Digestion and the importance of bacteria

In a nutshell

Digestion is when food is broken down in preparation for absorption. It is comprised of two main processes: mechanical and chemical. This summary takes a closer look at the process of absorption and explores the role of bacteria in digestion.

Mechanical digestion

This type of digestion begins in the mouth in a process called mastication or chewing. Teeth break down food mechanically (physically) to increase its surface area for chemical digestion. 

Tip: Mechanical digestion also includes churning in the stomach and contraction in the small intestine. 

Chemical digestion

This stage of digestion involves enzymes which are biological catalysts. Enzymes break down large food molecules into smaller molecules.

Absorption

The absorption of nutrients occurs in the small intestine and the large intestine.

Absorption in the small intestine

Large food molecules are broken down into smaller absorbable nutrients by enzymes. The nutrients are then absorbed into the villi and diffuse into the blood, where they are circulated to cells that need them.

Villi​

In the small intestine (1), there are thousands of finger-like projections called villi (2). Villi have three adaptations that help them absorb nutrients efficiently. They are one cell thick, they have a good blood supply (3.) and they have a large surface area.​

Absorption in the large intestine

Water is absorbed through the wall of the large intestine through a process called diffusion.

Bacteria and digestion

There are lots of bacteria found in the alimentary canal that play an important role in digestion. Some bacteria are only found in specific areas or in specific quantities, if they move or overgrow they can cause illness. Functions of bacteria include:

• Making vitamins that keep humans healthy.
• Breaking down food that can't be digested by enzymes.
• Controlling the growth of harmful bacteria.