16. DIGESTION AND ABSORPTION
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.
ABSORPTION
OF DIGESTED PRODUCTS
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.