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Topic H.2 – Digestion H.2.1 - State that digestive juices are secreted into the alimentary canal by glands, including salivary glands, gastric glands in the stomach wall, the pancreas and the wall of the small intestine. The alimentary canal of the digestive system includes the mouth, esophagus, stomach, small intestine, and large intestine. Food travels in one direction through the alimentary canal and is digested along the way, nutrients are absorbed and waste products are formed. The digestive system also contains accessory organs which contribute secretions that aid in the chemical digestion of food. These accessory glands include the salivary glands, gastric glands, pancreas, liver and glandular cells in the intestinal wall. Remember the primary role of digestion is to convert the large macromolecules we eat into their much smaller subunits that can then be absorbed into our cells. These accessory glands contribute digestive secretions or “juice” that aid in the breakdown of macromolecules. The following chart summarizes the digestive glands and their secretions. Digestive gland Secretion Saliva containing salivary Salivary gland amylase Mucus, hydrochloric acid Gastric glands and pepsinogen Pancreatic juice containing a protease, more amylase and lipase. Pancreas Also contains a form of hydrogen carbonate to neutralize stomach acids Liver Bile Intestinal glandular Variety of digestive cells enzymes Action Begins digestion of polysaccharides Begins digestion of proteins in the stomach Continues digestion of carbohydrates and proteins that began in the mouth and stomach. Lipase digests fats once they have been emulsified by bile. Emulsifies lipids Completes digestion of the macromolecules H.2.2 - Explain the structural features of exocrine gland cells. An exocrine gland is a collection of cells that produce a product and release it into a very specific location. An exocrine gland cell contains ducts which transfer a secretion from the gland to a specified location. More often than not the secretion is a protein either in the form of a digestive enzyme or a hormone. Due to the fact that the secretions from an exocrine gland are often proteins these glands tend to contain high numbers of organelles that are involved in the synthesis and processing of proteins. This would include high numbers of ribosomes, endoplasmic reticulum, Golgi bodies, vesicles, and mitochondria. The pancreas is an example of an exocrine gland that secretes enzymes into a duct. The exocrine cells of the pancreas are grouped around the end of a very small branch of the much larger pancreatic duct known as a ductule. The ductules empty into larger and larger ducts until the secretions reach the pancreatic duct and can then be released into the small intestine. The grouping or arrangement of the exocrine glands around the ductule is called an acinus. H.2.3 - Compare the composition of saliva, gastric juice and pancreatic juice. Saliva Solvent is water Gastric Juice Solvent is water Pancreatic Juice Solvent is water Amylase Mucus Amylase Mucus Hydrochloric acid Bicarbonate Pepsin in the form of pepsinogen Trypsin in the form of trypsinogen Lipase Source: Heinemann Baccalaureate Biology Higher. New York : Heinemann International Inc., 2007. H.2.4 - Outline the control of digestive juice secretion by nerves and hormones, using the example of secretion of gastric juice. This concept had its beginnings when Ivan Pavlov performed his classic experiment with salivating dogs and how the body prepares itself for digestion of food. You may recall that the dogs would begin to salivate at the occurrence of a stimulus that signaled food was on its way. Digestive juices are not continuously secreted. They are only released when they are needed to hydrolyze molecules. For example, digestive enzymes in the stomach are released once a stimulus is received. As we saw earlier, the sight and smell of food can stimulate the release of gastric juices. Presence of food in the stomach stimulates receptors in the stomach wall to send a message to the brain which in turn signals the secretion of more gastric juices. There is also a hormone called gastrin involved in the control of the secretion of gastric juices. When the stomach is distended (widened due to presence of food) it promotes the production of gastrin. Gastrin is a hormone that causes a sustained release of gastric fluid, most of all hydrochloric acid. H.2.5 - Outline the role of membrane-bound enzymes on the surface of epithelial cells in the small intestine in digestion. Most enzymes which enter the alimentary canal catalyse their specific hydrolytic reaction by mixing with the substrates that have been consumed. They often have a limited molecular lifespan and are normally digested themselves or eliminated as wastes. There are some exceptions which are membrane-bound digestive enzymes which are produced by and remain in the membranes of the cells (epithelial) lining the small intestine. An example of such an enzyme would be maltase. Maltase breaks down the disaccharide maltose into two glucose molecules. Maltase remains embedded within the membranes of the inner epithelial cells of the villi and microvilli. As maltose comes in contact with the active site of maltase the enzymes catalyses the hydrolysis reaction. The advantage of the membrane bound enzymes is that once the reaction has been catalysed the end products are exactly where they need to be in order to be absorbed. H.2.6 - Outline the reasons for cellulose not being digested in the alimentary canal. Despite the fact that many mammals are herbivores, no species of mammal including humans produces an enzyme that can digest cellulose, a polysaccharide composed of thousands of glucose molecules and a major structural component of plant cell walls. Mammals known as grazers (i.e. cows) contain a huge number of mutualistic bacteria in their intestines which produce an enzyme known as cellulase that can hydrolyze cellulose into glucose. Despite the number of bacteria present these grazers still cannot get a high yield of energy from plant material so they must continually ingest large amounts of plant matter. Due to the fact that we do not contain cellulase producing bacteria and we cannot produce cellulase on our own, most plant material we consume exits the body in our feces. H.2.7 - Explain why pepsin and trypsin are initially synthesized as inactive precursors and how they are subsequently activated. Pepsin and trypsin are known as proteases; enzymes which catalyze the hydrolysis of peptide bonds in proteins. The difficulty is that proteases cannot distinguish between a protein that has been ingested and a structural protein that is part of the human body. In order to control the hydrolysis of essential proteins these proteases are initially released in an inactive molecular form. Pepsin is released as pepsinogen and trypsin is released as trypsinogen. The inactive molecular form of pepsin has 44 additional amino acids attached to the primary structure of the enzyme. When pepsinogen is released into the stomach it is exposed to hydrochloric acid which removes the extra 44 amino acids converting pepsinogen into pepsin, the active protease. There is a lining of mucus that protects the lining of the stomach from being digested by pepsin and hydrochloric acid. Pepsinogen Pepsin Trypsinogen is released from the pancreas via the pancreatic duct and enters the small intestine at the duodenum. When the partially digested food from the stomach enters the small intestine it stimulates the release of an enzyme known as enterokinase. Enterokinase converts trypsinogen into its active form trypsin. H.2.8 - Discuss the roles of gastric acid and Helicobacter pylori in the development of stomach ulcers and stomach cancers. It was believed for a long time that stomach ulcers were caused by too much hydrochloric acid in the stomach perhaps brought on by stress. Until very recently scientists believed that the acidic level in the human stomach was too high for any organism to survive there. Dr. Barry J. Marshall and Dr. J. Robin Warren isolated bacterial cells from the intestines of patients with stomach ulcers in 1982-83. This has lead us to what we now know and understand about this strain of bacteria known as Helicobacter pylori. These bacteria can survive in the human stomach by burrowing beneath the mucus layer and infecting the cells of the lining of the stomach. They use the enzyme urease to create ammonia which neutralizes the stomach acids. The infection in the cells leads to gastritis and stomach ulcers. Patients can be treated with antibiotics but those who have had gastritis for 20-30 years have a significant increase in their risk of stomach cancer as compared to the general population. H.2.9 - Explain the problem of lipid digestion in a hydrophilic medium and the role of bile in overcoming this. Lipids serve many important functions within the human body (think phospholipids!) however they are difficult to digest due to their insolubility in water. As we saw earlier the solute for digestive enzymes is water and lipids are not water soluble. This means we have hydrophobic molecules in a hydrophilic medium. When lipids are exposed to an aqueous environment they tend to stick together or coalesce. When the molecules clump together it decreases the surface area in comparison to the volume of molecules. This means that the lipid molecules on the outside of this cluster of lipids can be digested but the enzymes cannot reach the lipid molecules in the interior. This problem is overcome by the addition of bile to the small intestine. Bile is produced by the liver and stored in the gall bladder. Bile molecules have both a hydrophobic and hydrophilic end which allows them to be partially soluble in both water and lipids. This means that bile molecules can wedge themselves inside that large lipid globule, break them apart and prevent them from coalescing again into the large lipid molecule. This process is known as the emulsification of lipids. The now much smaller lipid molecules can be digested by lipase.