" 

 == Edit request on 6 May 2013 == 

 ≈ 

  

  

  

  
 Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance.  Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones. 

 When food enters the mouth, digestion of the food starts by the action of mastication, a form of mechanical digestion, and the wetting contact of saliva. Saliva, a liquid secreted by the salivary glands, contains salivary amylase, an enzyme which starts the digestion of starch in the food. After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called a bolus. It will then travel down the esophagus and into the stomach by the action of peristalsis. Gastric juice in the stomach starts protein digestion. Gastric juice mainly contains hydrochloric acid and pepsin. As these two chemicals may damage the stomach wall, mucus is secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals. At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis, which are waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes. After some time (typically an hour or two in humans, 4–6 hours in dogs, somewhat shorter duration in house cats), the resulting thick liquid is called chyme.  When the pyloric sphincter valve opens, chyme enters the duodenum where it mixes with digestive enzymes from the pancreas, and then passes through the small intestine, in which digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine.  Water and minerals are reabsorbed back into the blood in the colon (large intestine) where the pH is slightly acidic about 5.6 ~ 6.9.  Some vitamins, such as biotin and vitamin K (K2MK7) produced by bacteria in the colon are also absorbed into the blood in the colon.Functions of the colon Waste material is eliminated from the rectum during defecation. 

 ==Digestive systems== 

 Digestive systems take many forms. There is a fundamental distinction between internal and external digestion. External digestion is more primitive, and most fungi still rely on it.Dusenbery, David B. (1996). “Life at Small Scale”, pp. 113-115. Scientific American Library, New York. ISBN 0-7167-5060-0. In this process, enzymes are secreted into the environment surrounding the organism, where they break down an organic material, and some of the products diffuse back to the organism. Later, animals form a tube in which internal digestion occurs, which is more efficient because more of the broken down products can be captured, and the internal chemical environment can be more efficiently controlled.Dusenbery, David B. (2009). Living at Micro Scale, p. 280. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6. 

 Some organisms, including nearly all spiders, simply secrete biotoxins and digestive chemicals (e.g., enzymes) into the extracellular environment prior to ingestion of the consequent ""soup"". In others, once potential nutrients or food is inside the organism, digestion can be conducted to a vesicle or a sac-like structure, through a tube, or through several specialized organs aimed at making the absorption of nutrients more efficient. 

 [[image:Bacterial Conjugation en.png|right|thumb|250px|Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cel