The digestive process can be initiated by sight, smell, taste, and even the thought of experiencing those things. Although some enzymatic digestion begins in the mouth, most of this process occurs in the small intestine. We will begin our discussion of digestion by starting with the carbohydrate, which provides 45% of the total energy needs of the American diet. Carbohydrates are classified into thre e groups:
monosaccharides, disaccharides and polysaccharides. The small intestine has the ability to absorb monosaccharides but not disaccharides or polysaccharides. Therefore, enzymes are necessary to convert the disaccharides and polysaccharides to monosaccharides prior to absorption. About 50% of dietary carbohydrate is in the form of starch which is the storage form of carbohydrates in plants. The storage form of carbohydrates in animals is the polysaccharide glycogen. Both plants and meat are polymers of glucose molecules. Most of the dietary carbohydrates that are disaccharides are ingested as sucrose or lactose (40%). Dietary monosaccharides are fructose and glucose, and make up the remaining 10%.
Remember, only monosaccharides can be absorbed from the small intestine into the blood, therefore all carbohydrates must be enzymatically digested to their simplest form (monosaccharide) before transport can take place. Some polysaccharides cannot be broken down at all because our bodies lack the necessary enzymes. These polysaccharides are known as fiber. Fiber is found in all kinds of plants, for example, the outer covering of corn kernels is composed of fiber, thus, if you don't chew the corn it can move through the entire digestive system without receiving a single scratch, in fact it won't even change color. Still, fiber is a very important component for the digestive system as it helps keep the stool loose and moving. Stool that doesn't move through the digestive system can be very unpleasant.
The process of digestion is really a discussion of enzymes and their ability to cleave or break bonds. Polysaccharides are put together using three different kinds of bonds between the monomers, these bonds are called: alpha 1- 4, alpha 1- 6 and beta 1- 4 linkages. The first enzyme that a carbohydrate will encounter is found in the salivary secretions of the mouth and is known as salivary amylase. Salivary amylase can hydrolyze (break) alpha 1- 4 bonds but is quickly inactivated by the stomach acid. The majority of carbohydrate digestion occurs in the small intestine through the actions of pancreatic amylase. Pancreatic amylase is also specific for most alpha 1- 4 bonds but since carbohydrates are a combination of all types of bonds the digestion is incomplete.
Amylase only acts on internal bonds in the polysaccharide chain, it cannot cleave individual glucose molecules from the chain. To complete digestion the small intestine has specific enzymes that are located on the apical membranes of the enterocytes (small intestinal epithelial cells), called brush border enzymes. These enzymes are able to hydrolyze alpha 1- 4 bonds left by amylase as well as alpha 1- 6 bonds and some beta 1-4 bonds. You have probably heard of some of these brush border enzymes as they are named after the carbohydrate that they are most specific for, for example, lactase, sucrase and maltase. Lactase is specific for the disaccharide lactose, and sucrase is specific for the disaccharide sucrose. Maltase digests maltose, which is a product of the action of amylase on starch and glycogen. The end result is that all of the ingested carbohydrates are converted to their simplest form; glucose, galactose or fructose.
Most animals do not contain enzymes that can break beta 1- 4 bonds. These bonds are found in fiber, however, certain types of bacteria can breakdown the bonds. Cows or animals that eat grass (high in fiber) have large amounts of bacteria in different chambers of their stomachs that help them break down the fiber to usable sources of monosaccharides. Even though we cannot breakdown fiber, it is still an important component of a healthy diet to help keep the stool loose and moving. High fiber diets have also been shown to reduce the risk of colon cancer and to decrease absorption of cholesterol. Individuals that are lactose intolerant have stopped making the brush border enzyme lactase and therefore have lost the capacity to digest lactose. This is actually the normal process since most mammals do not consume milk as adults. It is only humans that are descendants of groups that have domesticated cattle and goats that are not predominantly lactose intolerant. Undigested carbohydrates that are washed down to the large intestine can cause a plethora of unwanted side effects, such as diarrhea, cramping and extreme flatulence, the latter is only desirable at a few unique events like scout camp or when you stay up too late with your friends.
Following digestion and, in order to be moved from the lumen to the blood, the three monosaccharides, glucose, galactose or fructose, must first enter in through the apical membrane of the enterocyte and then exit through the basal membrane to complete absorption. Glucose and galactose are brought through the apical membrane through a Na+ co-transporter known as the sodium-glucose transport proteins (SGLT). This co-transporter is a secondary active transporter that is driven by the Na+ gradient established by the primary active transporter Na+/K+ ATPase. Fructose cannot be transported with Na+ but instead is moved through the membrane by facilitated diffusion through a transporter called GLUT 5. Once inside the cell a single basally located transporter is used to transport glucose, galactose or fructose called GLUT 2.
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