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• Carbohydrates are called carbohydrates because they are essentially hydrates of carbon
(i.e. they are composed of carbon and water and have a composition of (CH2O)n.
• The major nutritional role of carbohydrates is to provide energy and digestible
carbohydrates provide 4 Kcal/g
• No single carbohydrate is essential, but carbohydrates do participate in many required
functions in the body.
• Monosaccharides
o Do not need hydrolysis before absorption
o Very little in most foods
• Di- and poly-saccharides
o Relatively large molecules
o Must be hydrolyzed prior to absorption
o Hydrolyzed to monosaccharides
Only monosaccharides can be absorbed
fructose glucose Galactose**
(fructose-glucose) (glucose-glucose) (glucose-galactose)
* Galactose does not occur in foods singly but only as part of lactose
Three types of monosaccharides…
…join together to make three types of disaccharides
maltose lactosesucrose
•Mouth:
o Digestion of CHO begins in the mouth
o During mastication, salivary alpha amylase:
Breaks starches down to maltose, dextrins,
isomaltose,
Optimum pH 6.7
Requires Cl- for its activity
Plays only a small role in breakdown because of the
short time food is in the mouth
o The chemical digestion of carbohydrates, which begins in
the oral cavity, is terminated due to a decrease in pH
CHO digestion stops in the stomach because the
high acidity inactivates salivary alpha amylase
Pancreas
o Further digestion by pancreatic enzymes occurs in the small intestine: when the acidic stomach contents reach the small intestine, they are neutralized by bicarbonate secreted by the pancreas
o At alkaline pH pancreatic alpha amylase continues the starch digestion:
Hydrolyzes alpha 1-4 linkages between glucose residues
Major importance in hydrolyzing starch and glycogen to maltose
Polysaccharides Disaccharides(maltose, isomaltose)
Amylase
Small intestine
o The final digestive processes occur at the small intestine
and include the action of several disaccharidases.
o Disaccharidases are secreted through and remain
associated with the brush border of the intestinal
mucosal cells.
Disaccharides MonosaccharidesBrush Border Enzymes
Maltose Glucose + GlucoseMaltase
LactoseLactase
Glucose + Galactose
Sucrose Glucose + FructoseSucrase
Isomaltose Glucose + GlucoseIsomaltase
Small intestine
Monosaccharides, the end product of CHO digestion, enter the capillaries of the intestinal villi
Monosaccharides travel the liver via the portal vein
In liver, galactose and fructose are converted to glucose
Distributed to tissue through circulation
Extra glucose is stored as glycogen in the liver and skeletal
muscles
Insulin is not required for the uptake of glucose by the
intestinal cells
o Absorption of glucose= 100% (taken as standard)
o Absorption of galactose = 110%
o Absorption of fructose = 43%
The maximal rate of glucose absorption from intestine is 120gm/hr
Transport of glucose into cells (Active transport) • energy-requiring process that transports glucose “against” a
concentration gradient. The energy is used by the sodium-potassium pump that requires the enzyme ATPase (Adenosine triphosphatase).
• carrier-mediated process in which the movement of glucose is coupled to the concentration gradient of Na+
• Na+, is transported into the cell at the same time. Called: cotransport
• The carrier is a sodium-dependent–glucose transporter or SGLT.
• Occurs in the epithelial cells of the intestine, renal tubules, and part of the blood brain barrier.
1- Insulin independent transport system:
o Not require insulin for glucose uptake
o Mediated through carrier protein
o Present in brain, RBCs, hepatocytes,
intestinal mucosa, renal tubules and cornea
2- Insulin-dependent transport system
o Require insulin: muscles and adipose tissue
o Insulin increases the number of glucose
transporters in tissues containing insulin
receptors
Glucose enter cells by two ways:
Passive transport (facilitated diffusion):Na+-independent facilitated diffusion transport
• This system is mediated by a family of 14 glucose transporters in cell membranes. They are designated GLUT-1 to GLUT-14
• These transporters exist in the membrane in two conformational states
• Extra cellular glucose binds to the transporter, which then alters its conformation, transporting glucose across the cell membrane.
• The glucose transporters display a tissue-specific pattern of expression. For example, GLUT-3 is the primary glucose transporter in neurons.
• Requires no energy since it goes with the gradient concentration (from high-outside the cell- to low)
• Glucose binds to receptor on carrier protein; Latter changes shape then releases solute on other side of membrane
Fructose and pentoses are absorbed by this
mechanism.
Glucose and galactose can also use the same
transporter if the concentration gradient is favorable.
Active transport is much more faster than passive
transport.
There is also sodium – independent transporter
(GLUT-2) that is facilitates transport of sugars out of
the cell i.e. to circulation.
Summary of types of functions of most important glucose transporters:
SiteFunction
Intestine and renal
tubules.
Absorption of glucose by
active transport (energy is
used by Na+- K+ pump)
SGLT-1
Intestine and spermFructose transport and to a
lesser extent glucose and
galactose.
GLUT -5
- Intestine and renal tubule
- β cells of pancreas-liver
Transport glucose out of
intestinal and renal
cells circulation
GLUT - 2
Monosaccharides (glucose, galactose and fructose) resulting from carbohydrate digestion are absorbed and undergo the following:
a- Uptake by tissues (liver):
After absorption the liver takes up sugars, where galactose and fructose are converted into glucose.
b- Glucose utilization by tissues:
Glucose may undergo one of the following fate:
Fate of absorbed sugars
1. Oxidation: through
Major pathways (glycolysis and Krebs' cycle)
for production of energy.
Hexose monophosphate pathway: for
production of ribose, deoxyribose and NADPH + H+
Uronic acid pathway: for production of glucuronic
acid, which is used in detoxification and enters in
the formation of mucopolysaccharide.
2. Storage:
o As glycogen (glycogenesis) in the liver and muscles
mainly.
o As TG (lipogenesis) in adipose tissues
3. Conversion: to substances of biological importance:
o Ribose, deoxyribose RNA and DNA.
o Amino sugars
o Non essential amino acids
o Fatty acids
o Fructose
o Glucuronic acid
o Galactose essential for formation of
lactose , glycolipids, mucopolysaccharides