Wow! What a roller coaster! I never knew digestion would be such an agitating process. First, we were chewed up by the teeth. Some enzymes joined us in the mouth, but they werenít very nice. They kept trying to break us down, and some of the glucans I was bonded to were pulled away. Next, we went down a long tube called the esophagus. The muscles in the walls of the tube were very pushy! We were then admitted into the stomach by a valve called the Sphincter. He was kind of paranoid, and did not let us through until he was sure there was enough room for us. Inside the stomach, it was very dark. All of a sudden, these glands started releasing slimy mucus, water, and strong acid on us. I was pretty scared because I couldnít see what was going on, and I only had one glucan who was still bonded to me to talk to. After about four hours in that horrible stomach, the muscles in its walls finally pushed us on to part of the small intestine called the duodenum. At first, I thought this organ was going to be much better because the intestinal juices started to neutralize those strong stomach acids that were hanging around us. Soon, however, some more glands started letting loose pancreatic enzymes, which were even meaner than those stomach juices. Some guy named Amylase came and broke the bond between me and the other glucan, so now Iím not even part of a disaccharide anymore. When we had all been broken down into simple sugars again, we were pushed on into the small intestine. The walls of the small intestine were covered with little finger-like projections called villi. There were so many that they looked like a pink carpet. Then, the whole organ started churning so much that I thought I was going to be sick. I looked for a way out and was able to pass through the walls of one of the villi into a capillary. I was so glad to get out of those disgusting organs! Now the blood in the capillary is carrying me along to my next destination. Itís not a bad way to travel, so I might as well go with the flow!
The process of digestion breaks down complex food molecules (such as the starch in cereal) that the body cannot readily use into simpler, smaller molecules (glucose) which the body can absorb and use to fuel other life processes. The digestive tract performs this mission like a machine. Each organ has a specific job and depends on the others to perform properly. In the mouth, initial physical and chemical break-down of starch begins. The stomach continues physical break-down and moves the food into the duodenum, the first part of the small intestine. The duodenum and pancreas work together to complete the chemical break-down of starch into single glucose molecules. Finally, the small intestine absorbs the glucose molecules, allowing them to pass into the bloodstream. The blood then carries the simple sugars to the rest of the body to use for energy. Each part of the "machine" of digestion must work properly in order for starch to be broken down into useful glucose.
The process of digestion begins in the mouth as big pieces of food are broken down into smaller ones by the chewing and grinding of the teeth. This process helps to expose more of the foodís surface area to chemical digestion. The saliva in the mouth moistens the food so it will travel more smoothly down the digestive tract. Ptyalin, an enzyme in saliva, begins to break down starch into simpler sugars. It accomplishes this depolymerization through a process known as hydrolysis. In hydrolysis, a water molecule is inserted between the two glucans which are bonded together. This breaks the glycosidic bond between them by "capping" the free reactive ends with the H and the OH. A starch, therefore, enters the mouth as a polysaccharide and leaves as a less complex sugar -- usually a disaccharide called maltose.
Food exits the mouth when a person swallows. While you are chewing, the tongue shapes your food into a ball, called a bolus. When you are ready to swallow, the tongue pushes the bolus onto the pharynx of the throat. The pharynx is a tube which has two purposes. It carries food to the stomach and air to and from the lungs. When you swallow, there are, therefore, two flaps which must block the airway so the food will go down the right tube. The uvula flaps upward and closes off the airway leading to the nose. The epiglottis folds over to cover the opening to the trachea, the air pipe that leads to the lungs. The bolus must enter the only uncovered opening, the esophagus, which leads to the stomach.
The esophagus is a short (about 10 in) tube which runs behind the heart before curving to meet the stomach. Peristalsis, or wave-like contractions of the muscles in the outer walls of the digestive tract, carries the bolus down the esophagus. At the lower end of the esophagus, the muscles are particularly strong and can pinch the tube shut to prevent food in the stomach from re-entering the esophagus. The end of the esophagus is closed off from the stomach by the cardiac sphincter, a ring-shaped muscle that draws in like a purse string and is located near the heart. When the bolus reaches the end of the esophagus, the cardiac sphincter relaxes and allows the food to pass through into the stomach.
The walls of the stomach are made up of strong muscles which mix and churn the food in the stomach by making rhythmic contractions (about three per minute). These strong walls are covered by a lining filled with tiny glands. These glands secrete gastric juices such as water, enzymes, and acid to aid in the digestive process. Hydrochloric acid (HCl) , which is strong enough to burn a hole through a carpet, is one of the stomachís secretions. It helps to soften food and kill any germs it may contain. The HCl in the stomach is so strong, in fact, that it would digest the stomach itself if it were not for another secretion: mucus. This thick, gooey liquid helps protect the delicate cells in the stomachís lining and also moistens food. The cells in the stomachís lining still wear out, however, and are constantly being replaced. The entire lining of the stomach is replaced every three days. Enzymes are another secretion which the stomach produces. Enzymes aid in digestion by speeding up the chemical reactions that break down food. The enzyme that breaks down starch, however, cannot work in the stomach because the pH is too low (the stomach is too acidic). Starches, therefore, do not break down further until they reach the small intestine.
After about four hours, the food in the stomach has been mixed with gastric juices and is churned into a thick, soupy liquid called chyme. The wave-like contractions of peristalsis then move the chyme downward. With each contraction, the pyloric sphincter, the ring-like muscle which closes off the stomach from the intestines, opens and allows a small amount of chyme to enter the duodenum, or first part of the small intestine.
The duodenum is a C-shaped tube which is about 10 inches (25 cm) long. When the highly acidic chyme enters the duodenum, it neutralizes the HCl with an alkaline secretion. (If the chyme is not neutralized enough, the acid attacks the gut wall, causing a duodenal ulcer.) The pancreas also sends secretions into the duodenum. Two of the most important components of pancreatic juice are sodium bicarbonate (NaHCO3), which helps neutralize the acid in chyme, and amylase, an enzyme which helps break down complex carbohydrates into simple sugars. Amylase continues the work begun in the mouth by ptyalin and completes the process of breaking down a starch into single glucose molecules. Ptyalin breaks down a polysaccharide (starch) into a disaccharide (maltose). Amylase finishes the break-down by splitting the two glucose molecules in maltose into single glucans. It does this through the process of hydrolysis. Like ptyalin in the mouth, Amylase inserts a water molecule between the two glucans which are bonded together. This breaks the glycosidic bond between them by "capping" the free reactive ends with the H and the OH. The two glucose molecules are now separate monosaccharides.
The walls of the small intestine are covered with millions of finger-like projections called villi. These projections increase the amount of surface area in the small intestine for final digestion and absorption to take place by about 600 times. Inside of each villus there is a network of blood capillaries and lymph vessels. Because the starch has been broken down into small monosaccharides, the simple sugars can pass right through the intestinal lining into the thin capillary walls. The glucose molecules are then carried off by the blood flowing through the capillary network.