Digestion And Absorption Of Carbohydrate

Digestion And Absorption Of Carbohydrate


DIGESTION AND ABSORPTION OF CARBOHYDRATE

Forms of carbohydrate: 

  • Starch (Abundant form), Sucrose & Lactose

Digestion Process:

  • Starts in mouth itself.
  • Starch – Polysaccharide composed entirely of glucose units linked by 1, 4 – alpha linkage & 1, 6 branching point.

Enzymes involved:

  • Salivary amylase (Ptyalin)
  • Pancreatic amylase 
  • Both salivary and pancreatic amylases are activated by Cl-.
  • By hydrolysis of amylase enzymes.
  • Optimal pH is 6-7(vs acidic in stomach).
  • Salivary amylase is more sensitive to pH.
  • Inactive if not optimal.

Mechanism of starch hydrolysis:

  • Amylase attacks only internal, 1-4 α glucose-glucose bonds.
  • Spares 1-6 linkages & terminal 1-4 α linkages.

End products:

  • Maltose – disaccharide, two glucose residues linked by 1:4 α bonds.
  • Maltotriose – trisaccharide, three glucose residues linked by 1:4 α bonds.
  • Alpha limit dextrins – Polymers of glucose containing 8 glucose molecules with 1-6 α linkages
Digestion in Intestine:
  • Further digestion of starch derivatives (disaccharides) occurs by enzymes (oligosaccharides).
  • Present in small intestinal epithelial cell brush border.

Enzymes involved:

Maltase (c-glucosidase):

  • Breaks 1:4 linkages in maltose & maltotriose
  • Releases glucose – 2 & 3 glucose molecules from maltose & maltotriose respectively.

Isomaltase (α -limit dextrinase)

  • Breaks 1: 6 α linkages of alpha-limit dextrin & releases glucose.
  • Only enzyme that attacks 1:6 α linkages

Sucrose-isomaltase:

  • Sucrase α -limit dextrinase
  • Bifunctional enzyme that catalyzes the hydrolysis of sucrose & isomaltose (α -limit dextrin).

Sucrase:

  • Present in brush border of intestinal epithelium
  • Hydrolyses Sucrose into Fructose & Glucose
  • Sub-type: Sucrase α-limit dextrinase
  • Lactase (β-glucosidase) – Hydrolyses Lactose into Galactose

Trehalase:

  • Hydrolyzes trehalose (1:1 α – linked dimer of glucose) into two glucose molecules.
  • Trehalose is found in mushrooms
ABSORPTION OF DIGESTED CARBOHYDRATES:
End products –
  • Glucose, Galactose & Fructose needs absorption.

Absorption mainly in:

  • Small intestine, especially in proximal part of jejunum.
  • Also in distal jejunum & ileum.
Mechanism:
  • Rapid absorption – Seen with monosaccharides specially hexoses (glucose, fructose, galactose).

1. Absorption of Glucose & galactose:

  • By secondary active transport (cotransport) with Na2+.
  • Absorbed from lumen against concentration gradient 

Carrier protein:

  • Cotransporter SGLT-1.
  • Glucose & galactose are transported from enterocytes out into interstitium

By GLUT-2 located in basolateral membrane.

2. Absorption of Fructose:

  • By facilitated diffusion
  • Absorbed along concentration gradient
  • Carrier protein-

GLUT-2.

  • Fructose is transported from enterocytes out into interstitium.
Fate of end products:
  • Absorption of hexose sugars (glucose, fructose & galactose)
  • From interstitium diffuse into capillaries.
  • Pentose sugars{D-xylose & D-arabinose}:
  • Absorbed by simple diffusion in proximal small intestine (jejunum).
Exam Question
 

DIGESTION AND ABSORPTION OF CARBOHYDRATE

  • Starch – Polysaccharide composed of glucose units linked by 1, 4 – alpha linkage & 1, 6 branching point.

Enzymes involved:

  • Salivary amylase (Ptyalin).
  • Pancreatic amylase.
  • Both activated by Cl
  • Optimal pH is 6-7. (vs acidic in stomach).

Mechanism of starch hydrolysis:

  • Amylase attacks only internal, 1-4 α glucose-glucose bonds.
  • Spares 1-6 linkages & terminal 1-4 α linkages.

End products:

  • Maltose
  • Maltotriose
  • Alpha limit dextrins

Digestion in Intestine:

  • Digestion of starch derivatives (disaccharides),
  • Enzymes (oligosaccharides) present in brush border of small intestinal epithelial cells.

Enzymes involved:

1. Maltase (c-glucosidase):

  • Breaks 1:4 linkages in maltose & maltotriose

2. Isomaltase (α -limit dextrinase)

  • Only enzyme that attacks 1:6 α linkages

Sucrose-isomaltase:

  • Sucrase α -limit dextrinase.
  • Bifunctional enzyme that catalyzes hydrolysis of sucrose & isomaltose (α -limit dextrin).

3. Sucrase:

  • Present in brush border of intestinal epithelium
  • Hydrolyses Sucrose into Fructose & Glucose

4. Lactase (β-glucosidase):

  • Hydrolyses Lactose into Galactose

ABSORPTION OF DIGESTED CARBOHYDRATES:

1. End products of carbohydrate digestion –

  •  Glucose, Galactose & Fructose needs absorption.

2. Absorption mainly in:

  • Small intestine, especially in proximal part of jejunum.

3. Mechanism:

  • Rapid absorption – Seen with monosaccharides specially hexoses (glucose, fructose, galactose).

3a. Absorption of Glucose & galactose:

  • By secondary active transport (cotransport) with Na2+.
  • Absorbed from lumen against concentration gradient 

Carrier protein:

  • Cotransporter SGLT-1.
  • Transportation by GLUT-2 located in basolateral membrane.

3b.  Absorption of Fructose:

  • By facilitated diffusion
  • Absorbed along concentration gradient
  • Carrier protein- GLUT-2.
  • Fructose is transported from enterocytes out into interstitium by GLUT-2.
3c.  Absorption of Pentose sugars:
  • Pentose sugars {D-xylose & D-arabinose}:
  • By simple diffusion in proximal small intestine (jejunum).
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