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Carbohydrates

A. Dietary carbohydrates

 

These are :

  1. Monosaccharides : fructose, glucose
  2. Disaccharides : lactose (milk sugar), sucrose (cane sugar or table sugar)
  3. Polysaccharides :  

The only digestible polysaccharides in humans are the starches (starches are polymers of glucose). Dietary

starches can be of :

  1. Animal origin :
    1. ​​​​Glycogen :
       
      Glycogen is mostly straight in structure (with glucose molecules attached to each other by 1:4 alpha linkage); there is some side-branching also (here the linkage is by 1:6 alpha linkage)
  2. Plant origin :
    1. Amylopectin :
       
      This is the main dietary starch (constituting more than 80 to 90%).This is just like glycogen, with even fewer side branches. 
    2. Amylose :
       
      This has no side branches.
    3. Cellulose :
       
      This cannot be digested in humans.
    4. Digestion
       
      Final end products :
       
      The end products of carbohydrate digestion are the monosaccharides fructose, glucose and galactose. Only monosaccharides can be absorbed from the GIT.  

B. Site :

Although starch digestion begins in the mouth (by salivary alpha amylase), almost all starch digestion occurs in small intestine.

 

C. Enzymes :

  1. Salivary alpha amylase (ptyalin)
  2. Pancreatic alpha amylase
  3. Small intestine brush border enzymes :

Alpha-dextrinase (also called isomaltase), sucrase, maltase, lactase, trehalase (Note : alpha-dextrinase and sucrase are separate subunits of a single protein) Alpha amylase (salivary and pancreatic) digestion : The alpha amylase acting on starch can break only the 1:4 alpha linkages; They cannot break 1:6 alpha linkages, terminal 1:4 alpha linkages and 1:4 alpha linkages next to branching points.

Thus, the end products of amylase digestion are not monosaccharides; the end products are :

  1. Oligosaccharides
  2. Maltose (a disaccharide)
  3. Maltriose (a trisaccharide)
  4. Alpha-dextrins (these are glucose polymers containing on an average 8 glucose molecules containing 1:6 alpha linkages)

The above end products are further digested by the brush border enzymes.

 

Note : the activator for salivary alpha amylase is chloride ion.                                             

Brush border enzyme digestion                                                   


This is shown in the table below :

Enzyme

Acts on

End products

Alpha dextrinase or isomaltase (this is the main enzyme for breaking 1:6 alpha linkages)

Alpha dextrins, maltose, maltriose

Glucose

Sucrase

Sucrose, maltose, maltriose

Sucrose : glucose and fructose

Maltose : 2 glucose

Maltriose : 3 glucose

Maltase

Maltose, maltriose, alpha dextrins

Glucose

Lactase

Lactose

Glucose, galactose

Trehalase

Trehalose (this is a 1:1 alpha linkage dimer of glucose

2 glucose

 

D. Absorption

Site :

Absorption of the monosaccharides occurs in the small intestine.

Amount of monosaccharides that can be absorbed:

Absorption is not regulated. The intestine can absorb more than 5 kg of dietary sucrose/day. Almost all the glucose and galactose present in the intestine can be absorbed. The maximal rate of glucose absorption from intestine is about 120 gram/hr.


‘GLUT’ SERIES (Glucose Transporters)

GLUT 1

Blood brain barrier, brain, placenta, kidney etc

GLUT 2

cells of is lets of pancreas, liver, epithelial cells of small intestinal / renal tubules

GLUT 3

Brain, placenta, kidney etc

GLUT 4

Insulin – stimulated glucose uptake in skeletal muscle / adipose tissue

GLUT 5

Fructose transport in jejunum / sperm

GLUT 6

 

GLUT 7

Liver, G-6-P in Endoplasmic reticular

 

Glucose absorption

Site

Transport mechanism

Insulin

Intestine

SGLT

No effect

Kidney

SGLT

No effect

Muscle (SK muscle / cardiac

muscle)

GLUT 4

Favour

Adipose

GLUT 4

Favour

Liver

(Hexokinase)

Favour

 

Insulin does not affect the absorption of glucose in:

Kidney, intestine, RBC, brain.

 

E. Mechanism

  1. Glucose/galactose (hexoses)
    1. From lumen to enterocyte These are absorbed by sodium-dependent secondary active transport (the transporter is a symport and is called SGLT or sodium linked glucose transport) 

F. Salient features of SGLT:

  1. Just like the GLUT series of transporters, SGLT also crosses the membrane 12 times (with its COOH and NH2 terminals on the cytoplasmic side of the membrane)
  2. It is present in the kidney and intestine
  3. It is not affected by insulin
  4. It transports glucose and galactose
  5. It has 3 binding sites :
    1. 2 for sodium and
    2. 1 for glucose or galactose From enterocyte to the interstitium

This is by GLUT 2. From the interstitium, it diffuses into blood.

  1. Fructose (hexose)
     
    Its absorption is independent of sodium. It is transported by facilitated diffusion by GLUT series of transporters. 2
     
    GLUT series of transporters are involved :
    1. from lumen to enterocyte : GLUT 5
    2. from enterocyte to interstitium : GLUT 2
       
      Fructose absorption occurs rapidly because most of the fructose is converted into glucose and lactic acid within the enterocyte; this maintains a high concentration gradient for diffusion.
  2. ​​​Pentoses
     
    These are absorbed by simple diffusion.





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