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Functions of the alimentary canal

The alimentary canal is a hollow tube that processes food to make it usable by the body. It is also known as the gastrointestinal tract (GI tract) or the gut.

Food, such as bread, is broken down in the canal. The useful products need to be separated from waste, and transported to the rest of the body.

The most important nutrients that are obtained from food are glucose, amino acids and fatty acids. These are called the final end products of digestion.

The alimentary canal is responsible for processing food to allow useful substances to enter the bloodstream, and expelling the remaining material.
The alimentary canal is responsible for processing food to allow useful substances to enter the bloodstream, and expelling the remaining material.

The alimentary canal is isolated from the rest of the body. This prevents waste products and microorganisms from entering the blood stream.

Processing of food in the alimentary canal is divided into separate stages.

Process Definition
Ingestion Physical intake of food
Digestion Breakdown of large, insoluble matter into small soluble molecules
Absorption Movement of digested molecules from the gut into the blood stream
Assimilation Processing of absorbed nutrients for use by the body
Egestion Removal of undigested material and unabsorbed material from the gut
The alimentary canal forms a factory line for the different digestive processes.
The alimentary canal forms a factory line for the different digestive processes.

Digestion, the process of breaking down food into usable parts, has two separate types:

Physical digestion uses physical means to reduce the size of pieces of food. This can also be called mechanical digestion.

Chewing with your teeth is a form of physical digestion.

By breaking the food into smaller pieces, physical digestion increases the surface area that enzymes can attack.

During chemical digestion, enzymes break large molecules into small molecules that can be absorbed and used by the body.

Proteins are composed of hundreds of amino acids. Chemical digestion separates protein into its amino acids, which can then be used to make new proteins.

Chewing is an example of physical digestion.
Chewing is an example of physical digestion.

There are three main classes of enzymes in the digestive system responsible for breaking down different types of molecules:

  • Proteases such as trypsin

    $$$\text{protein $\rightarrow$ amino acids}$$$

  • Amylases such as salivary amylase

    $$$\text{carbohydrate $\rightarrow$ glucose}$$$

  • Lipases such as pancreatic lipase

    $$$\text{fat $\rightarrow$ fatty acids + glycerol}$$$

In most cases, more than one enzyme is needed to break down a large molecule into its final breakdown product.

Starch (a common carbohydrate) is first broken down into maltose by salivary amylase and then to glucose by maltase (produced by the small intestine).

The body needs to have enzymes that digest protein, carbohydrates and fats.
The body needs to have enzymes that digest protein, carbohydrates and fats.