Metabolism of Carbohydrates
Objectives
- study utilization of glucose and other carbohydrates in the body
- study the various mechanisms and fate of glucose in the body
- study the energetics of the various mechanisms
Digestion of Carbohydrates
Dietary carbohydrates principally consist of the polysaccharides: starch and glycogen. It also
contains disaccharides: sucrose, lactose, maltose and in small amounts monosaccharides like
fructose and pentoses. Liquid food materials like milk, soup, fruit juice escape digestion in
mouth as they are swallowed, but solid foodstuffs are masticated thoroughly before they are
swallowed.
1. Digestion in Mouth
Digestion of carbohydrates starts at the mouth, where they come in contact with saliva during
mastication. Saliva contains a carbohydrate splitting enzyme called salivary amylase (ptyalin).
Action of ptyalin (salivary amylase)
It is α - amylase, requires Cl- ion for activation and optimum pH 6-7. The enzyme hydrolyzes α-
(1,4) glycosidic linkage at random, from molecules like starch, glycogen and dextrins, producing
smaller molecules maltose, glucose and disaccharides maltotriose. Ptyalin action stops in
stomach when pH falls to 3.0
2. Digestion in Stomach
No carbohydrate splitting enzymes are available in gastric juice. HCl may hydrolyze some
dietary sucrose to equal amounts of glucose and fructose.
3. Digestion in Duodenum
Food reaches the duodenum from stomach where it meets the pancreatic juice. Pancreatic juice
contains a carbohydrate-splitting enzyme pancreatic amylase.
Action of pancreatic
Amylase
It is also an α - amylase, optimum pH 7.1. Like ptyalin it also requires Cl- for activity. The
enzyme hydrolyzes α-(1,4) glycosidic linkage situated well inside polysaccharide molecule.
Other criteria and end products of action are similar of ptyalin.
1. Digestion in Small
Intestine
Action of Intestinal Juice
a. pancreatic amylase:
It hydrolyzes terminal α-(1,4), glycosidic linkage in polysaccharides and Oligosaccharide
molecules liberating free glucose molecules.
b. Lactase
It is a β- glycosidase, its pH range is 5.4 to 6.0. Lactose is hydrolyzed to glucose and
galactose.
Lactose Intolerance
Lactose is hydrolyzed to galactose and glucose by lactase in humans (by β- Galactosidase in
Bacteria).Some adults do not have lactase.Such adults cannot digest the sugar.It remains in the
intestines and gets fermented by the bacteria. The condition is called as Lactose intolerance.
Such patients suffer from watery diarrhea, abnormal intestinal flow and chloeic pain. They are
advised to avoid the consumption of Lactose containing foods like Milk.
C. Maltase
The enzyme hydrolyzes the α -(1,4) glycosidic linkage between glucose units in maltose
molecule liberating two glucose molecules. Its pH range is 5.8 to 6.2.
D. Sucrase
PH ranges 5.0 to 7.0. It hydrolyzes sucrose molecule to form glucose and fructose.
Absorption of Carbohydrates
Products of digestion of dietary carbohydrates are practically completely absorbed almost
entirely from the small intestine.
Absorption from proximal jejunum is three times grater than that of distal ileum. It is also proved
that some disaccharides, which escape digestion, may enter the cells of the intestinal lumen by
“pinocytosis” and are hydrolyzed within these cells. No carbohydrates higher than the
monosaccharides can be absorbed directly in to the blood stream.
Mechanism of Absorption
Two mechanisms are involved:
1. Simple Diffusion
This is dependent on sugar concentration gradients between the intestinal lumen. Mucosal cells
and blood plasma. All the monosaccharides are probably absorbed to some extent by simple
‘passive’ diffusion.
2. “Active “Transport Mechanisms
- Glucose and galactose are absorbed very rapidly and hence it has been suggested that they are absorbed actively and it requires energy.
- Fructose absorption is also rapid but not so much as compared to glucose and galactose but it is definitely faster than pentoses. Hence fructose is not absorbed by simple diffusion alone and it is suggested that some mechanism facilitates its transport, called as” facilitated transport”.