Digestive System and Digestion
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The Digestive System and Process BIOLOGY 11 The Components of the Digestive System Figure 24.1 Functions of the digestive system Ingestion Mechanical Digestion Secretion Absorption Excretion processing Explain why digestion of large food molecules is essential Most food is solid and in the form of large complex molecules which are insoluble and chemically inert (not readily usable) As food was synthesized by other organisms, it contains materials not suitable for human tissue these need to be separated and removed Large molecules need to be broken down into smaller molecules that can be readily absorbed across membranes and into cells Small molecules can be reassembled into new products (e.g. amino acids can be reassembled to make new proteins) Digestive enzymes Enzymes are globular proteins that control biological reactions. Digestive enzymes speed up the breakdown (hydrolysis) of food molecules into their ‘building block’ components. These reactions occur outside of the cells lining the gut. Explain the need for enzymes in digestion Enzymes are biological catalysts which speed up the rate of a chemical reaction (e.g. digestion) by lowering the activation energy Enzymes allow digestive processes to occur at body temperature and at sufficient speed to meet the organism's survival requirements Enzymes are specific for a given substrate and so can allow digestion of certain molecules to occur independently of others Naming and classification of enzymes There are 2 systems used for naming enzymes: The suffix ‘-ase’ is used with the root name of the substance being acted upon, for example, when sucrose (sugar) is digested, it is acted upon by an enzyme called sucrase. The type of chemical reaction involved as the enzyme functions, for example, when sucrase acts on sucrose, it breaks it into a molecule of glucose and a molecule of fructose. This reaction involves adding a water molecule to break a chemical bond and so the enzyme is a hydrolase. All digestive enzymes belong to this hydrolase class. Enzymes are classified according to the type of chemical reaction catalysed. All digestive enzymes are hydrolases, whereas most of the enzymes involved in energy release for muscular contraction are oxidation-reduction enzymes such as oxidases, hydrogenases and dehydrogenases. Digestive Juices and Enzymes Saliva Gastric juice Pancreatic juice Intestinal enzymes Bile from the liver Substance Digested Product Formed The Structure of the Digestive Tract Figure 24.3 Movement of digestive materials Visceral smooth muscle shows rhythmic cycles of activity Pacemaker cells Peristalsis Waves that move a bolus Segmentation Churn and fragment a bolus Peristalsis Figure 24.4 Control of the digestive system Movement of materials along the digestive tract is controlled by: Neural mechanisms Parasympathetic Hormonal Enhance Local and local reflexes mechanisms or inhibit smooth muscle contraction mechanisms Coordinate stimuli response to changes in pH or chemical The Regulation of Digestive Activities Figure 24.5 The mouth opens into the oral or buccal cavity Its functions include: Analysis of material before swallowing Mechanical processing by the teeth, tongue, and palatal surfaces Lubrication Limited digestion The tongue primary functions include: Mechanical Assistance Sensory processing in chewing and swallowing analysis by touch, temperature, and taste receptors The pharynx Common and air Lined passageway for food, liquids, with stratified squamous epithelium Pharyngeal muscles assist in swallowing Pharyngeal Palatal constrictor muscles muscles Histology of the esophagus Distinctive include features of the esophageal wall Non-keratinized, Folded stratified squamous epithelium mucosa and submucosa Mucous secretions by esophageal glands A muscularis with both smooth and skeletal muscle portions Lacks serosa Anchored by an adventitia The Esophagus Figure 24.10a-c The Swallowing Process Figure 24.11ah Functions of the stomach Bulk storage of undigested food Mechanical breakdown of food Disruption of chemical bonds via acids and enzymes Production of intrinsic factor Digestion and absorption in the stomach Preliminary digestion of proteins Pepsin Permits digestion of carbohydrates Very little absorption of nutrients Some drugs, however, are absorbed Mucous secretion containing several hormones Entero-endocrine cells G cells secrete gastrin D cells secrete somatostatin The Stomach Figure 24.12b The Stomach Lining Figure 24.13a, b The Stomach Lining Figure 24.13c, d Histology of the stomach Gastric glands Parietal cells Intrinsic Chief factor, and HCl cells Pepsinogen Pyloric glands The Secretions of Hydrochloric Acid Figure 24.14 The Phases of Gastric Secretion Figure 24.15a The Phases of Gastric Secretion Figure 24.15b The Phases of Gastric Secretion Figure 24.15c Small intestine Important digestive and absorptive functions Secretions bladder and buffers provided by pancreas, liver, gall Three subdivisions: Duodenum Jejunum Ileum Ileocecal sphincter Transition between small and large intestine Regions of the Small Intestine Figure 24.16a Histology of the small intestine Plicae Transverse Villi Fingerlike projections of the mucosa Lacteals Terminal folds of the intestinal lining lymphatic in villus Intestinal glands Lined by entero-endocrine, goblet and stem cells The Intestinal Wall Figure 24.17a The Intestinal Wall Figure 24.17b, c The Intestinal Wall Figure 24.17d, e Intestinal juices Moisten chyme Help buffer acids Maintain digestive material in solution Small Intestine Duodenal produce glands (Brunner’s glands) mucus, buffers, urogastrone Ileum aggregated lymphoid nodules (Peyer’s patches) Intestinal movements Peristalsis Segmentation Gastroenteric reflexes Initiated by stretch receptors in stomach Gastroileal reflex Triggers relaxation of ileocecal valve The pancreas Pancreatic duct penetrates duodenal wall Endocrine functions Insulin and glucagons Exocrine functions Majority of pancreatic secretions Pancreatic juice secreted into small intestine Carbohydrases Lipases Nucleases Proteolytic enzymes The Pancreas Figure 24.18ac The liver Performs metabolic and hematological regulation and produces bile Histological organization Lobules containing single-cell thick plates of hepatocytes Lobules unite to form common hepatic duct Duct meets cystic duct to form common bile duct The Anatomy of the Liver Figure 24.19a The Anatomy of the Liver Figure 24.19b, c Liver Histology Figure 24.20a, b The gallbladder Hollow, Stores, PLAY pear-shaped organ modifies and concentrates bile Animation: Accessory Organ The Gallbladder Figure 24.21a, b Coordination secretion and absorption Neural and hormonal mechanisms coordinate glands GI activity stimulated by parasympathetic innervation Inhibited by sympathetic innervation Enterogastric, gastroenteric and gastroileal reflexes coordinate stomach and intestines The Activities of Major Digestive Tract Hormones Figure 24.22 Functions of the large intestine Reabsorb feces Absorb Store water and compact material into vitamins produced by bacteria fecal matter prior to defecation The Large Intestine Figure 24.23a The Large Intestine Figure 24.23b, c The rectum Last portion of the digestive tract Terminates Internal at the anal canal and external anal sphincters Histology of the large intestine Absence of villi Presence of goblet cells Deep intestinal glands Physiology of the large intestine Reabsorption in the large intestine includes: Water Vitamins – K, biotin, and B5 Organic wastes – urobilinogens and sterobilinogens Bile salts Toxins Mass movements of material through colon and rectum Defecation reflex triggered by distention of rectal walls The Defecation Reflex Figure 24.25 Processing and absorption of nutrients Disassembles organic food into smaller fragments Hydrolyzes carbohydrates, proteins, lipids and nucleic acids for absorption Carbohydrate digestion and absorption Begins in the mouth Salivary and pancreatic enzymes Disaccharides Brush and trisaccharides border enzymes Monosaccharides Absorption of monosaccharides occurs across the intestinal epithelia Lipid digestion and absorption Lipid digestion utilizes lingual and pancreatic lipases Bile salts improve chemical digestion by emulsifying lipid drops Lipid-bile Micelles salt complexes called micelles are formed diffuse into intestinal epithelia which release lipids into the blood as chylomicrons Protein digestion and absorption Low pH destroys tertiary and quaternary structure Enzymes used include pepsin, trypsin, chymotrypsin, and elastase Liberated amino acids are absorbed Absorption Water Nearly all that is ingested is reabsorbed via osmosis Ions Absorbed transport via diffusion, cotransport, and active Vitamins Water soluble vitamins are absorbed by diffusion Fat soluble vitamins are absorbed as part of micelles Vitamin B12 requires intrinsic factor Digestive Secretion and Absorption of Water Figure 24.27
