example, proteins are broken down into their ‘building block’ amino acids. Once released, these small molecules can then be absorbed through the gut wall and into the bloodstream.

An enzyme is a protein that can control the rate of biochemical reactions. In enzymatic hydrolysis reactions, an enzyme incorporates a water molecule across the bond, allowing it to break.


The basic building blocks of carbohydrates are simple sugars like glucose and fructose. The bonds holding these sugars together are called glycosidic bonds.


The basic building blocks of proteins are amino acids. The bonds that hold amino acids together are known as peptide bonds. To break the peptide bonds in a protein, a hydrolysis reaction is needed similar to that involved in breaking up carbohydrates. Enzymes known as proteases are needed to break up the protein.

The following example shows how a peptide bond can be broken. A protease enzyme catalyses this step:

Two amino acids are released – glycine and alanine. Note how the water molecule splits, with OH added to form glycine and the H added to form alanine.

Fats and oils

These macronutrients are formed from a combination of glycerol and 3 fatty acids.

For example, part of the saturated fat found in common foods like meats and dairy products is known as glyceryl tristearate. It has the following structure:

To convert it into a form that can be absorbed into the bloodstream, the bonds indicated with ~ need to be broken. This involves an enzyme called a lipase as well as water molecules. The products from this reaction are shown below.

Fatty acids are either saturated or unsaturated:

  • Saturated fatty acids have only single C–C bonds present in the chain.
  • Unsaturated fatty acids have at least one double C=C bond present in the chain.
    Published 5 September 2011