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Digestion, Absorption and Metabolism of Carbohydrates

Digestion, Absorption and Metabolism of Carbohydrates

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288 Section 4 t Digestive System

TABLE 45.1: Digestion of carbohydrates

Area



Juice



Enzyme



Substrate



End product



Mouth



Saliva



Salivary amylase



Polysaccharides – cooked

starch



Disaccharides – dextrin and

maltose



Stomach



Gastric juice



Gastric amylase



Weak amylase



The action is negligible



Pancreatic juice



Pancreatic amylase



Polysaccharides



Disaccharides

– Dextrin, maltose and maltriose



Sucrase



Sucrose



Glucose and fructose



Maltase



Maltose and maltriose



Glucose



Lactase



Lactose



Glucose and galactose



Dextrinase



Dextrin, maltose and maltriose Glucose



Trehalase



Trehalose



Small

intestine



Succus entericus



Glucose represents 80% of the final product of carbo­

hydrate 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



Glucose



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.

Metabolism of carbohydrates is given in the form of

schematic diagram (Fig. 45.1).



„ DIETARY FIBER

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

antioxidants

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

gallstones



7. Dietary fiber is also suggested to prevent or to

treat some disorders such as constipation, bowel

syndrome, diabetics, ulcer and cancer.



Digestion, Absorption

and Metabolism

of Proteins

„

„

„

„



Chapter



46



PROTEINS IN DIET

DIGESTION

ABSORPTION

METABOLISM



„ 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

gluten

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

Area



Juice



Enzyme



Substrate



End product



Mouth



Saliva



No proteolytic enzyme



Polysaccharides –

cooked starch



Disaccharides – dextrin and

maltose



Stomach



Gastric juice



Pepsin



Proteins



Proteoses, peptones, large

polypeptides



Proteoses

Peptones



Dipeptides

Tripeptides

Polypeptides



Carboxypeptidases A and B



Dipeptides

Tripeptides

Polypeptides



Amino acids



Dipeptidases



Dipeptides



Tripeptidases



Tripeptides



Amino peptidases



Large polypeptides



Trypsin

Pancreatic juice

Small

intestine

Succus entericus



Chymotrypsin



Amino acids



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



(Chapter 41).



„ 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

facilitated diffusion.



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).



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