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Digestion
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Digestion is the process of metabolism where by a biological entity processes a substance in order to chemically and mechanically convert the substance for the body to use. In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to break down starches. The stomach continues to break food down mechanically and chemically through the churning of the stomach and mixing with enzymes. Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with excretion.
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Digestion in humans, as in other animals, is the process by which food containing nutrients such as proteins, fats, and carbohydrates is eaten and broken down to its components. These components are absorbed from the small intestine and dispersed into the circulation for use by various organs and cells. The body is ... provided with the molecules from which energy, as calories, is used for metabolism--the chemical processes by which the body builds and recycles bones, blood, muscles, nerves, and organs. These nutrients also provide certain components that the body is unable to make, such as vitamins and minerals, salts, and certain essential amino acids to build proteins and fatty acids required for cell function that the body does not make. Failure to provide any of these leads to deficiency diseases. In the United States, the average diet provides about 11 percent of calories as protein, 46 percent as fat, and 43 percent as carbohydrate.
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Digestion of proteins continues in the duodenum, the first segment of the small intestine. As in fat digestion, the pancreas helps the process by secreting the pancreatic protease enzymes trypsin and chymotrypsin. Like pepsin, trypsin breaks down a protein into single amino acid molecules, through a process called hydrolysis. During hydrolysis, a water molecule is inserted between the two amino acids which are bonded together. This breaks the bond between them. After breakdown, the amino acids are small enough to pass through the intestinal lining into tiny veins (capillaries) in the villi (the finger-like projections on the wall of the small intestine).
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Digestion begins once food is broken down by the teeth, tongue and saliva and passes through the esophagus to the stomach. Once food enters the stomach the break down continues. Slight amounts of chemical processes occur while food is being broken down in the stomach, most pertaining to protein, by the enzymes in the stomach. Once food has been broken down in the stomach it passes to the small intestine where further breakdown occurs by specialized enzymes and helpful bacteria. In the small intestine the useful particles are then absorbed into the bloodstream where they are used by the body, while the remaining elements are ultimately expelled as waste.
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Digestion is usually divided into mechanical manipulation and chemical action. In most vertebrates, digestion is a multi-stage process in the digestive system, following ingestion of the raw materials, most often other organisms. The process of ingestion usually involves some type of mechanical manipulation. Digestion is separated into five separate processes: 1) Ingestion: Placing food into the mouth, 2) Mechanical digestion: Mastication, the use of teeth to tear and crush food, and churning of the stomach. 3) Chemical digestion: Addition of chemicals (acid, bile, enzymes, and water) to break down complex molecules into simple structures, 4) Absorption: Movement of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion, 5) Elimination: Removal of undigested materials from the digestive tract through defecation. Underlying the process is muscle movement throughout the system, deglutition and peristalsis.
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Digestion is usually divided into mechanical processing to reduce the size of food particles and chemical action to further reduce the size of particles and prepare them for absorption. In most vertebrates, digestion is a multi-stage process in the digestive system, following ingestion of the raw materials, most often other organisms. The process of ingestion usually involves some type of mechanical and chemical processing. Digestion is separated into four separate processes:
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Digestion