The small intestine is in the gastrointestinal tract. It is
an organ between the stomach and the large intestine. It is approximately 23ft
long and is made up of three structural parts; the duodenum, jejenum and ileum.
The small intestine is mainly involved in the digestion and
absorption of nutrients. It receives pancreatic secretions and
bile through the hepatopancreatic duct which also supports with its functions.
Digestion is a process in which ingested food is chemically
broken down into absorbable molecules. During absorption there is movement of
nutrients, water and electrolytes from the lumen of the small intestine into
the cell, then into the blood.
The three carbohydrate products which are absorbed
by the small intestine include; glucose, galactose and fructose.
Digestion of starch begins in the mouth, by salivary
amylase. Much of carbohydrate digestion occurs in the stomach and duodenum. The
main enzyme is pancreatic amylase, which produces disaccharides from
starch by digesting the 1-4 glycosidic bonds. The disaccharides produced are
all converted to glucose by brush border enzymes. The disaccharides that naturally
occur in food do not require amylase to break them down. Brush border enzymes
(lactase, sucrase, trehalase) hydrolyse these compounds into molecules of
glucose, galactose and fructose.
Glucose and galactose are both absorbed across the apical
membrane by secondary active transport (along with Na+) through the
Sodium-Glucose cotransporter (SGLT1). Both glucose and galactose exit the cell
via GLUT2 receptors across the basolateral membrane into the blood.
Fructose enters the cell by facilitated
diffusion via GLUT5 and is transported into the blood via GLUT2
Digestion of protein begins in the stomach with the action
of pepsin, it breaks down protein into amino acids and oligopeptides.
The process of digestion is completed in the small intestine with brush border
and pancreatic enzymes. They split the oligopeptides into amino acids,
dipeptides and tripeptides.
Amino acids are absorbed through a Sodium cotransporter, in
a similar mechanism to the monosaccharides. They are then transported across
the basolateral membrane through facilitated diffusion. Di and
tripeptides are absorbed via separate H+ dependent cotransporters and
once inside the cell are hydrolysed to amino acids.
Lipids are hydrophobic in nature, and therefore are poorly
soluble in the aqueous environment of the digestive tract. Lipid digestion is
started by lingual and gastric lipases, but this only digest 10% of
The remaining lipids are digested in the small intestine.
The fat goblets are emulsified by bile into smaller chunks, called
micelles, which have a much larger surface area.
Pancreatic lipase, phospholipase
A2 and cholesterol ester hydrolase (3 major enzymes involved in
lipid digestion) hydrolyse the micelles, breaking them down
into fatty acids, monoglycerides, cholesterol and lysolecithin.
The products from digestion are released at the apical
membrane and diffuse into the enterocyte. Inside the cell, the products are re-esterified to
form the original lipids, triglycerides, cholesterol and phospholipids. The
lipids are then packaged inside apoproteins to form a chylomicron.
The chylomicrons are too large to enter circulation, so they enter lymphatic
system via lacteals.