Biology AS revision notes – The Digestive System

Summary of the digestion process

Food enters the mouth and is physically broken down by teeth into smaller food pieces to ingest the food and provide a large surface area for chemical digestion. (The salivary glands pass their secretions which contain amylase to break starch down into maltose, into the mouth via the duct). The pharynx is responsible for the food going down the right passage through the oesophagus. The epiglottis prevents food getting into the respiratory system.

The food goes down the oesophagus which carries food from the mouth to the stomach and is made up of a thick muscular wall so that it is adapted for transport.

The food goes to the stomach which is a muscular sac with an inner layer. The stomach stores and digests foods and has glands that produce enzymes (protease) to digest proteins. The other glands produce mucus to stop the stomach being digested by its own enzymes.

The muscles in the stomach wall also churn the food (physical digestion).

The food is then further digested by enzymes that are produced within the walls of the small intestine and by the glands that pour their secretions into it. The small intestine is a long muscular tube with the inner walls folded into villi to increase surface area. The surface area of the villi has microvilli to further increase the surface area for absorbing products into the bloodstream.

The liver is made up of glandular tissue and produces bile and the gallbladder stores the bile.

The pancreas is a large gland that produces alkaline pancreatic juice which contains proteases to digest proteins, lipases to digest lipids, and amylase to digest starch.

The large intestine is a muscular tube that absorbs water and the rectum is where the faeces are stored before being removed via egestion.

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