Short notes on the digestion of carbohydrates in man
The food of human beings consists of mainly two types of carbohydrates namely- polysaccharides and disaccharides. As both these are complex molecules they require to be broken down into simple substances before they can be absorbed by the cells of the small intestine. The normal polysaccharides in the human diet include starch, cellulose, dextrin and glycogen.
Among the disaccharides, the most important ones are the ordinary sugar i.e., sucrose and lactose or milk sugar. In the human digestive system there are no enzymes for the digestion of the cellulose hence it passes out of the large intestine along with the faecal matter.
Many of the herbivorous animals like cattle, have enzymes to digest cellulose. The carbohydrates are digested by enzymes such as carbohydrases to monosaccharides. The process of carbohydrate digestion is initiated in the mouth itself and is completed in the small intestine.
In the buccal cavity, at the contact of food materials, the salivary glands secrete the enzyme salivary amylase or ptyalin. Ptylain breaks starch, glycogen and other carbohydrates into disaccharides like maltose. Further digestion of carbohydrates does not take place in the buccal cavity as food is retained here for a very short time.
The digestion of carbohydrates in the buccal cavity is more mechanical than chemical. However it has been estimated that about 30% of starch is hydrolysed in the mouth. The process of breaking up of the carbohydrates is called hydrolysis because the bonds between the units of sugars are broken down by the addition of water (hydro=water, lysis=breakdown).
There is no digestive process in the pharynx or the oesophagus. Further digestion begins when the carbohydrates reach the stomach. The salivary digestion however continues until the ptyalin enzyme is denatured by the acid of the gastric juice.
The hydrochloric acid present in the gastric juice helps partly in the digestion of carbohydrates. But there are no carbohydrate digesting enzymes in the gastric juice. The food then reaches the intestine. In the intestine two juices act on the carbohydrates. These are the pancreatic juice and intestinal juice.
The pancreatic juice produced in the pancreas by the influence of a harmone called secretin consists of carbohydrate digesting enzymes. The main enzyme is the pancreatic amylase. Under the influence of this enzyme starch, glycogen and others get broken down into maltose and other disaccharides. The intestinal juice has six carbohydrases. These are
1 .Intestinal amylase which digests the remaining starchy material to disaccharides.
2. Maltase. This splits the disaccharides maltose to monosaccharide, glucose.
3 .Isomaltase breaks isomaltose to glucose.
4. Sucrase breaks sucrose into glucose and fructose.
5. Limit dextrinase breaks up dextrines into glucose.
6. Lactase hydrolyses lactose into glucose and galactose.
As a result of this, the carbohydrates get completely digested into their monosaccharide units such as glucose, fructose and galactose. There is no mechanism for the digestion of cellulose in the human intestine. As a result of this, cellulose in unaltered form goes out through the faecal matter.