Food has several general components. Macronutrients include carbohydrates, proteins and fats, each with subcategories. Micronutrients include vitamins, minerals and other chemicals needed in small amounts. Nutrients are typically bound together in large chemical compounds. These large molecules must be broken down by the digestive system into smaller units to be absorbed. The oral cavity, stomach and intestines -- along with other organs such as the pancreas -- are designed to break down and absorb nutrients. After absorption, the cells of the body can utilize the nutrients.
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The process of breaking down food into usable nutrients begins in the oral cavity. The tongue, lips and cheeks move the food around in the mouth, exposing it to the teeth. The teeth mash and grind the food. The saliva moistens the food and exposes it to enzymes such as salivary amylase, which breaks down the bonds between long branching carbohydrate molecules. Saliva also causes the chewed food to stick together, forming a bolus.
The stomach contains hydrochloric acid. This kills many bacteria and it also helps the enzymes in the stomach to work. Stomach enzymes such as pepsin break down protein into amino acids. Other types of molecules are not as thoroughly digested as protein until reaching the small intestine. After digestion in the stomach, the food moves through the pylorus into the small intestine.
The liver releases bile and the pancreas releases digestive enzymes into the first part of the small intestine, called the duodenum. The duodenum also has its own enzymes. Bile helps to dissolve fats and neutralize stomach acid. Enzymes break down the remaining protein into amino acids, and carbohydrates and fats into individual molecules. Then, the cells of the intestine absorb the nutrients and pass them into the blood stream, where they are processed by the liver.
Once in the body, fats, carbohydrates and amino acids can be stored by such tissues as the skeletal muscle and the liver. Alternatively, they can be biochemically broken down by the cells in order to form the energy-carrying molecules of the cell, such as adenosine triphosphate, or ATP.