Biology - Unit 1 - Topic 2

2.1 Enzymes and the digestive system:

The human digestive system is made up of a long muscular tube and its associated glands. The glands produce enzymes that break down large molecules into smaller ones ready for absorption.

Major parts of the digestive system:

The oesophagus carries food from the mouth to the stomach. It is therefore adapted for transport rather than for digestion or absorption. It is made up of a thick muscular wall.

The stomach is a muscular sac with an inner layer that produces enzymes. Its role is to store and digest food, especially proteins. It has glands that produce enzymes which digest proteins. Other glands in the stomach wall produce mucus. The mucus prevents the stomach being digested by its own enzymes.

The small intestine is a long muscular tube. Food is further digested in the small intestine by enzymes that are produced by its walls and by glands that poor its secretions into it. The inner walls are folded into villi which give them a large surface area and the surface area is increased further by millions of tiny projection called microvilli on the epithelial cells of each villus.  This adapts the small intestine for its purpose of absorbing the products of digestion into the bloodstream.

The large intestine absorbs water. Most of the water that is reabsorbed comes from the secretions of the many digestive glands. The food within the large intestine therefore becomes drier and thicker and forms faeces.

The rectum is the final section of the intestines. The faeces are stored here before periodically being removed via the anus in a process called egestion.

The salivary glands are situated near the mouth. They pass their secretions via a duct into the mouth. These secretions contain the enzyme amylase, which breaks down the starch into maltose.

The pancreas is a large gland situated below the stomach. It produces a secretion called pancreatic juice. This secretion contains proteases to digest proteins, lipases to digest lipids and amylase to digest starch.

Physical Digestion: Food that is large is broken down into smaller pieces by means of structures such as the teeth. This not only makes it possible to ingest the food but also provides a large surface area for chemical digestion. Food is churned by the muscles in the stomach wall and this also physically breaks it up.

Chemical Digestion: chemical digestion involves breaking down large, insoluble molecules into smaller soluble ones. It is carried out by enzymes. All digestive enzymes function by hydrolysis. Hydrolysis is the splitting up of molecules by adding water to the chemical bonds that hold them together. The general term for such enzymes is hydrolases. Enzymes are specific and so it follows that more than one enzymes is needed to break down a large molecule. Usually one enzyme splits a large molecule into sections and these sections are the hydolyesed into smaller sections and then hydrolysed into smaller molecules by one or more additional enzymes. There are different types…