CARBOHYDRATES IN DIET
Human diet contains three types of
carbohydrates:
1. POLYSACCHARIDES
Large polysaccharides are glycogen,
amylose and amylopectin, which are in the form of starch (glucose
polymers). Glycogen is available in non-vegetarian diet. Amylose and
amylopectin are available in vegetarian diet because of their plant origin.
2. DISACCHARIDES
Two types of disaccharides are
available in the diet.
i. Sucrose (Glucose + Fructose), which
is called table sugar or cane sugar
ii. Lactose (Glucose + Galactose),
which is the sugar available in milk.
3. MONOSACCHARIDES
Monosaccharides consumed in human diet
are mostly glucose and fructose.
Other carbohydrates in the diet
include
i. Alcohol
ii. Lactic acid
iii. Pyruvic acid
iv. Pectins
v. Dextrins
vi. Carbohydrates in meat.
Diet also contains large amount of cellulose, which cannot be
digested in the human GI tract so it is not
considered as a food for human beings.
IN THE MOUTH
Enzymes involved in the digestion of
carbohydrates are known as amylolytic enzymes. The only amylolytic enzyme present in
saliva is the salivary amylase or ptyalin.
IN THE STOMACH
Gastric juice contains a weak amylase, which plays a minor
role in digestion of carbohydrates.
IN THE INTESTINE
Amylolytic enzymes present in the
small intestine are derived from pancreatic juice and succus entericus.
Amylolytic Enzyme in
Pancreatic Juice
Pancreatic juice contains pancreatic amylase.
Amylolytic Enzymes in
Succus Entericus
Amylolytic enzymes present in succus
entericus are maltase,
sucrase, lactase, dextrinase and trehalase.
FINAL PRODUCTS OF CARBOHYDRATE
DIGESTION
Final products of carbohydrate
digestion are monosaccharides, which are glucose, fructose and galactose. Glucose
represents 80% of the final product of carbohydrate digestion. Galactose and fructose
represent the remaining 20%.
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 is absorbed into blood by
means of facilitated
diffusion. Some
molecules of fructose are converted into glucose. Glucose is absorbed as
described above.
METABOLISM OF CARBOHYDRATES
Metabolism is the process in which
food substances undergo chemical and energy transformation. After digestion and
absorption, food substances must be utilized by the body. The utilization
occurs mainly by
oxidative process in which the
carbohydrates, proteins and lipids are burnt slowly to release energy. This
process is known as catabolism. Part
of the released energy is utilized by tissues for physiological actions and
rest of the energy is stored as rich energy phosphate bonds and in the form of
proteins, carbohydrates and lipids in the tissues. This process is called
anabolism.
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