Digestion – breaking the large into the small

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.

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Action of sucrase on sucrose

Sucrose binds to the active site on sucrase, and this puts stress on the bond between the 2 sugars that make up sucrose. The bond breaks, releasing glucose and fructose.

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.

Carbohydrates

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

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Hydrolysis reaction

Hydrolysis reactions use up water molecules to break bonds. In this example, a water molecule (HOH) is used to supply an OH to one side of the breaking bond and an H to the other. Sucrase is one of the many digestive enzymes released into the small intestine.

Proteins

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:

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Breaking a peptide bond

This example shows how a peptide bond can be broken.

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:

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Conversion of glyceryl tristearate

To absorb the energy from components within saturated animal fat, glyceryl tristearate, the human body needs to undergo a chemical digestion process to break the bonds holding the molecular ‘building blocks’ within the food together.

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.