Digestion and Absorption – The Process of Egestion | NCERT Solutions for Class 11 Biology
NCERT Biology Class 11 : The small intestine leads into the large intestine, which is wider as compared to the small intestine. At the junction of the small and the large intestine, there is a sac-like structure that is known as caecum, and a blind tube called the appendix. In herbivores, the caecum stores food material where bacteria are able to break down the cellulose. This function no longer occurs in the human cecum, so in humans, it is simply a dead-end pouch forming a part of the large intestine. The appendix also has no specific function in food digestion, but in humans, the appendix may get contaminated or infected with germs or foreign bodies. This causes severe inflammation and pain in the appendix and leads to appendicitis. All the digested food contents are absorbed in the small intestine, but the remaining unabsorbed parts are transferred in the large intestine, which consists of largely cellulose, live and dead bacteria, and dead cells from the lining of the alimentary canal, mucus, bile pigments, small amounts of nitrogenous wastes, and the water. As materials pass along the colon, water is absorbed from it. The unabsorbed matter for example bacteria etc. become more solid. The solid matter then passes on to the rectum (the final section of the large intestine, terminating at the anus) as faeces. The faeces are expelled out through the anus by the process of egestion.
Egestion is the discharge or expulsion of undigested material (food) from a cell in case of unicellular organisms, and from the digestive tract via the anus in case of multicellular organisms.
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Mechanism of Egestion:
Peristalsis (muscular contractions and relaxations that take place in the digestive tract), gradually pushes the indigestible materials of the small intestine into the large intestine or colon. Normally 1500 ml of chyme passes into the large intestine per day. The colon absorbs most of the water. It also absorbs electrolytes, including sodium and chloride from the chyme.
The epithelial cells of the colon also excrete certain salts such as iron and calcium from the blood. Escherichia coli (bacterium) live in the colon which feeds on the undigested matter. This bacterium, in turn, produces vitamin B12 (cobalamin), vitamin K, vitamin В1 (thiamine) and vitamin B2 (riboflavin) which are absorbed by the wall of the colon. Consequently, the chime converts into semisolid faeces.
The faecal matter is stored in the rectum, the lower part of the large intestine, as the pellets of faeces enter the rectum; distension of rectal wall induces the feeling of defecation due to a “defecation reflex”. This reflex initiates peristalsis in the last part of the colon (sigmoid colon) and the rectum, forcing the faeces towards the anus. As the faeces reach anus, involuntary relaxation of the internal anal sphincter and voluntary relaxation of external anal sphincter cause defecation. Defecation is the final act of digestion, by which organisms eliminate solid, semisolid, or liquid waste material from the digestive tract via the anus.
The faeces consist of about three-fourth water and one- fourth solid matter. Of the solid matter are about 3 percent bacteria, 10 to 20 percent fat, 2 to 3 percent protein, about 15 percent inorganic matter and 30 percent undigested roughage and dry constituents of digested juices. Dead mucosal cells, mucus, and cholesterol also occur in the faeces. Its brown color is due to brown pigments, stercobilinogen, and stercobilin, which are derivatives of bilirubin.
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