Digestion is the breaking down of macro food molecules into smaller components, to a form that can be, for example absorbed, into the blood stream. There are two types of digestion: mechanical digestion and chemical digestion. First, I will explain the mechanical digestion, which is the first process that happens in the overall all digestion activity.
It is important to note that mechanical digestion isn't just another process we need to complete, there is a specific and important purpose for it; the purpose of mechanical digestion is essentially to improve the access of digestive enzymes. In mechanical digestion the food goes into the mouth, which is broken down into molecules by the teeth. After this happens, the broken down food goes through the esophagus, that uses the peristalsis (muscle) which contracts and pushes the food into the stomach. Inside the stomach, muscles work to transform the food into chyme (partly digested food).
During and subsequent to mechanical digestion, chemical digestion takes place. I say during mechanical digestion because at the same time the food is being chewed in the mouth, the salivary glands produce saliva, in the saliva, salivary amylase is present; the salivary amylase serves to catalyze the hydrolysis of starch into simpler compounds, like the disaccharide maltose. Also, at the same time the muscles of the stomach contract to make chyme, the lining produces HCI and pepsin to digest the proteins into polypeptides. Proceeding, the chyme is moved into the small intestine. In the small intestine numerous tasks occur; to make it easier I will list them.
1. The CCK produced in the lining of the small intestine goes into the pancreas. In doing so, the pancreas releases pancreatic amylase, pancreatic lipase, and pancreatic trypsin. Each of these three substances released have a task. The pancreatic amylase cuts starch into disaccharides, the pancreatic lipase cuts triglycerides, and the pancreatic trypsin cuts the polypeptides into dipeptides.
2. The intestinal lining produces sucrose, maltase, and lactase. These released substrates are then digested into monosaccharides (fructose, glucose, galactose).
3. The CCK produced in the lining of the small intestine goes to the gallbladder (in the liver); the gallbladder stores bile, so as the CCK reaches it, bile is released. Then, when the bile is released into the small intestine, the bile emulsifies the fats to act (work/ react) on the triglycerides to become fatty acids and glycerol.
4. The blood flowing along the lining of the small intestine absorbs fatty acids with the help from the lacteals. This blood is flowing in villi, which absorbs amino acids and monosaccharides (and some mineral and vitamins). What helps the function of absorption is that the villus intestinal wall has many folds, increasing the surface area, the surface of villus is close to the blood vessels so materials can easily diffuse, and the villus wall consists of single layer of cells.
Once all these processes occur in the small intestine, the digested food is moved into the large intestine. In the large intestine the following happens:
1. Blood absorbs some water, vitamins, and minerals
2. The rectum stores the rest of water, cellulose, and enzymes that weren’t absorbed
And finally…the rest is released through the anus!
I personally had a very hard time in understanding the whole digestion process in terms of chemical actions that occur, like the breaking down amylase, protease, and lipase. After going back to my notes to write this blog post I believe I have a more clear understanding on what happens chemically in digestion. However, I still think that I have to commit myself into studying this aspect because I know I don't have a full understanding and comprehension.
