How Carbohydrates are Absorbed:
Undigested lactose is the cause of the symptoms related to LI. Lactase is the enzyme that breaks down lactose into simpler sugars, so that like fructose and other sugars, it is transported via the Transport Mechanism. situated in the small intestine.
Lactose is a sugar. So is sucrose, found in cane sugar, fructose found in fruits, maltose found in the brewery industry, starch found in potatoes and glucose found in our blood. Sugars come under the Biochemical name of carbohydrates and they are a very essential source of energy for every living being. However, carbohydrates is the name to cover many different sugars, of different sizes, weights, shapes and properties but they come under three families, called:
- monosaccharides - made up of one single block of sugar
- disaccharides - made up of two blocks of sugars joined together
- polysaccharides - made up of three or more of sugars joined together
The main monosaccharides are glucose, fructose and galactose. Adsorption of sugars in healthy persons occurs almost exclusively in the the small intestine. Adsorption is limited to the monosaccharides glucose, fructose and galactose. They are large molecules and can only cross cell membranes when a "carrier protein" is present. These are very specific and do not react with and transport other sugars.
Starch is a long chain of many monosaccharides joined together, hence called polysaccharide. Starch is a large number of glucose joined together. They can be joined together as one long chain, called Amylose or in branches called Amylopectine. Cellulose and Glycogen are other forms of starch derived from another type of glucose.
Disaccharides are the sugars that are of main interest to us. They consist of two sugars joined together. Due to their size they are not readily adsorbed by the enterocyte. The transport mechanism is not suitable for them, hence the digestive system produces enzymes to break these sugars into two single ones. The most common disaccharide is the sucrose (right). It is made up of glucose (the left part) and fructose (the right part) cleaved together. The image represents a chemical configuration of the two sugars linked together. Fortunately the body produces a continuous flow of sucrase, the enzyme that breaks down the linkage. The two sugars on their own, now monosaccharides will be easily adsorbed into the small intestine via the transport mechanism, and hence no symptoms occur.
Similarly, lactose (right) is a disaccharide but with glucose and galactose linked together. Lactase is the enzyme that breaks down the linkage. Lactase, whose biochemical name is known as lactase-phlorizin hydrolase (LPH), is found in the brush border of the small intestine when produced.
Lactase at Work
Polysaccharides and disaccharides must be digested to monosaccharides prior to absorption and the key players in these processes are the brush border hydrolases, which include maltase, lactase and sucrase. Dietary lactose and sucrose are "ready" for digestion by their respective brush border enzymes. Starch is first digested to maltose by amylase in pancreatic secretions and, in some species, saliva.
|Lactase: enzyme found in microvilli cells of small intestine that hydrolyzes lactose.|
Dietary lactose and sucrose, and maltose derived from digestion of starch, diffuse in the small intestinal lumen and come in contact with the surface of absorptive epithelial cells covering the villi where they engage with brush border hydrolases:
- maltase cleaves maltose into two molecules of glucose
- lactase cleaves lactose into a glucose and a galactose
- sucrase cleaves sucrose into a glucose and a fructose
Once broken down into the simple form of sugars, they are now readily adsorbed. Glucose and galactose are taken into the enterocyte by cotransport with sodium using the same transporter. Fructose enters the cell from the intestinal lumen via facilitated diffusion through another transporter.
Compared to other mammalian species, human milk has the highest concentration
of the disaccharide lactose. The enzyme lactase is a membrane bound enzyme
located in the brush border or microvilli of the small intestine. Lactase
has two activities. It hydrolyzes phlorizin, a disaccharide found in roots
and bark of plants of the family Rosaceae and some seaweeds and it also
hydrolyzes β-galactoside or put simply, lactose.
The image on the left shows the linkage of glucose and galactose. Lactase hydrolyses (binds water chemically H2O) the linkage. Once lactose is hydrolysed by the interaction of the lactase enzyme, the enzyme then moves onto the next lactose molecule and does the same each time. Enzymes are powerful chemicals. The body does not need to produce enzymes in large quantities because the way they work is very efficient and economical. Enzymes are not chemically changed or altered after the reaction takes place, they act as a catalyst and move on to the next lactose molecule.
The result of the hydrolysed lactose disaccharide is the formation of glucose and galactose as seen in the lower image on the left. These monosaccharides are readily adsorbed by the transport mechanism of the enterocytes in the small intestine.
The Transport Mechanism
Adsorption of glucose, fructose or galactose (monosaccharides), entails transport from the intestinal lumen, across the epithelium and into blood. The transporter that carries glucose and galactose into the enterocyte is the sodium-dependent hexose transporter, known more formally as SGLUT-1. As the name indicates, this molecule transports both glucose and sodium into the cell and in fact, will not transport either alone.
The small intestinal epithelial cells (right) adsorb glucose, galactose and fructose from the intestinal lumen, then export those sugars into blood. These cells have at least three hexose transporters (coloured in blue, pink and green). The apical membrane contains the sodium-glucose co-transporter SGLUT1, which allows the cell to take up glucose and galactose by co-transport with sodium, and GLUT5, which mediates absorption of fructose. On the basolateral plasma membranes is GLUT2, which allows diffusion of all three of these hexoses out of the cell into extra cellular fluid and ultimately, into blood. The body then utilizes them for energy.