The Digestive System Heterotrophs take in organic molecules synthesized by other animals.         1. Digestion provides energy needed to carry out routine metabolic activities and maintain homeostasis.         2. The digestive tract:             a. ingests food,             b. breaks down food into small molecules that can cross plasma membranes.             c. absorbs these nutrient molecules, and             d. eliminates nondigestible remains.

Incomplete versus Complete Digestive System Incomplete gut Digestive system contains only a mouth, a pharynx, and an intestine. Digestive enzymes in gastrovascular cavity allow some extracellular digestion. Digestion is finished intra-cellularly by cells that line the cavity; food diffuses to nearby cells. Digestive system lacks regions of specialized function.

Incomplete versus Complete Digestive System Complete digestive tract is composed of a tube with a mouth at one end and an anus at the other end. Different regions have specialized functions (e.g., ingestion, mechanical digestion, etc.). Muscular pharynx draws in food with sucking action. Crop is storage area with expansive walls. Gizzard has thick muscular walls to grind food. Digestion occurs in intestine, outside of cells Surface area for absorption is increased by intestinal folding. Undigested remains exit the body at anus

Continuous versus Discontinuous Feeders  Continuous feeders, often called a filter feeder. Water moves into a mantle cavity through an incurrent siphon and deposits particles on gills. Cilia move particles to labial palps which direct them into mouth and to stomach. Digestive enzymes from a digestive glands help complete digestion. Marine fanworms are sessile filter feeders; only small particles are consumed. Baleen whales are active filter feeders; baleen (fringe) filters small krill from water.

Continuous versus Discontinuous Feeders Squids are an example             a. Head of a squid has ten arms; two arms seize prey and bring it to the squid's mouth.             b. Beak-like jaws and a radula (toothy tongue) reduce food to pieces.             c. Esophagus leads to a stomach that holds food until digestion is complete.             d. Discontinuous feeders require a storage region in their gut.

Adaptation to Diet Animals are: herbivores (eat plants) or; carnivores (eat animals) or; omnivores (eat both). Mammal dentition differs according to mode of nutrition. Omnivores, including humans, have dentition that accommodate a vegetable diet and a meat diet. Omnivore teeth include incisors (shearing), canines (tearing), premolars (grinding), and molars (crushing). Herbivores have large, flat premolars and molars for grinding plant matter.

Adaptation to Diet Grazers (e.g., horses) also have sharp incisors for clipping off grass and leaves. Hard to digest plant material requires extensive grinding to disrupt plant cell walls. Animals that feed on plants may have long and complex digestive tracts and bacteria in their digestive tracts that can digest cellulose, producing nutrients that an animal can use. Some grazers have a rumen to digest chewed grasses; partially digested cud is rechewed. Carnivores' pointed incisors and canines tear off pieces small enough to swallow. Meat is rich in protein and fatty acids and is easier to digest than plant material. Carnivores have fewer molars for grinding and a shorter digestive tract with less specialization.

Human Digestive Tract 1. Human digestive tract is complete and complex 2. Each part of the digestive system has a specific functio 3. Digestion of food in humans is an extracellular process. 4. Enzymes are secreted into the digestive tract by nearby glands which never contain food themselves. 5. Digestion requires a cooperative effort by production of hormones and actions of nervous system.

Human Digestive Tract Mouth Human dentition has many specializations because humans are omnivores. Food is chewed in the mouth and mixed with saliva. Three pairs of salivary glands secrete saliva by way of ducts into the mouth. Salivary amylase is enzyme that begins starch digestion; maltose is common end product. Salivary amylase starch + H2O     maltose Food is manipulated by a muscular tongue with touch and pressure receptors. Food is chewed and mixed with saliva to form a bolus in preparation for swallowing.

Human Digestive Tract Pharynx and the Esophagus Digestive and respiratory passages come together in pharynx, then separate. During swallowing, path of air to lungs could be blocked if food entered trachea. Epiglottis covers opening into trachea as muscles move bolus through pharynx into esophagus. Esophagus is a muscular tube that moves swallowed food to stomach by peristalsis.

Human Digestive Tract Stomach Stomach stores a liters of partially digested food freeing humans from continual eating. Gastric juice produced by cells of gastric glands. Walls of the stomach contract vigorously and mix food with juices secreted when food enters. Gastric juice contains hydrochloric acid and another digestive substance, pepsin. Gastric juices are produced independently of protective mucous secretions.

Human Digestive Tract Stomach Hydrochloric acid (HCl) lowers pH of the gastric contents to about 2. Epithelial lining of the stomach has millions of gastric pits leading to gastric glands. This acid kills most bacteria and other microorganisms. Low pH also stops activity of salivary amylase and promotes activity of pepsin. Pepsin is a hydrolytic enzyme that acts on proteins to produce peptides. Pepsin protein + H2O     peptides

Human Digestive Tract Stomach A thick layer of mucus protects wall of the stomach and first part of duodenum from HCl and pepsin. Ulcers develop when lining is exposed to digestive action; recent research indicates this is usually due to infection by Helicobacter pylori bacteria. Stomach contents, a thick, soupy mixture, are called chyme. At base of the stomach is a narrow opening controlled by a sphincter (a circular muscle valve). When the sphincter relaxes, chyme enters duodenum; a neural reflex causes the sphincter to contract closing off the opening. Duodenum is first part of the small intestine.             c. The sphincter relaxes and allows more chyme to enter the duodenum.             d. The slow, rhythmic pace with which chyme exits the stomach allows thorough digestion.

Human Digestive Tract Small Intestine Human small intestine is a coiled muscular tube about three meters long. Mucous membrane lining has ridges and furrows; surfaces are covered by villi. Villi are finger-like projections whose surface cells are covered by microvilli. Microvilli are minute projections, a brush border, of surface cells of intestinal villi. Ridges, furrows, villi, and microvilli greatly increase effective surface area of small intestine. As chyme enters duodenum, proteins and carbohydrates are partly digested; no fat digestion occurs.

Human Digestive Tract Small Intestine Additional digestion is aided by secretions from liver and pancreas. Bile is a secretion of liver temporarily stored in gallbladder before sent to duodenum. Bile emulsifies fat; bile is a green byproduct of the breakdown of hemoglobin. Bile contains bile salts that help in emulsification of fat. Emulsification breaks fat globules into microscopic droplets. Bile salts fat       fat droplets This increases fat digestion by increasing surface area of fat globules exposed to enzymes.

Human Digestive Tract Small Intestine Pancreatic juice secreted by pancreas contains the following: 1) Sodium bicarbonate [NaCO3] that neutralizes acidity of chyme; pH of small intestine is slightly basic; 2) Pancreatic amylase that digests starch to maltose; pancreatic amylase starch + H2O       maltose     3) trypsin and other enzymes that digest protein to peptides; trypsin protein + H2O      peptides     4) Lipase that digests fat droplets to glycerol and fatty acids. lipase fat droplets + H2O      glycerol + fatty acids

Human Digestive Tract Small Intestine Epithelial cells of villi produce intestinal enzymes attached to plasma membrane of microvilli. Intestinal secretions complete digestion of peptides and sugars; peptides are digested by peptidases to amino acids; peptidases peptides + H2O      amino acids maltose from the first step in starch digestion is converted by maltase to glucose; maltase maltose + H2O      glucose

Human Digestive Tract Small Intestine Absorption by Villi    1. Small intestine is specialized for absorption by the huge number of villi that line the intestinal wall. 2. If a smooth tube, intestine would have to be 500-600 m long to have a comparable surface area. 3. Each villus contains blood vessels and a lymphatic lacteal. 4. Lacteal is lymphatic vessel in an intestinal villus that aids in absorption of fats. 5. Sugars and amino acids enter villi cells and are absorbed into bloodstream. 6. Glycerol and fatty acids enter villi cells; reassembled into fat molecules, they move into lacteals. 7. Absorption involves diffusion and active transport requiring expenditure of cellular energy.

Control of Digestive Juices Gastrin is produced by cells in gastric glands of stomach wall; stimulates gastric glands and increases gastric motility; its secretion is stimulated by a meal rich in protein. Secretin is produced by cells in duodenal wall; stimulates pancreas to secrete fluids rich in NaCO3 into duodenum; secretion is stimulated by acid chyme. Cholecystokinin (CCK) produced by duodenal wall stimulates pancreas to increase pancreatic juice and liver to increase output of bile; causes gallbladder to release bile; secretion is stimulated by fats. Gastric inhibitory peptide (GIP) from duodenal wall inhibits gastric gland secretion and stomach motility.

Accessory Organs Pancreas Pancreas lies deep within abdominal cavity, just below stomach, and rests on posterior abdominal wall. It is an elongated and somewhat flattened organ. As an endocrine gland, it secretes glucogon and insulin hormone into bloodstream. As an exocrine gland, it secretes pancreatic juice. 1) Pancreatic juice contains sodium bicarbonate that neutralizes acidic chyme. 2) Digestive enzymes digest carbohydrates, fats and proteins.

Accessory Organs Liver Liver is a large glandular organ that fills the top of abdominal cavity, just below diaphragm. Liver has numerous functions: 1) It detoxifies blood by removing and metabolizing poisonous substances. 2) It makes plasma proteins including albumin and fibrinogen. 3) Liver destroys old red blood cells; converts hemoglobin to bilirubin and biliverdin in bile. 4) It produces bile stored in gallbladder before entering duodenum to emulsify fats.

Accessory Organs Liver 5) It stores glucose as glycogen; breaks down glycogen to maintain constant blood glucose concentration. Blood vessels from large and small intestines lead to liver as hepatic portal vein. Liver maintains blood glucose level at 0.1% by removing glucose from hepatic portal vein to store as glycogen; when needed, glycogen is broken down and glucose enters hepatic vein. 6)Liver produces urea from amino groups and ammonia. Amino acids can be converted to glucose but deamination (removal of amino groups) must occur. Using complex metabolic pathway, liver converts amino groups to urea. Urea is most common human nitrogenous waste it is transported by blood to kidneys.

Large Intestine Large intestine is region following the small intestine. It has four parts: cecum, colon, rectum, and anal canal. Appendix This is finger-like projection extending from cecum, a blind sac at junction of small and large intestine. It may play a role in fighting infections. If infected appendix bursts, it results in general abdominal infection (peritonitis). 4. 1.5 liters of water enter digestive tract daily from drinking; another 8.5 liters enter from various secretions. About 95% of this total liquid is reabsorbed by small intestine; remainder by cells of colon. If water is not reabsorbed, it causes diarrhea which can cause serious dehydration and ion loss.

Large Intestine Large intestine functions in ion regulation, absorbing salts plus vitamin K produced by intestinal bacteria. Large intestine that terminates at the anus, an external opening. Feces Feces consists of 75% water and 25% solid matter. One-third of the solid matter is intestinal bacteria. Remainder is undigested wastes, fats, organic material, mucus, and dead cells from intestinal lining.