Human digestive system is a pipe-like structure which starts from the mouth and ends in anus. In between there are some organs like stomach, small intestine and large intestine where digestion of foods occurs. There are some glands which help in digestion e.g. salivary gland, liver and pancreas.
Digestion means breaking down the food into those molecules that can be absorbed by the body.so if we divide food into the macro categories they will be 1)carbohydrate, 2)protein and 3)fat.
In carbohydrates, there can be these types of bonds –
Starch and glycogen : starch and glycogen are storage polymers of glucose in plants and animals respectively. In starch and glycogen the glucose molecules are bonded by glycosidic bonds ( alpha 1-4 glycosidic bond and alpha 1-6 glycosidic bond). Our body cannot absorb the big molecules as starch, so it has to be broken to free glucose molecules for absorption. Amylase enzyme is released by the salivary gland and pancreas can break these bonds of starch and glucose.
Lactose : Lactose is a disaccharide mainly found in milk. Lactose is disaccharide of glucose and galactose by glycosidic bond. It can be broken by lactase brush border enzymes in the wall of the small intestine.
Sucrose : Sucrose is a disaccharide of glucose and fructose and it is found in fruits, vegetables and the sugar we use is also sucrose. Sucrose can be broken down into glucose and fructose by brush border enzyme sucrase present in the inner lining of the small intestine.
Maltose : Maltose is a disaccharide of two glucose molecules. It is also known as malt sugar found in barley, wheat, corn, etc several grains. It can be broken down by amylase (released by salivary gland and pancreas) , maltase brush border enzyme in small intestine lumen.
Protein : Protein is made up of amino acids bonded by peptide bonds. The bonds need to be broken in order to be absorbed in the bloodstream. Protein digestion occurs in the stomach and small intestine. Stomach gastric gland releases pepsinogen which is a precursor of pepsin enzyme. Protein digesting enzymes are released as zymogens so that they do not digest their own glands and tissues. Zymogens need to be activated in the right environment. Pepsinogen is released by stomach chief cells and it is converted into active enzyme pepsin in the acidic environment of the stomach. HCL released by parietal cells of the stomach lowers the ph of the stomach environment. Other protein digesting zymogens are released by pancreas and those are trypsinogen, chymotrypsinogen and procarboxypeptidase. Trypsinogen is converted into active enzyme trypsin by a brush border enzyme enterokinase present on the wall of the small intestine. Then trypsin further activates chymotrypsinogen to chymotrypsin and trypsin also activates procarboxypeptidase to carboxypeptidase. Trypsinogen, chymotrypsin and carboxypeptidase all of them contribute to protein digestion in the small intestine.
Fat or lipid : The lipid is made up of one glycerol backbone with several fatty acids attached to it by ester bonds. The whole lipid cannot be absorbed by the body. So it has to be broken down into free fatty acids and glycerols. The enzyme that can break down lipids is lipase enzyme. Lipase is found in the mouth and also secreted by the small intestine. The lipase found in the mouth part of the digestive tract is secreted by the minor salivary gland Von Ebner gland. Von ebner gland is situated in the oral cavity towards the back side of the tongue. This lipase is called lingual lipase. The main digestion of lipids occurs in the small intestine. Pancreas exocrine glands release lipase enzyme which is secreted in the small intestine. Another main factor that helps in lipid digestion intensively is Bile. Bile is secreted by the liver and stored in gallbladder then released in the small intestine. Bile contains bile salt. Bile salt is a surface active agent. It reduces the surface tension of large lipid globules and thus works as a dispersant. The large lipid globule breaks down into smaller droplets due to decreased surface tension caused by bile salt. The breakdown of larger lipid globules into smaller droplets is highly beneficial for lipid digestion as the smaller droplets of lipids provide larger surface area for the lipase enzyme to work. The bile salt works as an emulsifier and prevents the coalescence of smaller droplets to larger one. So the bile salt helps to maintain the small lipid droplets to help digestion of lipids by lipase.
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