Digestion is the breaking down of large insoluble food molecules into small soluble molecules that the body can absorb. The main organs involved with digestion are mouth, gullet, stomach, small intestine, large intestine and rectum. The digestive process involves:
- mechanical break down by chewing
- chemical breakdown using enzymes and acid. Enzymes are biological agents that speed up reactions.
The main chemicals (nutrients) in food are proteins, carbohydrates and fats which the digestive system turns into amino acids, sugars (such as glucose and maltose), glycerol and acids.
- proteins are digested by enzymes from the stomach, pancreas and small intestine into amino acids
- carbohydrates are digested by enzymes from the mouth, pancreas and small intestine into sugars
- fats are digested by enzymes from the pancreas and small intestine into glycerol and acids
Digestion begins in the mouth with the breaking down of the food by chewing and by the amylase enzyme contains in saliva which digest the large starch molecules. The food is swallowed down the gullet (also called the oesophagus) to the stomach where hydrochloric acid and protease enzyme break down large protein molecules. The hydrochloric acid kills the bacteria as well as provide the low pH for the protease enzyme to work. This partly digested food is passed into the small intestine where enzymes from the pancreas continue the digestion into small molecules:
- carbohydrases convert starch into simple sugars (glucose and maltose)
- protease converts proteins into amino acids
- lipase converts fats into fatty acids and glycerol
The small molecules are sbsorbed through the lining of the gut into the bloodstream, and the blood transports these small molecules around the body where they are pass into the cells and used. The small intestine is lined with millions of villi that protude out to absorb the small molecules into the blood. Villi have a very thin outer layer of cells, a good supply of blood and a large surface area for absoption.
Undigested food such as cellulose (plant material that humman can't digest) is passed into the large intestine where water is absorbed and faeces are formed. The faeces are stored in the rectum and egest out of the body through the anus (known as egestion).
Egestion is getting rid of primarily unused materials such as faeces. Excretion is getting rid of metabolic (chemical) wastes such as urine, sweat and CO2.