Digestive System Parts and Function

Digestion All organisms are composed of four complex biological molecules: lipids (or fats), proteins, carbohydrates, and nucleic acids. For consumers such as humans, these molecules must be broken down into their component parts. Lipids to fatty acids, proteins to individual amino acids, and carbohydrates into simple sugars The chemical breakdown of complex biological molecules into their component parts is the process of digestion.

The digestive system In most animals the digestive system is made up of a tube (alimentary canal) that runs more or less the length of the body. Generally the food moves in one direction and different parts are responsible for doing different jobs in the digestive process. There are also accessory organs that are important in digestion that connect to the alimentary canal via ducts.

Principle Parts of Alimentary Canal Mouth- mechanical breakdown of food; tasting; secretion of salivary glands (salivary amylase) Esophagus- muscular tube that connects the mouth with the stomach Stomach- large muscular storage organ; functions in storage, mixing, some secretions (acid and pepsinogen) Small intestine (3 parts) Duodenum, jejunum, ileum Receives bile, pancreatic amylase, other secretions; absorption of nutrients (most sugars absorbed here) Large intestine- reabsorption of water; bacteria in colon produce Vit. K Anus- external opening surrounded by sphincter muscle

Accessory Organs Connected to Digestive System Liver- has many functions including regulation of amino acids in blood, production of glycogen (a storage molecule) and bile, converting ammonia to urea Pancreas- secretion of amylase and insulin (lack on insulin may cause diabetes) Gall Bladder- storage of bile

Salivary Glands Esophagus Stomach Pancreas Large Intestine Appendix Small intestine Gall Bladder Liver

Connections to the Circulatory System Mesenteric Veins- veins connected to capillary bed closely tied with the intestine Hepatic Portal System- major blood vessel that takes blood from the intestine to the capillary bed in the liver Circulatory system- major system that transports nutrients to the rest of the body Absorption of nutrients from the small intestine occurs with the aid of villi and microvilli in the small intestine which increase surface area for diffusion

The Villi of the small intestine

Enzymes They are biological catalysts which greatly increase the rate of a chemical reaction but are not themselves changed during the process Enzymes are central in the digestion of many substances including carbohydrates, fats and proteins. In most animals, the digestive enzymes are secreted into a special extracellular (outside of the cells) cavity called a gut where digestion actually takes place These smaller molecules can then be absorbed by the circulatory system and distributed to cells throughout the body.

Importance of Proteins Proteins are important as a structural element in bones, cartilage, hair, feathers, nails, and cell membranes. They are also important as enzymes, hormones, antibodies, and in oxygen transport in red blood cells. Proteins are formed by the linkage of amino acids into polypeptides.

Digestion of Proteins Any enzyme that digests proteins is called a protease Chemical digestion of proteins begins in the stomach The stomach is very acidic (has a low pH, 1.5 - 7) Pepsin is the primary digestive enzyme in the stomach The small intestine carries out further digestion with trypsin, which is secreted by the pancreas As proteins are digested, the polypeptide chains unravel and break up into small chains of amino acids called peptides

Importance of Lipids They are fats and oils which are a fundamental component of cell membranes and may be used for energy storage or insulation A characteristic feature is that they do not dissolve in water

Digestion of Lipids It begins in the small intestine by making the molecules more compatible with water so that the digestive enzymes can access them. This is accomplished by breaking up the lipid into small droplets which can be distributed in the water of the small intestine This process is referred to as emulsification Bile which is produced by the liver, stored in the gall bladder and pumped into the small intestine when lipids are present is responsible for emulsification Once emulsified, they may be digested into their subunits (glycerol and fatty acids) by digestive enzymes called lipases. They are produced in the pancreas and secreted into the small intestine

Importance of Carbohydrates These include simple sugars such as glucose and sucrose and polysaccharides such as starch and cellulose They are important as structural compounds and as a source of energy that can be used as ATP Starch is a complex polysaccharide made in plants cells for the storage of energy Foods such as potatoes and pumpkins are rich in starch and can be good sources of energy Cellulose is one of the most common carbohydrates and can be found in the cell walls of plants Human digestive system is unable to break down cellulose and is the largest component of dietary fiber

Digestion of carbohydrates The digestion begins by converting polysaccharides (long chains of simple sugars) and disaccharides (two sugars linked together) into monosaccharides (simple sugar units) that can be absorbed by body cells It begins in the mouth and is completed in the small intestine (they are not digested in the stomach) Amylase is the enzyme responsible for digesting starch It can be found in the mouth in one’s saliva as well as in the small intestine secreted by the pancreas