The Human Digestive System (AQA GCSE Biology) : Revision Note
Digestion: basics
The digestive system is an example of an organ system in which several organs work together to digest and absorb food
During digestion large, insoluble molecules in food are broken down into smaller, soluble molecules that can be absorbed into the bloodstream
These small soluble molecules are used by cells for the release of energy, or to build other molecules
Tissues & organs of the human digestive system
The organs of the digestive system work together to digest and absorb food
Structure | Function |
|---|---|
Mouth | Food is ingested here and the teeth break it apart during mechanical digestion, increasing its surface area for chemical digestion |
Salivary glands | Saliva is secreted into the mouth The enzyme amylase in saliva begins to digest starch into maltose |
Oesophagus | This tube connects the mouth to the stomach Contractions of the walls of the oesophagus force the food downwards; this is peristalsis |
Stomach | Churning of the muscular stomach walls continues the process of mechanical digestion Protease enzymes begin protein digestion Hydrochloric acid provides a suitable pH for the enzymes and also destroys any pathogens in food |
Liver | Bile is produced here; it aids the digestion of fats, as well as neutralising stomach acid as it exits the stomach |
Gall bladder | Bile is stored here before being released into the duodenum via the bile duct |
Pancreas | Amylase, protease and lipase enzymes are produced here before being released into the small intestine |
Small intestine | The acidic stomach contents are neutralised by bile and become slightly alkaline Enzymes complete chemical digestion Food and water are absorbed into the blood via villi |
Large intestine | Remaining water is absorbed from food into the blood, and the solid waste left behind in the colon forms faeces |
Rectum | Faeces are stored here prior to egestion |
Anus | Faeces leave the body via the anus; this is egestion |
Examiner Tips and Tricks
Note that the details about the tissues and organs of the digestive system are assumed knowledge from the Key Stage 3 Science syllabus.
While you won't be examined directly on the content in this section, it provides essential background to aid understanding of the rest of this topic.
The importance of bacteria in digestion
The large intestine is home to hundreds of species of bacteria
These bacteria form a microbial ecosystem that play an essential role in human digestion of food by, e.g.:
breaking down substances we can’t digest
supplying essential nutrients
providing competition with any harmful bacteria to restrict their growth
Examiner Tips and Tricks
Note that the details about bacteria in digestion are assumed knowledge from the Key Stage 3 Science syllabus.
While you won't be examined directly on the content in this section, it provides essential background to aid understanding of the rest of this topic.
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