DIGESTION OF CARBOHYDRATES
In the diet, carbohydrates are present as complex polysaccharides (starch,
glycogen), and to a minor extent, as disaccharides (sucrose and lactose).
Polysaccharides are hydrolysed to monosaccharide units in the
gastrointestinal tract.
This process of digestion starts in mouth by the salivary alpha-amylase.
However, the time available for digestion in the mouth is limited because
the gastric hydrochloric acid will inhibit the action of salivary amylase.
In the pancreatic juice another alpha-amylase is available which will
hydrolyse the glycosidic linkages randomly in small intestine, so as to
produce smaller subunits like maltose, isomaltose and branched or
unbranched oligosaccharides.
The cells of brush border of intestine contain the enzymes, sucrase,
maltase, isomaltase and lactase which hydrolyse the corresponding
disaccharides into monosaccharides which are then absorbed.
Clinical Application; Lactose Intolerance
Lactase (which produced from enterocytes in brush border of intestine)
hydrolyses lactose to glucose and galactose. Deficiency of lactase leads to
lactose intolerance. In this condition, lactose accumulates in the gut
leading to irritant diarrhea and flatulence.
ABSORPTION OF CARBOHYDRATES
Only monosaccharides are absorbed by the intestine. Glucose has specific
transporters:
1. Co-transport of glucose from Lumen to Intestinal Cell
This process is mediated by Sodium Dependent Glucose Transporter-1
(SGluT-1) , which carries glucose along with sodium. This sodium is
later expelled by the sodium pump with utilization of energy. So energy
is needed indirectly.
Clinical application: Common treatment for diarrhea is oral rehydration
fluid which contains glucose and sodium, presence of glucose in oral
rehydration fluid allows uptake of sodium to replenish body sodium
chloride which is deficient due to diarrhea
2. Uniport System Releases Glucose into Blood
The same intestinal epithelial cells have a different transport mechanism
on the membrane facing capillaries. Intestinal cells release glucose into
blood stream by the carrier mechanism called Glucose Transporter Type
2 (GluT2).This transporter is not dependent on sodium. It is a uniport,
facilitated diffusion system.
GluT2 (facilitated transport) also involved in absorption of glucose from
blood stream to cells. GluT2 is present in intestinal epithelial cells, liver
cells, beta cells of pancreas and kidney. Since GluT2 has a high Km (low
affinity) for glucose, its presence in beta cells is ideally suited for sensing
a high glucose level and releasing insulin. So this mechanism enables the
pancreas to monitor the glucose level and adjust the rate of insulin
secretion.
3. Glucose Transporter 4
GluT4 is the major glucose transporter in skeletal muscle and adipose
tissue. GluT4 is under the control of insulin. But other glucose
transporters are not under the control of insulin.
Clinical application: Insulin induces the movement of intracellular
GluT4 molecules to the cell surface and thus increases glucose uptake in
muscle and fat cells. In diabetes, entry of glucose into muscle is only half
of normal cells and because glucose is the preferred source of energy for
most of the body tissues and brain cells derive the energy mainly from
glucose so when glucose metabolism is deranged, life threatening
conditions may occur.
Fate of absorbed sugars:
Monosaccharides (glucose, galactose and fructose) resulting from
carbohydrate digestion are absorbed and undergo the following:
A. Uptake by 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 fates:
1. Oxidation: through one of these pathways
a) Major pathways (glycolysis and Krebs' cycle) for production of
energy.
b) Hexose monophosphate pathway: for production of ribose,
deoxyribose and NADPH + H+
c) Uronic acid pathway, for production of glucuronic acid, which is
used in detoxification and enters in the formation of mucopolysaccharide.
2. Storage: in the form of:
a) Glycogen: by glycogenesis.
b) Fat: by lipogenesis.
3. Conversion: to substances of biological importance, like
a) Ribose, deoxyribose
RNA and DNA.
b) Lactose
milk.
c) Glucosamine, galactosamine
mucopolysaccharides.
d) Glucoronic acid
mucopolysaccharides.
e) Fructose
in semen.
Aerobic respiration
‐ the process by which a cell uses O2 to "burn"
molecules and release energy
C
6
H
12
O
6
+ 6O
2
>> 6CO
2
+ 6H
2
O
This reaction takes place over the course of three major reaction
pathways:
1. Glycolysis
2. The Krebs Cycle
3. Electron Transport Phosphorylation
