The mouth is the start of the digestion system. Food is ingested into the body here, and digestion begins immediately. One enzyme is present in the mouth. It is called amylase contained in the saliva, which is produced in the salivary glands. Amylase breaks down starch into maltose, a simple sugar, whilst the food is being chewed. It does not normally have time to complete this breakdown. Mineral salts are also present in saliva, and these ensure that the mouth is at the optimum pH for amylase to work. Chewing is a process that occurs in the mouth and it is very important in speeding up the rate that enzymes break down substances, both here and later on in the process. Every time you chew a piece of food it splits it in half, increasing the surface area. This means that the enzyme has more surface on which to act, speeding up the digestion process.
Food travels down the oesophagus from the mouth to get to the rest of the digestive system. The method in which it is transported is called peristalsis. In peristalsis the muscles around the oesophagus close behind the ball of food (called a bolus) to push it onwards.
The bolus then reaches the stomach. The stomach is the place in the digestive system where protein is digested. An enzyme called pepsin is responsible for breaking down proteins into polypeptides. Food stays in the stomach for 1-2 hours, allowing a lot of protein to be broken down. The stomach produces acidic (pH 1-2) gastric juices that provide the correct pH for the pepsin enzyme and also kill bacteria. The stomach also churns, mixing the gastric juices and broken down food, to form chyme. The chyme then leaves through a sphincter at the bottom of the stomach, into the duodenum.
The duodenum is the part of the small intestine near the stomach. Pancreatic juice and bile are mixed with the chyme coming from the stomach. Pancreatic juice is produced in the pancreas and enters the duodenum through the pancreatic duct. It contains the enzymes amylase, which finishes off the breakdown of starch to maltose, trypsin, which also breaks down proteins to polypeptides and lipases which break down emulsified fats into fatty acids and glycerol. Sodium hydrogencarbonate (NaHCO3) is also present, and that produces the optimum pH for this set of enzymes, as it is an alkali. As mentioned previously, the fats need to be emulsified to be broken down by lipases. Bile, which is produced by the liver and enters the duodenum through the bile duct does this. Bile also stains the food brown.
The ileum is the last point where food is digested. No enzymes are actually secreted into the ileum, they are present on the ileum walls. In addition to lipases, maltase, sucrase and lactase are present, and these break down complex sugars into simple sugars. Peptidases are also in the ileum, their substrate is polypeptides, produced by the protease enzymes, which are digested into amino acids. The products of all the enzymes are absorbed by the ileum wall, into the bloodstream. The remainder of the food continues into the colon (large intestine) where water is removed, stored in the rectum and then egested though the anus.
