Crystal Choy
Carbohydrates, Fats, and Proteins
Carbohydrate digestion begins in the mouth. When you chew, salivary glands produce
salivary amylase that breaks down starch into smaller polysaccharides and maltose. Then,
carbohydrates enter the stomach. No carbohydrate digestion takes place in the stomach, but
salivary amylase is destroyed by acids. While the carbohydrates travel to the small intestines,
pancreatic amylase is secreted by the pancreas to the small intestines. Pancreatic amylase
converts the remaining starch that didn’t get broken down into maltose. Also, in the small
intestines, maltase, sucrase, and lactase break down disaccharides into monosaccharides that
enter the bloodstream. Monosaccharides in the bloodstream travel to the liver. In the liver, the
monosaccharides are converted to glucose, where cells can use for energy. Leftover glucose is
stored as glycogen in the liver. Carbohydrates that do not get broken down still, like fiber, are
still undigested as it enters the large intestines. It passes through the large intestines and leaves
our bodies through feces.
Fat digestion starts through the mouth. Salivary glands uses the enzyme, lingual lipase to
break down some of the triglycerides. After swallowing the food, fats travel down the esophagus
to the stomach. In the stomach, fats are mixed and broken down into fat droplets. Gastric lipase
breaks down some triglycerides. Up to this point, only a small portion of fats are digested. Most
of the fat digestion occurs in the gallbladder and small intestines. The gallbladder creates bile for
the small intestine to hydrolyze pancreatic enzymes in order to break large fat droplets into
smaller droplets. The gallbladder compresses because of CCK and secretin. In the small
intestines, triglycerides are broken further down into monoglycerides and fatty acids. Then, lipiddigesting enzymes change cholesterol esthers and phospholipids into components. Lastly,
micelles bring the components to the small intestines for absorption.
The process of protein digestion begins through the mouth. Protein is chewed and
moistened by saliva. Then it is passed to the stomach, where proteins are denatured by the acid in
the stomach, called hydrochloric acid. The acid triggers pepsin in order to break down the
proteins into smaller substances, like single amino acids and polypeptides that can be useful for
mucosal cells. After it goes through the stomach, it enters the small intestines. Enzymes called
proteases, which are created by the pancreas, and cells in the wall of small intestines break down
proteins into amino acids. The last step of the process is the liver. The amino acids travel to the
liver. The liver monitors the amino acids but are not stored there. They are transported to other
cells.