Absorption, Assimilation and Egestion - Practice Questions & MCQ

• Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph. 
• It is carried out by passive, active or facilitated transport mechanisms. 
• Small amounts of monosaccharides like glucose, amino acids and some electrolytes like chloride ions are generally absorbed by simple diffusion. 
• The passage of these substances into the blood depends upon the concentration gradients.
• However, some substances like glucose and amino acids are absorbed with the help of carrier proteins. 
• This mechanism is called facilitated transport.
• Transport of water depends upon the osmotic gradient. 
• Active transport occurs against the concentration gradient and hence requires energy
• Various nutrients like amino acids, monosaccharides like glucose, electrolytes like Na+ are absorbed into the blood by this mechanism.
• Fatty acids and glycerol being insoluble, cannot be absorbed into the blood.
• They are first incorporated into small droplets called micelles which move into the intestinal mucosa. 
• They are re-formed into very small protein-coated fat globules called chylomicrons which are transported into the lymph vessels (lacteals) in the villi. 
• These lymph vessels ultimately release the absorbed substances into the bloodstream.
• Absorption of substances takes place in different parts of the alimentary canal, like mouth, stomach, small intestine and large intestine. 
• However, maximum absorption occurs in the small intestine.

Summary of Absorption in Different Parts of the Digestive System:

• The absorbed substances finally reach the tissues which utilise them for their activities.
• This process is called assimilation.
• The digestive wastes, solidified into coherent faeces in the rectum initiate a neural reflex causing an urge or desire for its removal. 
• The egestion of faeces to the outside through the anal opening (defaecation) is a voluntary process and is carried out by a mass peristaltic movement.