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“From gum to bum” In this section, you will: identify the main structures and functions of the digestive system describe the physical and chemical processing of food through the digestive system and into the bloodstream identify enzymes and other chemicals involved in digestion describe, in general terms, how digested molecules enter the bloodstream The main role of the digestive system is to metabolize food 2 roles of metabolism are: Catabolic reactions: breakdown of molecules, energy is released Anabolic reactions: the making/formation of a molecule, energy is required In the digestive tracts the following occurs: PolmersÆmonomersÆabsorbed into the bloodÆcellsÆmitcondriaÆenergy (ATP) Unlike plants, heterotrophs must consume large organic molecules and break them down into smaller molecules that can be transported by the blood, absorbed and used in the body. This process of digestion involves four distinct stages: Ingestion: ‘eating,’ taking in food Digestion: 2 types: Mechanical: Physical breakdown of food into smaller pieces to increase surface area Chemical: breaking of the bonds by enzymes to make something new Absorption: movement of food particles into the blood Egestion: ‘pooping,’ removing waste Ingestion (eating) is the process of taking in nutrients and preparing them for transfer to the stomach. Teeth masticate (chew) food into small particles. This is an example of physical (mechanical) digestion. Salivary glands secrete saliva, which moistens and lubricates the food. Saliva also contains a starch digesting enzyme called amylase. Amylase digests polysaccharides into smaller sugars. This is an example of chemical digestion. The tongue positions food for chewing and later rolls the food into a bolus for swallowing. When the food is swallowed, the bolus passes from the pharynx to the esophagus. The epiglottis covers the trachea (windpipe) when swallowing so that food does not enter the respiratory tract. The food is pushed down the esophagus by a wave-like muscular contraction of the esophagus, called peristalsis. The stomach is a J-shaped pouch that has three main roles: Partially digest food chemically (particularly proteins). Physically digest food by churning & by dissolving nutrients. Store food in preparation for further digestion and absorption into the body. The top of the stomach is closed off by a sphincter muscle called the esophageal (or cardiac) sphincter. If this muscle is weak, it can lead to heartburn or acid reflux disorder. The normal stomach The stomach secretes 3 important substances into its lumen (interior): 1. Hydrochloric acid (HCl). Helps dissolve & break up substances, kills pathogens, and denatures (unfolds) proteins. 2. Mucous. Protects the stomach from the acid. 3. Pepsinogen. An inactive form of the enzyme pepsin, which digests proteins. Activated in the stomach by low pH. Stomach ulcer Acid Reflux – aka “heartburn” is caused by the movement of gastric juices (acidic) back into the esophagus. This is usually caused by failure of the cardiac sphincter to close. General Acid Reflux Shockwave™ Animation Peptic Ulcers – are caused by a breakdown of the protective mucous lining in the stomach. This causes lesions (sores) on the stomach lining that are irritated by the acidic secretions in the stomach. Ulcers are caused by stress, diet and the bacteria Heliobacter pylori. The stomach can be removed with any real harm occurring The mixture of partially digested food & stomach acid is called chyme. Chyme enters the first portion of the sm. intestine, called the duodenum, as it is released by the pyloric sphincter. In the duodenum, chyme is joined by secretions from the pancreas and liver. Bile, which is made by the liver and stored in the gall bladder, emulsifies (breaks and dissolves) fat droplets. Digestion involves further chemical breakdown of macromolecules into smaller components. Most digestion takes place in the first 25-30 cm of the small intestine, called the duodenum. Digestion is primarily a chemical process that is helped by a number of enzymes secreted by the pancreas, liver and gallbladder. Animations The pancreas secretes a number of important substances that aid in digestion. Secretion Role Function Bicarbonate (HCO3-) Base Neutralizes stomach acid Carbohydrases Digestive Enzymes Digest carbs Lipases Digestive Enzymes Digest lipids Erepsins Digestive Enzymes Digests small peptides Nucleases Digestive Enzymes Digests nucleic acids Trypsinogen Inactive enzyme Digests proteins once activated by enterokinase Enterokinase Activating enzyme Turns trypsinogen into trypsin. Hormones are chemical messengers that travel through the blood stream. Gastrin is released by the walls of the stomach and further stimulates HCl secretion. Secretin is released by the stomach and stimulates HCO3- production by the pancreas. CCK is released by the small intestine and stimulates bile secretion from the gall bladder. Enterogastrone is released by the sm intestine and inhibits stomach secretions. Nutrients are absorbed from the sm. intestine into the blood stream. The jejunum absorbs most nutrients, while the ileum absorbs lipids & fat soluble vitamins. Finger-like projections called villi point into the lumen of the intestine, which increase surface area. Microscopic projections on the villi, called microvilli, further increase surface area. Capillaries in the villi absorb most nutrients, while small vessels called lacteals, part of the lymphatic system, package & absorb fats. The liver breaks down hemoglobin from red blood cells and stores the products in the gallbladder. The resulting pigments give feces its characteristic color. The liver also stores glycogen and vitamins A, B12, and D. The liver also removes many toxins and harmful substances from the body (like alcohol). Gallstones – crystals of bile salt the form in the gallbladder. These may block the bile duct causing pain and impairing fat digestion. Jaundice – yellowing of the skin and tissues due to an accelerated destruction of red blood cells. Bile pigments are not excreted and accumulate in the body. Cirrhosis – chronic inflammation of the liver caused by nutritional deprivation or infection. Top: Glucose is actively transported into cells of the intestinal wall to move into the bloodstream. Middle: Amino acids are actively transported into the cells of the intestinal wall to move into the bloodstream. Bottom: Glycerol and fatty acid molecules diffuse into the cells of the intestinal wall where they are resynthesized into fats, coated with proteins, and move into lymph vessels for eventual transport into the bloodstream. The colon is mainly responsible for water reabsorption. Many water soluble vitamins are also absorbed at this time. There are also bacteria (such as E. coli) in the colon that make vitamins for us (particularly B & K). Solid wastes (feces) are stored in the rectum until ready for egestion (bowel movement) via the anus. Removal of colonal polyps. The human body takes in matter from the environment in the form of food and water. The human digestive system processes the food and water in order to obtain the macromolecules it needs for survival. Food passes through the digestive tract—the mouth, pharynx, esophagus, stomach, small intestine, and large intestine—during physical digestion. The accessory organs—the salivary glands, liver, gall bladder, and pancreas—supply chemicals that also contribute to the digestion of food as it passes through the digestive tract. The stomach supplies chemicals to aid digestion as well as generating physical contractions to physically break down food. The food is eventually liquefied into soluble units that can pass through cell membranes for transport via the circulatory system to all the cells in the body. The waste materials from the digestive process leave the body via the large intestine. The nutrients that food supplies include carbohydrates, lipids (fats), protein, and nucleic acids. Carbohydrates and lipids are broken down to supply energy; lipids also supply material for the cell membranes. Proteins are more structurally and functionally diverse than carbohydrates and lipids. They assist in transport, immunity, and muscle action and are used to make up most cellular structures. Nucleic acids direct growth and development. Enzymes speed up chemical reactions, particularly for the production of energy. Vitamins and minerals are organic and inorganic substances that enable chemical reactions to occur and aid in tissue development and growth and immunity. These substances are needed for a healthy, functional human body.
