Download Digestion of carbohydrate in small intestine

‫كلية الطب‬
‫المحاضرة السابعة‬
) GIT) physiology
‫المرحلة الثانية‬
Disorders of the Small Intestine:1-Malabsorption or “sprue''
nutrients are not adequately absorbed from the small intestine even though the food
has become well digested. Can occur when large portions of the small intestine have been
removed or Several diseases can cause decreased absorption by the mucosa like:A- Nontropical Sprue:One type of sprue called idiopathic sprue or(celiac disease) occur in children, its
autoimmune disease occur genetically ,or called gluten enteropathy in adult, results from
the toxic effects of gluten present in certain types of grains, especially wheat and rye.
Only some people are susceptible to this effect, gluten has a direct destructive effect on
intestinal enterocytes. This case either milder forms ,only the microvilli are destroyed, or
more severe forms, the villi themselves disappear ,this will lead to reducing the absorptive
area of the GIT tract. Removal of wheat and rye flour from the diet frequently results in
cure within weeks, especially in children with this disease.
B-Tropical Sprue.
A different type of sprue called tropical sprue frequently occurs and can often be treated
with antibacterial agents. Even though no specific bacterium has been implicated as the
cause, it is believed that this variety of sprue is usually caused by inflammation of the
intestinal mucosa resulting from unidentified infectious agents.
Digestion
I t is chemical breakdown of ingested food into absorbable molecules, The digestive
enzymes are secreted by salivary gland, gastric gland, pancreatic glands, and apical
membrane of intestinal epithelial cells.
Digestion of carbohydrates:Three major sources of carbohydrates exist in the normal human diet. These are sucrose
(table sugar) disaccharide, lactose (disaccharide) in milk and starches (polysaccharides)
present in all non animal foods, and grains.
Other carbohydrates ingested are glycogen, alcohol . The diet also contains a large amount
of cellulose, which is a carbohydrate, however, no enzymes capable of hydrolyzing it ,
cellulose are secreted in the human digestive tract. cellulose cannot be considered a food
for humans.
Digestion of carbohydrates in the mouth:When food is chewed, it is mixed with saliva, which contains the digestive enzyme ptyalin
(an a-amylase) secreted mainly by the parotid glands. This enzyme hydrolyzes starch into
the disaccharide maltose and other small polymers of glucose that contain three to nine
glucose molecules, the food remains in the mouth only a short time, 5 % of all the starches
will have become hydrolyzed by the time,when the food is swallowed..
Digestion of carbohydrate in the stomach:
In stomach active of salivary ptyalin can continue for as long as1 hour after the food has
entered the stomach untiI the contents of the body and fundus are mixed with stomach
secretions. The activity of salivary amylase is blocked by acid of gastric secretions. 30-40 %
of the starches will have been hydrolyzed mainly to maltose.
Digestion of carbohydrate in small intestine:
Pancreatic secretions contain a large quantities of a-amylase that continue splitting
starches into maltose and other small polymers of glucose. the carbohydrates are
converted into maltose and very small glucose polymers before passing the duodenum or
upper jejunum.
Hydrolysis of disaccharide and small glucose polymers: the brush border epithelial cells
lining the small intestine contain enzymes lactase, sucrase, maltase and a-dextrinase which
are capable of splitting the disaccharide lactose, sucrose and maltose into their
monosaccharide, Lactose splits into galactose and glucose. Sucrose splits into fructose and
a glucose. Maltose and other small glucose polymers all split into multiple molecules of
glucose ,which are absorbed and delivered to the liver by way of the hepatic portal vein.
After the liver processes, the nutrients enter into the blood stream circulating throughout
the body .
Digestion of carbohydrate
Lactose intolerance:
It is the most common cause of carbohydrate malabsorption. It result from inability of the
intestinal mucosa to produce lactase in the infant. The diarrhea produce dehydration can
be life threatening.
Digestion of protein:
The dietary proteins are derived from meats and vegetables.
Digestion of proteins in stomach:
Stomach secret pepsinogen that is converted to pepsin in the presence of HCL. Pepsin
hydrolysis the peptide linkages between the amino acid. pepsin digest collagen, an
albuminoid type of protein that is affected little by other digestive enzymes. Collagen is
a major constituent of the intercellular connective tissue of meats. Gelatinase other
enzyme found in the stomach that liquefies gelatin. Pepsin digestion represents 10-30 %
of total protein digestion.
Digestion of protein by pancreatic secretion:
Most protein digestion occur in small intestine under proteolytic enzymes of the
pancreatic secretion. Trypsin, chemotrypsin, and carboxypolypeptidase which split protein
molecules into small polypeptides.
Digestion of peptides by epithelial peptidase of small intestine:
The brush border of small intestine contains several different enzymes for
hydrolyzing the peptides linkage of the remaining dipeptides and other small
polypeptides as they come in contact with epithelium of the villi.The enzymes
responsible areaminopolypeptidase and several dipeptidase. About 98 % of all the
proteins finally become either amino acids or dipeptides that can be absorbed
into the blood. All of these smaller protein fragments go directly to the liver by
the hepatic portal vein. Once in the liver one of three things happens to the
proteins:
1. It converts to glucose
2. It converts to fat
3. It is directly released into the blood as amino acids.
Protien digestion
Digestion of fat:
Dietary lipids include triglycerides, cholesterol and phospholipid. The lipids must be
solubilized to be digested and absorbed. small amount of triglycerides is digested in the
stomach by lingual lipase that is secreted by lingual glands in the mouth and swallowed
with the saliva. This amount of digestion is less than 10 % and generally unimportant
.The first step in fat digestion is physically to break the fat globules into very small sizes
which becomes water-soluble digestive enzymes can act on it. This process is called
emulsification of the fat, begins in the stomach when fat mixing with stomach digestion.
Most fat digestion begin in duodenum by pancreatic lipase which is most important one
and enteric lipase, lipase act on fat has been emulsified (fat droplet breakdown into
small sizes by (bile) so that become water soluble. Both the cholesterol esters and
phosphlipids are hydrolyzed by lipase in the pancreatic secretion. The bile salt play role
in converted cholesterol into monoglycerides and fatty acids, which is essential to
absorption of cholesterol. However 60 % of triglycerides can be digested and absorbed
even in the absence of bile salt. Pancreatic lipase hydrolyzes triglyceride molecules to
one molecule of monoglyceride and two molecules of fatty acid. When foods with high
lipid content enter the stomach, the hormone – gastric inhibitory peptide is released,
slowing down movement flow out of the stomach. This is why we feel full after eating
high fat foods.
Absorption
It is movement of nutrients, water and electrolytes from the lumen of the
intestine into the blood. There are two path for absorption, a cellular path and
paracellular path. In the cellular path the substance must cross the apical (luminal)
membrane, then enter the tight junction intestinal epithelial cell, and be extruded
from the basolateral membrane. In paracellular, through intercellular spaces
between intestinal epithelial cells, and to the blood. The structure of intestinal
mucous is suited for absorption of large quantities of nutrients though villi and
microvilli which increase the surface of a small intestine. The villi are largest in the
duodenum, where digestion and absorption occur and shortest in the terminal
ileum.. The villi and microvilli increases total surface area by 600 fold. The epithelial
cells of small intestine are replaced every 3-6 days.
Absorption of carbohydrates:
Glucose and galactose are absorbed in separate mechanisms involving Na-depend
cotransport. against an electrochemical gradient by secondary active transport
,while Fructose is absorbed by facilitated diffusion.
Na-glucose cotransport and Na-galactose cotransport, using a gradient as the energy
source on basolateral membrane because the intracellular Na+ concentration is low
in intestinal cells ,as it in other cells ,Na+ moves into the cell along gradient. Glucose
and galactose are extracted across the basolateral membrane into blood by
facilitated diffusion. Fructose is transported across luminal membrane by facilitated
diffusion, then extruded into blood. Figure(13)
lumen
Epithelial cell of the small intestine
blood
Figure(13)absorption of monosaccharide
Absorption of protein:
 Amino acid are transported from the lumen into the cell by Na-amino acid
cotransporter in apical membrane energy by a gradient.
 There are four separation cotransporters; each one for neutral, acidic, basic, and
other amino acids.
 The amino acids then are transported across the basolateral membrane into the
blood by facilitated diffusion.
 Again by separation mechanism most ingested protein is absorbed by intestinal
epithelial cells in dipeptides and tripeptide form and free amino acids.
 Inside the cell, most of dipeptides and tripeptides are hydrolyzed to amino acid by
cystosolic peptidases,

the remaining dipeptides and tripeptides are Absorbed unchanged. Figure(14).
lumen
Epithelial cell of the small intestine
blood
Figure(14)absorption of protein
Absorption of lipid
1- the product of lipid digestion are solubilized in the intestinal lumen in mixed micelles
except glycerol, which is water soluble.
The outer layer of micelle which is cylindrical shape is composed of bile acids.
2- The micelles diffuse to apical membrane of the intestinal epithelial cells. The lipids are
released from the micelle diffuse down concentration gradients into the cell. The micelles
do not enter the cell.
The bile acid are left behind in the intestinal lumen. Most of the digested lipid is
absorbed by the mid jejunum. The work of bile acid is completed in side the intestine
before they are returned to the liver via entrohepatic circulation .
3- The product of lipid (fatty acids) are re-esterified on the smooth endoplasmic reticulum
to form the original ingested lipid, triglyceride, cholesterol ester and phospholipids.
4- Inside cells, lipids are packaged with lipid-carrying particle called chylomicrons. The
chylomicrons , with an average diameter of 1000 A°, phospholipid cover 80 % of outside of
it, and remaining 20 % of surface covered with apoproteins which are synthesized by
intestinal epithelial cells, are essential for absorption of it.
5- The chylomicrons are packaged in secretary vesicles on the Golgi apparatus. The
secretary vesicles migrate to the basolateral membranes and there is exocytosis of the
chylomicrons. They are too large to enter vascular capillaries, they can enter the
lymphatic capillaries. The lymphatic circulation carries them to the thoracic duct which
empties into the blood stream. Figure (15)
lumen
Epithelial cell of small intestine
Figure (15)absorption of lipid