Absorption and Assimilation of Carbohydrates – Biology Class 11 Online Notes
Biology Class 11 : In this article, I have chosen the following topic “Absorption and Assimilation of Carbohydrates” from unit 5, Class 11 Biology by Online classes .
ABSORPTION AND ASSIMILATION OF CARBOHYDRATES
The metabolism of carbohydrates is the process of getting the carbohydrates in the foods we eat into the form that provides fuel to our body’s cells. This process involves digestion, absorption, and transportation to various cells of the body for its utilization (i.e. assimilation). Most commonly, carbohydrate metabolism results in the production of glucose molecules which are the most efficient source of energy for our muscles and our brains. Energy or fuel from our food is used for cell growth, repair, and normal cell functioning.Carbohydrates are broken down to provide glucose for energy. Digestion predominantly occurs with the help of enzymes lining the wall of the small intestine. Once absorbed, galactose and fructose are metabolized further by the liver to produce glucose and minimal amounts of other metabolites.
Carbohydrates are most commonly consumed as polysaccharides (e.g. starch, fibre or cellulose) or disaccharides (e.g. lactose, sucrose, galactose) and therefore need to be broken down into their simpler monosaccharide forms which the body can utilize.
The digestion process of polysaccharides such as starch will begin in the mouth where it is hydrolyzed by salivary amylase. The amount of starch hydrolyzed here is often quite small as most food does not stay in the mouth long. Once the food bolus reaches the stomach the salivary enzymes are denatured. As a result, digestion predominantly occurs in the small intestine with pancreatic amylase hydrolyzing the starch to dextrin and maltose.
Enzymes classed as glucosidases on the brush border of the small intestine break down the dextrin and maltase, lactase and sucrase convert the other disaccharides into their two monosaccharide units.
Absorption & transport
The monosaccharide units, glucose, galactose, and fructose are transported through the wall of the small intestine into the portal vein which then takes them straight to the liver. The mode of transport varies between the three monosaccharides. Absorption of glucose entails transport from the intestinal lumen, across the epithelium, and into the blood. The transporter that carries glucose and galactose into the enterocyte (epithelial cells that help in absorption of digested food in the small intestine) is the sodium-dependent hexose transporter, known more formally as SGLUT-1. As the name indicates, this molecule transports both glucose and sodium ion into the cell and in fact, will not transport either alone.
The essence of transport by the sodium-dependent hexose transporter involves a series of conformational changes induced by binding and release of sodium and glucose, and can be summarized as follows:
- The transporter is initially oriented facing into the lumen and thus, at this point, it is capable of binding sodium, but not glucose
- Sodium binds, inducing a conformational change that opens the glucose-binding pocket
- Glucose binds and the transporter reorients in the membrane such that the pockets holding sodium and glucose are moved inside the cell
- Sodium dissociates into the cytoplasm, causing glucose binding to destabilize
- Glucose dissociates into the cytoplasm and the unloaded transporter reorients back to its original, facing towards the lumen.
Watch animated videos on the same for a detailed explanation by clicking Biology notes for Class 11.
Fructose is not co-transported with sodium. Rather it enters the enterocyte by another hexose transporter (GLUT5).
Glucose, galactose, and fructose are transported out of the enterocyte through another hexose transporter (called GLUT-2) in the basolateral membrane. These monosaccharides then diffuse “down” a concentration gradient into capillary blood within the villus.
Once in the liver galactose and fructose are removed from the blood and converted into other metabolites. When eaten in moderate quantities, most fructose is taken up by the liver and converted to glucose, glycogen, and lactate. A fraction may also be oxidized or converted into fatty acids and uric acid. Only a small amount of fructose reaches the bloodstream, so blood fructose concentrations are always low. Galactose is primarily converted into glucose and stored as glycogen.
Figure: Absorption of glucose, galactose, and fructose from the small intestine into the blood.
Surplus glucose is initially stored as glycogen in the liver or muscles. The liver can store approximately 100g of glycogen which is used to maintain basal blood glucose levels between meals, whilst the muscles typically store 400-500g often used during movement. Once these reserves are saturated, excess glucose is converted to fat for longer-term storage.
The most notable exception to the carbohydrate metabolism explained above is dietary fibre. Dietary fibre, a type of polysaccharide, can be classed as either soluble (dissolves in water) or insoluble (cannot be dissolved in water). The body cannot digest or absorb dietary fibre like other carbohydrates. Instead, a portion is fermented by colonic gut bacteria. As a result, it passes relatively untouched through the digestive system and is removed in stools.
Visit previous article Biology Class 11 – Structure of the small intestine
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