Introduction to Digestion
The function of the Digestive System is to absorb body nutrients and to separate waste material for disposal. Several chemical and physical actions occur and the complete passage of a meal through the alimentary tract takes about 44 hours. Since nutrients cannot be absorbed in the way they are ingested, the digestive system breaks down nutrients to simpler chemical structures that can be absorbed by cell membranes.
Carbohydrates, lipids, proteins, water, vitamins and minerals mainly compose food. We are going to focus on the organic structures.
Digestion relies on gastrointestinal secretions for the breakdown of molecules. Secretions originate in specialized cells in the walls of the digestive tract and other organs of the gastrointestinal (GI) system. Substrates for secretions come from plasma, and blood and lymph carry out absorbed products. 20 to 25% of cardiac output (1250-1500 ml/min) goes to the GI system (30% goes directly to the liver through the Hepatic artery and the remaining 70% reaches the liver via the Portal vein, who collects blood from stomach, spleen, pancreas, and small and large intestines). Transfer of water and solutes across secretory cell membranes into duct lumen is accomplished by means of a variety of active and passive transport processes. Secretions are triggered by changes in pressure, or by chemical or neural stimulation. The GI system is innervated by the Autonomous Nervous System, with both parasympathetic and sympathetic fibers and also has an intrinsic nervous system composed of sensory neurons, interneurons, motoneurons and secretory neurons constituting a short reflex regulation of its activity. Motility of smooth muscle and secretion of exocrine glands have neural control.
Saliva, produced by salivary glands, is mainly composed of salivary amylase (ptyalin), mucous cells (mucin) and ion. Salivary amylase starts the breakdown of high-molecular-weight carbohydrates while mucous cells facilitate chewing, swallowing and speech. Saliva has a cleansing action mainly due to the bicarbonates (HCO3-) that neutralize acids. Mouth pH is typically 6.2 and rises to 7.4 when secretion increases. Neural stimulation, both parasympathetic and sympathetic, is excitatory to saliva secretion; but parasympathetic activity is mainly responsible for the 1 to 2 liters of saliva secreted each day. In presence of salivatory amylase and an alkaline media, starch is broken down into maltose and, in a minor degree, into glucose.
Food is in the mouth for a short period of time, but this is enough to start digestion of carbohydrates (starches). When food reaches the stomach, in spite of the acid media, starch digestion continues in the internal part of the bolus. 30 to 40% of the starch is digested by salivary amylase, and the remaining part is later helped along by pancreatic amylase. Disaccharides are later broken into monosaccharides by intestinal epithelial membrane.
When the stomach receives a bolus, carbohydrates are digested the fastest, intermediate are the proteins, and fats last for about 18 hours.
Gastric secretion, on the order of 2 to 3 liters a day, includes hydrochloric acid (HCl) and intrinsic factor, both of which are secreted by parietal cells, and pepsinogen, secreted by chief cells. Intrinsic factor is the only gastric secretory component essential for life, since it is needed for absorption of vitamin B12 required for formation of normal red blood cells. Hydrochloric acid has an important role decreasing the pH, allowing the pepsinogen to transform itself into pepsin and also killing bacteria. HCl production requires large amounts of ATP due to the steep hydrogen transport gradient.
Pepsin, which exists in a mildly acid media, will break proteins into peptides or short chains of amino acids.
The end products of this reaction are molecules of a size that the intestinal epithelium can break down or absorb. Production of gastric secretion is increased by parasympathetic (vagal) stimuli.
Several enzymes are generated by the pancreatic cells and reach the digestive tract at the duodenum. As they need a neutral pH to be active, pancreatic secretion includes alkaline components (1 to 1.5 liters a day) that compensate acidity from the stomach, obtaining an intestinal acid-base balance. Pancreatic enzymes attack carbohydrates, fats and proteins. Secretion is stimulated by the parasympathetic system. Pancreatic amylase breaks down the remaining starch and trypsin breaks down proteins, in the same way pepsin acts on proteins as explained above. Fats ingested are mainly neutral fats called triglycerides. Their digestion takes place only after the emulsification of their molecules. Bile salts have a polar (hydrophilic) end that is soluble in water and a non-polar (hydrophobic) end that is soluble in fats, and agitation from the small intestine helps to break fat into fragments. Water soluble pancreatic lipase breaks emulsified fat molecules into small molecules that can be absorbed by the intestinal mucosa.
Biliary secretion is also stimulated by parasympathetic (vagal) stimuli. 250 to 1,000 ml of bile are secreted daily and its role is to emulsify fats, as explained before.
About the reagents to be used:
Benedict`s solution is alkaline copper sulfate and sodium citrate (blue in color). When boiled in the presence of reduced sugars, cupric ion is reduced by sugars transforming it into an insoluble red cuprous oxide. The solution will turn yellow, orange, green or red, for increasing amounts of sugar, and will remain blue if no sugar is present. If the solution remains blue, it is said the result was negative.
Lugol`s solution is color sensitive to starches. In the presence of unhydrolyzed starch, the result is a dark blackish color; the more starch the darker the color.
Litmus solution indicates changes in pH; it is blue in alkaline solutions and red in acid solutions.
Biuret solution is used to detect unhydrolyzed proteins. In this situation, the solution develops a pink-violet color. When all proteins have been broken into amino acids, the test is negative, with no color development. Results of this test will be measured in the spectrophotometer at 540nm and zeroed with distilled water.