CARBOHYDRATES
Carbohydrates are the most abundant organic molecules in nature with the empiric formula for many of the simpler carbohydrates being (CH2O), hence the name "hydrate of carbon.
They have a wide range of functions which are:-
They provide a significant fraction of the energy in the diet of most organisms.
They act as a storage form of energy in the body.
They serve as cell membrane components that mediate some forms of intercellular communication.
Carbohydrates also serve as a structural component of many organisms, including the cell walls of bacteria, the exoskeleton of many insects, and the fibrous cellulose of plants.
Digestion and absorption of carbohydrates
For most humans, starch is the major source of carbohydrates in the diet. Digestion begins in the mouth, where salivary α amylase hydrolyzes the internal glycosidic linkages of starch, producing short polysaccharide fragments or oligosaccharides.
In the stomach, salivary α amylase is inactivated by the low pH, but a second form of α- amylase, secreted by the pancreas into the small intestine, continues the breakdown process. Pancreatic α amylase yields mainly maltose and maltotriose (the di- and trisaccharides of α (1→4) glucose) and oligosaccharides called limit dextrins, fragments of amylopectin containing
α (1→6) branch points.
Maltose and dextrins are degraded by enzymes of the intestinal brush border (the fingerlike microvilli of intestinal epithelial cells, which greatly increase the area of the intestinal surface).
Disaccharides must be hydrolyzed to monosaccharides before entering cells. Intestinal disaccharides and dextrins are hydrolyzed by enzymes attached to the outer surface of the intestinal epithelial cells:
The monosaccharides so formed are actively transported into the epithelial cells, then passed into the blood to be carried to various tissues, where they are phosphorylated and funneled into the glycolytic sequence.
Absorption of monosaccharides by intestinal mucosal cells
The duodenum and upper jejunum absorb the bulk of the dietary sugars. However, different sugars have different mechanisms of absorption. For example, galactose and glucose are transported into the mucosal cells by an active, energy-requiring process that involves a specific transport protein and requires a concurrent uptake of sodium ions. Fructose uptake requires a sodium-independent monosaccharide transporter (GLUT-5) for its absorption. All three monosaccharides are transported from the intestinal mucosal cell into the portal circulation by yet another transporter, GLUT-2.
Transport of Glucose into cells
Glucose cannot diffuse directly into cells, but enters by one of two transport mechanisms:
- A Na +- independent, facilitated diffusion transport system or a Na+–monosaccharide co-transporter system.
A. Na+-independent facilitated diffusion transport
This system is mediated by a family of at least fourteen glucose transporters in cell membranes. They are designated GLUT-1 to GLUT-14 (glucose transporter isoforms 1 to 14). These transporters exist in the membrane in two conformational states. Extracellular glucose binds to the transporter, which then alters its conformation, transporting glucose across the cell membrane. The glucose transporters are specific i. e GLUT-3 is the primary glucose transporter in neurons, GLUT-1 is abundant in erythrocytes and brain, but is low in adult muscle, whereas GLUT-4 is abundant in adipose tissue and skeletal.
In facilitated diffusion, glucose movement follows a concentration gradient, that is, from
a high glucose concentration to a lower one.
B. Na+- monosaccharide cotransporter system
This is an energy-requiring process that transports glucose against" a concentration gradient—that is, from low glucose concentrations outside the cell to higher concentrations within the cell.
This system is a carrier-mediated process in which the movement of glucose is coupled to the concentration gradient of Na+, which is transported into the cell at the same time. This type of transport occurs in the epithelial cells of the intestine, renal tubules, and choroid plexus.
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