A level Biology exam revision resources written by A level Examiners


The large intestine is made up of the following parts:

  • Appendix
  • Colon
  • Rectum
Figure 6
small intestine

The appendix is sack-like structure at the junction of the small and large intestines. The appendix is vestigial - i.e. it has no function. The colon and rectum are two parts of a muscular tube that contains large amounts of bacteria. Peristalsis moves the gut contents along the colon, and also compacts faeces in the rectum.

The wall of the colon secretes mucus that lubricates the colon, and prevents damage as the gut contents pass along. All the digestion of food is complete before the colon, however there is further breakdown of food here because the bacteria contained in the colon breaks down undigested food. These bacteria synthesise vitamins B and K, which are absorbed.

Up to this point the various secretions have all added water to the gut contents (saliva, gastric juice, intestinal juice and pancreatic juice). The gut contents are very watery, most of this water is absorbed in the colon.

The faeces formed in the colon are stored in the rectum then voided via the anus. The main components of faeces are:

  • undigested food
  • bile pigments (these give faeces it's characteristic brown colour)
  • bacteria
  • dead cells from the small intestine.

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