Digestion of Carbohydrates
Sources
of Carbohydrate in the Diet
The three major sources of carbohydrates are found
in the normal human diet. These are
·
Sucrose, which is the disaccharide known
popularly as cane sugar
·
Lactose, which is a disaccharide found
in milk
·
Starches, which are large
polysaccharides present in almost all non-animal foods, particularly in different
types of grains.
Other minor sources of carbohydrates ingested to a
slight extent are
·
Amylose
·
Glycogen
·
Alcohol
·
Lactic Acid
·
Pyruvic Acid
·
Pectins
·
Dextrins
·
Minor quantities of carbohydrate
derivatives in meats.
The diet also contains a large amount of cellulose,
which is a carbohydrate. However, no enzymes capable of hydrolyzing cellulose
are secreted in the human digestive tract.
Therefore cellulose cannot be considered a food for
humans.
Digestion
of Carbohydrates in the Mouth and Stomach
When food is chewed, it is mixed with saliva, which
contains the digestive enzyme ptyalin (α-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 as
given below –
then
then
However, the food remains in the mouth only a short
time, so probably not more than 5 percent of all the starches will have become
hydrolyzed by the time the food is swallowed. However, starch digestion
sometimes continues in the body and fundus of the stomach for as long as 1 hour
before the food becomes mixed with the stomach secretions.
Then activity of the salivary amylase is blocked by
acid of the gastric secretions because the amylase is essentially non active as
an enzyme once the pH of the medium falls below about 4.0. Nevertheless, on the
average, before food and its accompanying saliva do become completely mixed
with the gastric secretions, as much as 30 to 40 percent of the starches will
have been hydrolyzed mainly to form maltose.
Digestion
of Carbohydrates in the Stomach- No digestion takes
place in stomach except some breaking down and hydrolysis aided by HCl.
Digestion
of Carbohydrates in the Small Intestine
Digestion
by Pancreatic Amylase
Pancreatic secretion, like saliva, contains a large
quantity of α-amylase that is almost identical in its function with the
α-amylase of saliva but is several times as powerful. Therefore, within 15 to
30 minutes after the chyme empties from the stomach into the duodenum and mixes
with pancreatic juice, virtually all the carbohydrates will have become
digested.
In general, the carbohydrates are almost totally
converted into maltose and/or other small glucose polymers before passing
beyond the duodenum or upper jejunum.
Hydrolysis
of Disaccharides and Small Glucose Polymers
The enterocytes lining the villi of the small
intestine contain four enzymes (lactase, sucrase, maltase, and α-dextrinase),
which are capable of splitting the disaccharides lactose, sucrose, and maltose,
plus other small glucose polymers, into their constituent monosaccharides.
These enzymes are located in the enterocytes
covering the intestinal microvilli brush border, so the disaccharides are
digested as they come in contact with these enterocytes. Lactose splits into a
molecule of galactose and a molecule of glucose. Sucrose splits into a molecule
of fructose and a molecule of glucose. Maltose and other small glucose polymers
all split into multiple molecules of glucose.Thus, the final products of
carbohydrate digestion are all monosaccharides. They are all water soluble and
are absorbed immediately into the portal blood.
In the ordinary diet, which contains far more
starches than all other carbohydrates combined, glucose represents more than 80
percent of the final products of carbohydrate digestion, and galactose and
fructose each represent seldom more than 10 percent.
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