Absorption Of Carbohydrates Biochemistry Notes

Afza.Malik GDA

Biochemistry for Nurses Absorption of Carbohydrates

Absorption Of Carbohydrates Biochemistry Notes

Absorption Of Carbohydrates,Absorption of Disaccharides, Cori Theory About The Rate of Absorption for Different Carbohydrates,Mechanisms Of Absorption,Wilson and Crane’s Hypothesis of Active Transport,Factors Influencing Rate of Absorption.

Absorption Of Carbohydrates

    It is observed from carbohydrate digestion is complete when the food materials reach small intestine and all complex dietary carbohydrates like starch and glycogen and the disaccharides are ultimately converted to simpler monosaccharides

    All monosaccharides, products of digestion of dietary carbohydrates, are practically completely absorbed almost entirely from the small intestine

Absorption Rate in Small Intestine

    Rate of absorption diminishes from above downwards; proximal jejunum three times greater than that of distal ileum. 

Absorption of Disaccharides

    It is also proved that some disaccharides, which escape digestion, may enter the cells lining the intestinal lumen may be by pinocytosis; and are hydrolysed within these cells. 

    No carbohydrates higher than the monosaccharides can be absorbed directly into the bloodstream in normal health and if administered parenterally, they are eliminated as foreign bodies.

Cori Theory About The Rate of Absorption for Diffrent Carbohydrates

    Cori studied the rate of absorption of different sugars from small intestine in rat. Taking glucose absorption as 100, comparative rate of absorption of other sugars were found as follows:

Galactose(110) > Glucose (100) > Fructose (43) > Manose (19) > Xylose (15) > Arabinose (9)

    The above study proves that glucose and galactose are absorbed very fast; fructose and mannose intermediate rate and the pentoses are absorbed slowly. Galactose is absorbed more rapidly than glucose.

Mechanisms Of Absorption

Two mechanisms are suggested:

1. Simple diffusion: This is dependent on sugar concentration gradients between the intestinal lumen, mucosal cells and blood plasma. All the monosaccharides are probably absorbed to some extent by simple ‘passive’ diffusion.

2. “Active” Transport Mechanisms

    • Glucose and galactose are absorbed very rapidly and hence it has been suggested that they are absorbed actively and it requires energy.

  • Fructose absorption is also rapid but not so much as compared to glucose and galactose, but it is definitely faster than pentoses. Hence fructose is not absorbed by simple diffusion alone and it is suggested that some mechanism facilitates its transport, called as facilitated transport.

Wilson and Crane’s Hypothesis of Active Transport :Wilson and Crane have shown that sugars which are ‘actively’ transported have several chemical features in common. They suggested that to be actively transported sugar must have the following:

1:They must have a six-membered ring,

2:Secondly, they must have one or more carbon atoms attached to C 5

3:Thirdly, they must have a –OH group at C-2 with the same stereo configuration as occurs in D-glucose. –OH group and 5 hydroxymethyl or methyl group on the pyranose ring appear to be essential structural requirements for the active transport mechanism.

    Energy: It is provided by ATP, by the interaction of the sodium dependent sugar carrier and the sodium pumps, actively transported sugars are concentrated within the cell without any back leakage of the sugar into the lumen. 

    It is believed that sodium binding by the carrier-protein is pre-requisite for glucose binding. Sodium binding changes the conformation of the protein molecule, enabling the binding of glucose to take place and thus the absorption to occur. 

    It is presumed that analogous “carrier protein” exists for D-galactose also. This is a cotransport system Glucose Transporters (GLUT) Several glucose transporters GLUT-1 to 7 have been described in various tissues

Evidences in favour of the cotransport system of glucose absorption:

    • The dependence of the active transport of glucose upon the presence of sodium ions has been demonstrated in isolated loops of rat intestine by replacing the sodium of bathing fluid by K+ and lithium. Under these circumstances, the rate of glucose transport is markedly reduced and ultimately stopped.

    • Drugs such as strophanthin and ouabain which inhibit sodium pump also inhibit active transport of sugars.

    • Substances preventing the liberation of metabolic energy, such as dinitrophenol (DNP), also inhibits active transport of sugars.

    • Phloridzin, a glycoside inhibits glucose transport by probably displacing sodium from its binding site, as a result glucose cannot be bound and transported.

Absorption of Other Sugars

    • Sugars like D-fructose and D-mannose are probably absorbed by facilitated transport which requires the presence of carrier protein but does not require energy.

    • Other sugars like pentoses and L-isomers of glucose and galactose are absorbed passively by simple diffusion. Facilitated Transport Vs Active Transport Similarities

    • Both appear to involve carrier proteins.

    • Both show specificity.

    • Both resemble a substrate-enzyme type of reaction.

    • Both have specific binding sites for solutes.

    • ‘Carrier’ is saturable so it has maximum rate of transport.

    • There is a binding constant for solute.

    • Structurally similar competitive inhibitors block transport. Differences

    • Facilitated transport can act bi-directionally, whereas active transport is unidirectional.

    • Active transport always occurs against an electrical or chemical gradient and hence requires energy. Facilitated transport does not require energy. Mechanism of Facilitated Transport Ping-Pong’ mechanism explains facilitated transport.

    • Carrier Protein exists in two principal conformations depending on the solute concentration. Two forms are: – Pong state, and – Ping state.

     In the Pong state, it is exposed to high concentrations of solutes, and molecules of solutes bind to specific sites on the ‘carrier protein’. This occurs in lipid bilayer of the cell with high solute concentration.

    • In inner side, a conformational change occurs to Ping state and the solute is discharged to the side favouring new equilibrium.

    • The empty carrier protein then reverts to the original conformation “Pong” state to complete the cycle. Factors determining facilitated transport: Rate at which solutes enter a cell by facilitated transport is determined by the following factors:

    • Concentration gradient across the membrane.

    • The amount of “Carrier protein” available (key control system).

    • Rapidity of solute-carrier interaction.

    • Rapidity of conversion of conformation state from ‘Pong’ to ‘Ping’ and vice versa. 

Factors Influencing Rate of Absorption 

    1. State of mucous membrane and length of time of contact: 

If mucous membrane is not healthy, absorption will suffer. Similarly in hurried bowel, length of contact is less and as such absorption will be less.

    2. Hormones

    • Thyroid hormones: These increase absorption of hexoses and act directly on intestinal mucosa.

    • Adrenal cortex: Absorption decreases in adrenocortical deficiency, mainly due to decreased concentration of sodium in body fluids

    • Anterior pituitary: This affects absorption mainly through its influence on thyroids. Hyperpituitarism induces thyroid overactivity and vice versa.

    • Insulin: This has no effect on absorption of glucose.

    3. Vitamins:

    Absorption is diminished in states of deficiency of B-vitamins, viz, thiamine, pyridoxine and pantothenic acid.

    4. Inherited enzyme deficiencies:

    Inherited enzyme deficiencies like sucrase and lactase can interfere with hydrolysis of corresponding disaccharides and their absorption.

    Clinical Aspect Defects In Digestion And Absorption Of Carbohydrates (including inherited disorders)

1. Lactase Deficiency

    Some infants may have deficiency of the enzyme lactase and they show intolerance to lactose, the sugar of milk. Symptoms and signs seen in affected infants include:

• Diarrhea and flatulence

• Abdominal cramps

• Distension. Explanation: The above features are explained as follows:

    • As lactose of milk cannot be hydrolyzed due to deficiency of lactase enzyme, there occurs accumulation of lactose in intestinal tract, which is osmotically active and holds water, producing diarrhea.

    • Accumulated lactose is also fermented by intestinal bacteria which produce gas and other products, producing flatulence, distension and abdominal cramps. Types: Lactase deficiency can be of 3 types. Inherited Lactase Deficiency

    • Rare disorder

    • Symptoms of intolerance to milk such as diarrhea and wasting incident to fluid and electrolyte disturbances as well as inadequate nutrition, all develop very soon after birth     • Urine: Presence of lactose in urine is a prominent feature (lactosuria)

    • Treatment: Feeding of lactose-free diet results in disappearance of the symptoms and marked improvement.

Low Lactase Activity

    a. Primary low lactase activity: It is relatively a common syndrome. It is seen particularly among non-white population in USA as well as other parts of the world specially South East Asia including India. 

    Intolerance to lactose is not a feature in early life and appears later in life. It is presumed to represent a gradual decline in the activity of the enzyme lactase in susceptible individuals.

    b. Secondary low lactase deficiency: This is secondary to many GI conditions prevalent in tropics and non-tropical countries like:

• Tropical and nontropical sprue (Celiac disease)

• Kwashiorkor

• Colitis and chronic gastroenteritis Also can occur after surgery of peptic ulcer. 

2. Sucrase Deficiency Inherited deficiency of sucrase and isomaltase have been reported. Symptoms occur in early childhood with ingestion of sugars (cane sugar and table sugar) sucrose, a disaccharide. Symptoms and signs as in lactase deficiency.

3. Disacchariduria An increase in the excretion of disaccharides may be observed in some patients with disaccharidase deficiency. As much as 300 mg or more of disaccharides may be excreted in those people and in patients with intestinal damage (e.g. sprue and celiac disease).

4. Monosaccharide Malabsorption Inherited disorders in which glucose and/or galactose are absorbed very slowly have been reported. The reason probably is absence of “carrier protein” necessary for absorption of glucose/galactose.

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