Digestion1 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, saliva2 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 bolus3, which can then be easily swallowed.
This mechanical work on the food not only greatly increases its surface area4 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 enzymatic5 hydrolysis6 can break the bonds holding the molecular ‘building blocks’ within the food together. For example, proteins are broken down into their ‘building block’ amino acids7. Once released, these small molecules8 can then be absorbed through the gut wall and into the bloodstream.
An enzyme9 is a protein10 that can control the rate of biochemical11 reactions. In enzymatic hydrolysis reactions, an enzyme incorporates a water molecule12 across the bond, allowing it to break.
Carbohydrates
The basic building blocks of carbohydrates are simple sugars like glucose13 and fructose14. The bonds holding these sugars together are called glycosidic bonds.
Proteins
The basic building blocks of proteins are amino acids15. The bonds that hold amino acids together are known as peptide16 bonds. To break the peptide bonds in a protein, a hydrolysis reaction is needed similar to that involved in breaking up carbohydrates. Enzymes17 known as proteases are needed to break up the protein.
The following example shows how a peptide bond18 can be broken. A protease19 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 fat20 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 lipase21 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.
Related content
The human digestive system consists of a long muscular tube and several accessory organs. It is responsible for food ingestion22 and digestion, absorption of digestion products and the elimination of undigested materials.
Use Rate of digestion to explore how the digestion of food involves chemical reactions that break up large food molecules into their ‘building block’ components. There are a number of factors that affect the rates of these reactions.
Enzymes are globular proteins that control23 biological reactions. Digestive enzymes speed up the breakdown (hydrolysis) of food molecules into their ‘building block’ components. These reactions occur outside of the cells lining the gut.
Actvity ideas
Try these student activities and explore the actions of digestive enzymes: lactase (and lactose intolerance), proteolytic enzymes on party jellies and salivary amylase on cooked rice starch24.
Find out why we need carbohydrates, proteins and fats in our diet as well as the structure and function of these macronutrients25.
- digestion: The mechanical and chemical breaking down of food into smaller components that can be absorbed into a bloodstream.
- saliva: A watery fluid produced by the salivary glands in the mouth. It is made up of water, mucus, amylase, lipase and sodium bicarbonate.
- bolus: Semi-degraded food that has been chewed and mixed with saliva. Also known as cud, it is produced during the digestive process of rumination.
- surface area: The total area of an object or surface.
- enzyme: A complex protein that acts as a catalyst (speeds up chemical reactions) in specific biochemical reactions. For example, saliva contains an enzyme called amylase that can break down starch into simple sugars.
- hydrolysis: 'hydro' – involving water, 'lysis' – breaking down. When water is involved in the breakdown of a chemical compound into two smaller parts.
- amino acid: The basic building block of proteins. A short chain of amino acids is called a peptide, and a long chain of amino acids (normally more than 50) is called a protein.
- molecule: Two or more atoms bonded together. The molecule of an element has all its atoms the same. The molecule of a compound has two or more different atoms.
- enzyme: A complex protein that acts as a catalyst (speeds up chemical reactions) in specific biochemical reactions. For example, saliva contains an enzyme called amylase that can break down starch into simple sugars.
- protein: Any of a large class of complex compounds that are essential for life. Proteins play a central role in biological processes and form the basis of living tissues. They have distinct and varied three-dimensional structures. Enzymes, antibodies and haemoglobin are examples of proteins.
- biochemical: Any organic compound involved in living processes, for example, a protein, carbohydrate or lipid.
- molecule: Two or more atoms bonded together. The molecule of an element has all its atoms the same. The molecule of a compound has two or more different atoms.
- glucose: A simple sugar belonging to the group of carbohydrates called monosaccharides. It is the main form of carbohydrate used by the body.
- fructose: A sugar found in fruits.
- acid: A hydrogen-containing substance that is capable of donating a hydrogen ion to another substance.
- peptide: Two or more amino acids linked together to form a chain.
- enzyme: A complex protein that acts as a catalyst (speeds up chemical reactions) in specific biochemical reactions. For example, saliva contains an enzyme called amylase that can break down starch into simple sugars.
- peptide bond: A covalent bond found in proteins and polypeptides that links amino acids together.
- protease: A type of enzyme that speeds up the breakdown of proteins.
- saturated fat: The fatty acids present in this type of fat have only single bonds present between each of the carbon atoms in the molecule.
- lipase: An enzyme that breaks down fats.
- ingestion:
- The process of taking a substance into the mouth or body.
- The process or act of engulfing particles by cells.
- control: 1. Part of a scientific experiment in which no treatment has been applied in order to see whether there are any detectable differences to the experiment that did receive a treatment. 2. To hold in check or to curb.
- starch: A complex carbohydrate found chiefly in seeds, fruits, tubers, roots and stem pith of plants. Commonly found in foods, such as potatoes, wheat, rice and corn.
- macronutrient: 1. In human nutrition, a nutrient that is required in large amounts and provides the energy needed to maintain body functions and carry out the activities of daily life. There are three macronutrients – carbohydrates, proteins and fats. 2. In agriculture, a macronutrient is any of the chemical elements required by plants in relatively large amounts: carbon, oxygen, hydrogen, nitrogen, phosphorus, potassium, magnesium, sulfur and calcium.
