Carbohydrate Digestion And Absorption

Carbohydrate Digestion And Absorption

Explain how starch, sucrose and lactose are hydrolyzed in the gastrointestinal tract. Add a note on glucose absorption.
Answer:

Digestion of carbohydrates in the gastrointestinal tract.

1. Digestion in the mouth:

• While food is chewed in the mouth before it is swallowed it mixes with saliva.
• The chewed food is subjected to the action of salivary a-amylase (ptyalin)
• This a-amylase aids in digestion of starch to produce dextrin and maltose.

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2. Digestion in the stomach:

• The action of salivary amylase on starch and glycogen continues in the stomach.
• The HCI present in the gastric juice may hydrolyse some sucrose.

3. Digestion in the small intestine:

• Pancreatic amylase rapidly converts all form of starch into dextrin, maltose and maltroise.
• Its digestive activity is increased by presence of bile salts.
• Succus entericus contains enzymes for splitting disaccharides into monosaccharides.

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They are:

1. Sucrase-converts sucrose into glucose and fructose at pH 5-7.
2. Maltase-converts maltose into glucose at pH 5.S-6.2.
3. Lactase-converts lactose into glucose and galactose at pH 5.4-6.0.
4. Dextrinase-converts dextrin into glucose.

Absorption of glucose:

• Glucose is transported from the lumen of small intestine into the epithelial cells in the mucous membrane of small intestine by means of sodium co-transport.
• Glucose first combines with a ‘carrier’ and form glucose-carrier complex.
• This moves the glucose across the lipid barrier of cell membrane and releases the glucose inside the cell.
• The glucose and Na+ have the same carrier called sodium-dependent glucose transporter – 1.
• To concentrate the glucose within the cell, the carrier is coupled to a source of energy which is obtained by the binding process of sodium ion and the glucose molecule to the carrier protein.
• Sodium ion now moves laterally into the intercellular space utilizing the energy liberated by breakdown of ATP.
• The glucose is transported by glucose transporter – 2 into the interstitial space and then to blood capillaries.

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Factors effecting glucose absorption:

• Thyroid – increases absorption.
• Adrenal cortex deficiency – decreases absorption.
• In diarrhoea – absorption decreases.

Fate of glucose:

• 5% stored as glycogen in liver and muscle.
• 50-60%-catabolised in tissues.
• 30-40%-converted to fat and stored.

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