Medical brain notes 25

  Digestion   Introduction   The conversion of food into a form that can be absorbed by the body is called digestion. It describes how the body breaks down food and uses it for energy, cell repair and growth. It starts in the mouth, continues in the stomach and small intestine and is completed in the large intestine. The liver and pancreas add enzymes and juices that aid in this process. Carbohydrates are broken down to glucose, proteins to amino acids, fats to glycerol and fatty acids.

  Carbohydrates   The major carbohydrates present in our diet are starch, glycogen, sucrose, lactose , maltose and very little concentrations of fructose and pentose.   1. Digestion in mouth   Milk and other fluid items like juices escape digestion in the mouth as they do not reside in the mouth for a longer time, whereas, starch and glycogen containing solid foods are masticated with saliva thoroughly. Saliva contains ptyalin, an a amylase, which attacks the a 1-4 linkages resulting in the formation of monosaccharide glucose, disaccharide maltose and trisaccharide maltotriose. However, because of steric hindrance caused by the branches, some of the interior a 1-4 linkages are inaccessible for the enzyme. This results in the formation of a highly branched structure called as limit dextrin.   The optimum pH for salivary amylase is pH 6.7. Ptyalin needs chloride ions for their effective action.         Glycogen, Starch  ----...

  Proteins   Proteins in diet are from animal sources and vegetable sources. Animal sources like milk, dairy products, meat, fish, liver and eggs are rich sources of proteins.   Vegetable sources like cereals, pulses, peas, beans and nuts are rich in protein.   1. Digestion in mouth   There are no enzymes in mouth to degrade the protein.   2. Digestion in stomach   HCl : HCl secreted by the gastric mucosa destabilizes the secondary structures of the proteins such that it can be easily acted upon by the enzymes.   The proteolytic enzymes present in the gastric juice are pepsin, rennin, gastricin and gelatinase.   Pepsin : It is a potent proteolytic enzyme and is present in the gastric juices. It issecreted in the inactive zymogen form called as pepsinogen, which has a molecular weight of 42,500 daltons. In the acidic medium, pepsinogen is cleaved to pepsin and the reaction is favoured autocatalytically. Pepsin having a molecular weight of 34,5...

  Digestion of Lipids   As all the enzymes involved in digestion are water soluble, lipids pose a special problem due to their insoluble nature. The problem of fat digestion is solved by emulsification of fats (i.e). breaking of large fat particles to small particles such that there is an increase in the surface area, which facilitates the interaction of fats with the fat hydrolyzing enzyme lipase.   Dietary sources of fat are from animal as well as vegetable origin. Animal sources include dairy products like milk, butter, ghee, meat, pork, eggs and fish. Vegetable sources include various cooking oils. Vegetables sources are superior to animal sources because of the presence of the various polyunsaturated fatty acids.   1.  Digestion in mouth   Recently, a lingual lipase has been detected in mouth. Its optimum pH is 4 to 4.5. It acts on the food in the stomach, where it resides for some time. Lingual lipase is best suited to act on milk.   2.  Dig...