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Digestion, Absorption and Metabolism of Carbohydrates
288 Section 4 t Digestive System
TABLE 45.1: Digestion of carbohydrates
Polysaccharides – cooked
Disaccharides – dextrin and
The action is negligible
– Dextrin, maltose and maltriose
Glucose and fructose
Maltose and maltriose
Glucose and galactose
Dextrin, maltose and maltriose Glucose
Glucose represents 80% of the final product of carbo
hydrate digestion. Galactose and fructose represent the
ABSORPTION OF CARBOHYDRATES
Carbohydrates are absorbed from the small intestine
mainly as monosaccharides, viz. glucose, galactose
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
Metabolism of carbohydrates is given in the form of
schematic diagram (Fig. 45.1).
Dietary fiber or roughage is a group of food particles
which pass through stomach and small intestine, without
being digested and reach the large intestine unchanged.
Other nutritive substances of food are digested and
absorbed before reaching large intestine.
Characteristic feature of dietary fiber is that it is
not digestible by digestive enzymes. So it escapes
digestion in small intestine and passes to large intestine.
It provides substrate for microflora of large intestine and
increases the bacterial mass. The anaerobic bacteria
in turn, degrade the fermentable components of the
fiber. Thus, in large intestine, some of the components
of fiber are broken down and absorbed and remaining
components are excreted through feces.
Components of Dietary Fiber
Major components of dietary fiber are cellulose,
hemicelluloses, Dglucans, pectin, lignin and gums.
Cellulose, hemicelluloses and pectin are partially
degradable, while other components are indigestible.
Dietary fiber also contains minerals, antioxidants and
other chemicals that are useful for health.
Chapter 45 t Digestion, Absorption and Metabolism of Carbohydrates 289
FIGURE 45.1: Schematic diagram of carbohydrate metabolism
Source of Dietary Fiber
Source of dietary fiber are fruits, vegetables, cereals,
bread and wheat grain (particularly its outer layer).
Health Benefits of Dietary Fiber
1. By intake of high dietary fiber food, some disease
producing food substances may be decreased in
quantity or completely excluded in diet
2. Dietary fiber helps in weight maintenance because it
requires more chewing and promotes hunger satisfaction by delaying the emptying of stomach and by
giving the person a sense of fullness of stomach
3. Diet with high fiber content tends to be low in energy
and it is also useful in reducing the body weight
4. Dietary fiber increases the formation of bulk and
soft feces and eases defecation
5. It contains some useful substances such as
6. Some components of dietary fiber also reduce blood
cholesterol level and thereby, decrease the risk of
some diseases such as coronary heart disease and
7. Dietary fiber is also suggested to prevent or to
treat some disorders such as constipation, bowel
syndrome, diabetics, ulcer and cancer.
PROTEINS IN DIET
PROTEINS IN DIET
Foodstuffs containing high protein content are meat,
fish, egg and milk. Proteins are also available in wheat,
soybeans, oats and various types of pulses.
Proteins present in common foodstuffs are:
1. Wheat: Glutenin and gliadin, which constitute
2. Milk: Casein, lactalbumin, albumin and myosin
3. Egg: Albumin and vitellin
4. Meat: Collagen, albumin and myosin.
Dietary proteins are formed by long chains of amino
acids, bound together by peptide linkages.
DIGESTION OF PROTEINS
Enzymes responsible for the digestion of proteins are
called proteolytic enzymes.
IN THE MOUTH
Digestion of proteins does not occur in mouth, since
saliva does not contain any proteolytic enzymes. So, the
digestion of proteins starts only in stomach (Table 46.1).
TABLE 46.1: Digestion of proteins
No proteolytic enzyme
Disaccharides – dextrin and
Proteoses, peptones, large
Carboxypeptidases A and B
Chapter 46 t Digestion, Absorption and Metabolism of Proteins 291
FIGURE 46.1: Schematic diagram of protein metabolism
IN THE STOMACH
Pepsin is the only proteolytic enzyme in gastric juice
(Chapter 38). Rennin is also present in gastric juice. But
it is absent in human.
dipeptidases, tripeptidases and aminopeptidases
FINAL PRODUCTS OF PROTEIN DIGESTION
IN THE SMALL INTESTINE
Final products of protein digestion are the amino acids,
which are absorbed into blood from intestine.
Most of the proteins are digested in the duodenum and
jejunum by the proteolytic enzymes of the pancreatic
juice and succus entericus.
ABSORPTION OF PROTEINS
Proteolytic Enzymes in Pancreatic Juice
Pancreatic juice contains trypsin, chymotrypsin and
carboxypeptidases. Trypsin and chymotrypsin are
called endopeptidases, as these two enzymes break the
Proteins are absorbed in the form of amino acids from
small intestine. The levo amino acids are actively
absorbed by means of sodium cotransport, whereas,
the dextro amino acids are absorbed by means of
interior bonds of the protein molecules (Chapter 39).
Absorption of amino acids is faster in duodenum
and jejunum and slower in ileum.
Proteolytic Enzymes in Succus Entericus
METABOLISM OF PROTEINS
Final digestion of proteins is by the proteolytic
enzymes present in the succus entericus. It contains
Metabolism of proteins is given in the form of a schematic
diagram (Fig. 46.1).