Digestion and Absorption - Notes | Class 11 | Part 3: Digestion and Absorption of Food



Digestion is the conversion of complex insoluble food materials into simple and absorbable form. It includes mechanical processes such as mastication (chewing), deglutition (swallowing) & peristalsis (wave-like movement of food bolus through the gut by muscular contraction).

·   Digestion in buccal cavity: Only starch digestion.

Ptyalin converts starch (polysaccharide) into disaccharides. About 30% of starch is digested by ptyalin.

·   Digestion in stomach: Stomach stores food for 4-5 hrs.

It is mixed with gastric juice by the churning movements and is converted into acidic pasty form (chyme).

The gastric lipase hydrolyses a small amount of lipids.

· Digestion in small intestine (in Duodenum): Chyme is mixed with succus entericus, pancreatic juice & bile juice.

a) Action of bile: Bile helps in digestion by emulsification (conversion of fat into micelles or tiny droplets). It provides large surface area for the action of lipase on fat. Bile also activates lipase.

b) Action of pancreatic juice: Amylopsin (Pancreatic amylase) hydrolyses remaining starch into disaccharides.

Enterokinase (Enterokinin) secreted by intestinal mucosa activates trypsinogen to active trypsin. Trypsin activates chymotrypsinogen & procarboxypeptidase.

c) Action of intestinal juice: At duodenum region, the intestinal enzymes act on the products of above reactions.

In large intestine, there is no significant digestive activity. The functions of large intestine are:

a.  Absorption of water, minerals and certain drugs.

b. Secretes mucus for adhering waste (undigested) particles together and lubricating it for an easy passage.

Fully digested semi fluid and alkaline food formed in small intestine is called chyle.

The digestive activities like gastric & intestinal secretions are controlled by neural and hormonal mechanisms. The sight, smell or presence of food in buccal cavity stimulate salivary secretions. Gastric & intestinal mucosa secretes digestive hormones. They control secretion of digestive juices.


Absorption is the transfer of end products of digestion through the intestinal mucosa into blood & lymph.

It is 2 types- passive and active.

a)   Passive absorption (Passive transport): Absorption of nutrients from higher concentrated region to lower concentrated region without the expenditure of energy.

It includes osmosis (absorption of water) and diffusion (absorption of solute molecules).

     Diffusion is 2 types:

  i.   Simple diffusion: In this, molecules alone can be diffused. E.g. Small amounts of monosaccharides like glucose, amino acids, vitamins, electrolytes like Cl- etc.

 ii.   Facilitated diffusion: Diffusion with the help of carrier proteins. E.g. glucose, amino acids etc.

b)   Active absorption (Active transport): Absorption of nutrients from lower concentrated region to higher concentrated region (i.e. against concentration gradient).

It needs energy. E.g. absorption of amino acids, monosaccharides like glucose, electrolytes like Na+ etc.

Absorption of lipids

- Monoglycerides, diglycerides and fatty acids cannot be absorbed directly as they are insoluble in water.

- Bile salts and phospholipids convert them into small spherical water-soluble droplets called micelles.

-  They are reformed into small protein coated fat globules (chylomicrons). They are transported into lacteals in the villi. From the lymph, the chylomicrons enter the blood.

Absorption in different parts of alimentary canal

·   Mouth: Certain drugs.

·   Stomach: Water, simple sugars, some drugs & alcohol.

·   Small intestine (mainly Jejunum & Ileum): All nutrients including minerals & vitamins.

It is the chief area of absorption due to the presence of villi, its great length and coiled nature.

·   Large intestine: Water, some minerals & drugs.

The absorbed materials are incorporated into tissues for their activities. It is called assimilation.

The undigested substances like plant fibres, dead bacteria etc. form faeces. It enters caecum through the ileo-caecal valve, which prevents back flow of faeces.

Faeces are temporarily stored in rectum and are eliminated through anus. It is called egestion (defaecation).
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