Carbohydates

1. DIGESTION AND ABSORPTION OF CARBOHYDRATES
DIGESTION OF CARBOHYDRATES
 Digestion is a process involving the hydrolysis of large and complex organic
molecules of foodstuffs into smaller and preferably water-soluble molecules
which can be easily absorbed by the GIT for utilization by the organism.
 Digestion of macromolecules also promotes the absorption of fat soluble
vitamins and certain minerals.
 The principal dietary carbohydrates are polysaccharides (starch, glycogen)
disaccharides (lactose, sucrose) & monosaccharides (glucose, fructose)
 The digestion of carbohydrates occurs in the mouth & intestine
 The hydrolysis of glycosidic bonds is carried out by a group of enzymes called
glycosidases.
DIGESTION IN MOUTH
 Saliva contains carbohydrate splitting enzyme salivary amylase (ptyalin)
ACTION OF SALIVARY AMYLASE
 It is α – amylase, requires Cl- ions for activation & optimum pH 6.7 (6.6 to 6.8)
 Salivary amylase hydrolyses α 1-4 glycosidic bonds of polysaccharides,
producing smaller molecules maltose, glucose & trisaccharide, maltotriose.
 Salivary amylase action stops in stomach when pH falls to 3.0.
DIGESTION IN STOMACH
 No carbohydrate splitting enzyme in gastric juice.
 Some dietary sucrose may be hydrolysed to equimolar amounts of glucose &
fructose by HCL.
DIGESTION IN DUODENUM
 Food bolus in duodenum mixes with pancreatic juice.
 Pancreatic juice contains pancreatic amylase, similar to salivary amylase.
ACTION OF PANCREATIC AMYLASE
 It is an α-amylase, optimum pH 7.1, requires Cl- ions.
 It specifically hydrolyzes α1-4 glycosidic bonds & not on a1-6 bonds.
 It produces disaccharides (maltose, isomaltose) & oligosaccharides.
 The final digestion of di- & oligosaccharides to monosaccharides primarily
occurs at the mucosal lining of the upper jejunum.
DIGESTION IN SMALL INTESTINE
ACTION OF INTESTIAL JUICE
 Intestinal amylase: It hydrolyses terminal a 1-4-glycosidic bonds in
polysaccharides & oligosaccharides, liberating free glucose.
 Lactase: It is β-galactosidase, its pH range 5.4 to 6.0.
 Lactose is hydrolysed to glucose & galactose.

2.  Isomaltase: It catalyses a 1-6 glycosidic bonds, branching points, producing
maltose & glucose.
 Maltase: It hydrolyses a 1-4-glycosidic bonds between glucose units in maltose
& its pH range is 5.8 to 6.2.
 Sucrase: It hydrolyses sucrose to glucose & fructose &Its pH range is 5.0 to
7.0.
ABSORPTION OF CARBOHYDRATES
 Carbohydrates are absorbed from the small intestine mainly as
monosaccharides, viz. glucose, galactose and fructose.
ABSORPTION OF GLUCOSE
 Glucose is transported from the lumen of small intestine into the epithelial cells
in the mucus membrane of small intestine, by means of sodium cotransport.
Energy for this is obtained by the binding process of sodium ion and glucose
molecule to carrier protein.
 From the epithelial cell, glucose is absorbed into the portal vein by facilitated
diffusion. However, sodium ion moves laterally into the intercellular space.
From here, it is transported into blood by active transport, utilizing the energy
liberated by breakdown of ATP.
ABSORPTION OF GALACTOSE
 Galactose is also absorbed from the small intestine in the same mechanism
as that of glucose.
ABSORPTION OF FRUCTOSE
 Fructose absorption is simple.
 Does not require energy and Na+ ions.
 Transported by facilitated diffusion mediated by a carrier.
 Inside the epithelial cell, most of the fructose is converted to glucose
 The latter then enters the circulation.
 Pentoses are absorbed by a process of simple diffusion.
 Fructose is absorbed into blood by means of facilitated diffusion. Some
molecules of fructose are converted into glucose. Glucose is absorbed as
described above.

3. DISORDERS OF DIGESTION AND ABSORPTION OF
CARBOHYDRATES
LACTOSE INTOLERANCE
 This is condition, in which there is deficiency of the enzyme Lactase.
 The enzyme which hydrolyse lactose to glucose and galactose.
 Due this, lactose gets accumulates in the gut where it is a substrate for bacterial
fermentation in the large intestine with production of H2 and CO2 gases and low
molecular weight acids,like acetic acid ,propionic acid and butyric acids which
are osmotically active.
 Abdominal cramps and flatulence results from the accumulation of gases and
osmotically active products draw water from intestinal cells into lumen resulting
in Diarrhea and Dehydration.
SUCRASE DEFICIENCY
 This is condition,There is inherited deficiency of the enzyme Sucrase.
 The enzyme which hydrolyse sucrose to glucose and fructose.
 Symptoms occur in early childhood.
 These symptoms are same as lactose intolerance.
 Disacchariduria:
 Increase in the excretion of disaccharides may be observed in some patients
with disaccharidase deficiency.
REFERENCE
 Textbook of Biochemistry - DM.Vasudevan
 Textbook of Medical Biochemistry - MN Chatterjea
 ESsential of medical physiology by K sembulingam
AKSHAY PARMAR
Post Graduate Student | Department Of Biochemistry
School Of Science,
Gujarat University.