There are two types of digestion: mechanical and chemical. Mechanical digestion is when food is ground up using the teeth. Another example of mechanical digestion is when the food is churned by the movements in the alimentary canal. Chemical digestion is the breaking down of food using enzymes. Enzymes break large food molecules into smaller ones. As food is broken down, it needs to be transported through each part of the digestive system. To get from the mouth to the stomach, the food has to travel down the oesophagus. It is not gravity that pulls the food down, it is waves of peristalsis. Peristalsis is the contraction and relaxation of circular muscles in the wall of the gullet. Peristalsis is also used in the stomach to break up the chunks of food into a mush as part of mechanical digestion.

The point of digestion is to break down large, insoluble food molecules into smaller, soluble ones that can be easily absorbed into the bloodstream. Enzymes help to do this. There are different types of enzymes which each break down different food substances. Carbohydrates are broken down by the enzyme carbohydrase. It is found in saliva in the mouth and in the pancreatic juice that is poured into the small intestine from the pancreas. It breaks down starch molecules into glucose molecules. Proteins are broken down by the enzyme, protease. This is found in the pancreatic juice and also in gastric juice which is found in the stomach. This enzyme breaks protein molecules into amino acid molecules. Fats and oils, or lipids, are broken down by the enzyme lipase. This is found in the pancreatic juice. Lipase breaks fat molecules into fatty acid and glycerol molecules.

The juices that are found in the digestive system contain enzymes to break down food molecules. However, different enzymes work more efficiently when they are at their different optimum pHs. In the gastric juice found in the stomach, there is an enzyme called protease. The juice also contains hydrochloric acid to kill bacteria and to make the juice acidic; the optimum pH for protease. The enzymes that break down the food in the small intestine, however, work more efficiently in an alkaline. Therefore, a juice called bile is added. It is made by the liver and is stored in the gall bladder. It also emulsifies fats.

Once food has been digested, it needs to be absorbed into the blood and then taken to the cells. It is absorbed into the blood by diffusion. Diffusion is the movement of molecules from a high concentration to a low concentration. The part of the small intestine where this takes place is the ileum. It is specially adapted to allow rapid diffusion to take place:  It is very long to allow time for digestion and absorption.  It has a very large surface area, giving a bigger surface through which diffusion can take place. This is because it has villi and microvilli. These are finger like projections.  The walls of the villi are only one cell thick. This means that the food molecules can easily travel through it. Different food types pass into different places in the villi. Digested carbohydrates and proteins pass into the blood vessel in the villi, and are then transported in the circulatory system to all the cells of the body. Digestion fats (or lipids), however, pass into the lymph vessel to be stored under the skin.

Food that cannot be digested is passed into the large intestine. The first part of the large intestine is the called the colon. This is where the water in the large intestine is absorbed back into the blood, making the remaining food in the large intestine into semi solid faeces. This is stored in the part of the large intestine called the rectum. Finally, the faeces are passed out of the body altogether through the last part of the digestive system: the anus.

Lymph vessel Mouth Small intestine (ileum) Large intestine Stomach Blood vessel Oesophagus Small intestine (duodenum) RectumAnus Digested foodIndigestible food FatsCarbohydrates and proteins

CARBOHYDRATES FATS + OILS PROTEINS VITAMINS + SALTS WATER Glucose and simple sugars Glycerol and fatty acids Amino acids Keeping the body in good health Many uses in the body excess Excess stored in liver as glycogen Energy produced in cell respiration Stored under the skin Form proteins in the body Excess broken down in liver glucose urea Passed to the kidneys by blood for removal as urine Some out in sweat (C.H.O.) lipase (H2O) (various elements) (C.H.O.N.) protease (C.H.O.) carbohydrase