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Digestion Chemistry

Food plays an extremely important part in the lives of all humans. After ingestion, the food is mechanically broken down into smaller pieces and then chemically digested through the action of enzymes. The products of this digestion process are then absorbed into the bloodstream and transported to the liver for further processing. Remaining undigested material is then passed on to the large intestine where some of it undergoes bacterial fermentation. Any remaining waste material is passed on and prepared for elimination from the digestive system.

The GI tract

The human digestive system, also known as the gastrointestinal (GI) tract, consists of a long muscular tube and several accessory organs such as the salivary glands, pancreas and gall bladder. It is responsible for food ingestion and digestion, absorption of digestion products and the elimination of undigested materials.

We look at the main structural features of the GI tract and the role they play in the digestive process.


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

We look at mechanical digestion and the chemical digestion (enzymatic hydrolysis) of carbohydrates, proteins and fats.

Rate of digestion

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.

We look at how surface area, temperature and pH all influence the rate of digestion of large food molecules. The action of salivary amylase on starch is used as an example.

Large intestine function

Recent research has revealed that the large intestine and its resident bacterial population have key roles to play in determining our health and wellbeing. It is much more than just a waste storage facility.

We look at the structure of the large intestine and the four main functions it performs.

Unlocking the energy in foods

The foods we eat supply the energy needed by the body to drive its complex chemical, mechanical and electrical systems.

We look at the origin of this energy, how is it locked into food molecules, how is it released and how the energy content of foods can be calculated.

Energy requirements of the body

The macronutrients (carbohydrates, proteins, fats and oils) we consume in our diet help to supply the energy needed by the body to keep it working. The rate of energy release from macronutrients by chemical processes occurring in the body is known as metabolic rate.

We look at the concept of resting metabolic rate and a simple method of calculating it. In addition, the idea of balancing energy intake from eating food with output through resting metabolism, growth, exercise and digesting food is also considered.