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Small and Large Intestines J. Gilbert 4th March 2004 BiologyMad.com The Wall of the Small Intestine BiologyMad.com Small Intestine Functions: Digestion: neutralise acid from stomach, add digestive enzymes and bile, break proteins, carbohydrates and lipids to absorbable materials Absorption: 95% of food absorbed here Structure Regions: duodenum, jejunum, ileum Mucosa adaptations: villi containing blood and lacteal capillaries BiologyMad.com Major enzymes of digestion BiologyMad.com Accessory Organs: Aid Digestion and Absorption Pancreas: exocrine functions Secretes digestive enzymes and sodium bicarbonate Liver Produces bile (acts as emulsifier – begins fat breakdown Hepatic portal system: drains blood from digestive tract Metabolic functions: storage, synthesis, chemical processing Gallbladder: stores bile BiologyMad.com From the duodenum… The intestinal epithelium secretes sodium and chloride ions into the lumen Results in a water potential gradient Water potential in the first part of the lumen of the SI is lower than epithelial cells Water moves into lumen by osmosis Na+ Cl- water BiologyMad.com Sodium Ions All cells are required to maintain a low intracellular concentration of sodium. This is maintained in the intestinal epithelial cells by sodium pumps embedded in the membrane of the microvilli In rats, there are about 150,000 sodium pumps per SI epithelial cell, this means that 4.5 billion sodium ions move out of each cell per minute. This flow and accumulation of sodium is ultimately responsible for absorption of water, amino acids and glucose. BiologyMad.com Absorption of Proteins and Carbohydrates BiologyMad.com Absorption of Glucose Glucose is absorbed from the lumen by active transport This requires specific carrier molecules (sodium dependent glucose transporters) in the plasma membrane of epithelial. These proteins transport both sodium and glucose into the cell BiologyMad.com Absorption of Glucose BiologyMad.com Absorption of Glucose Once inside the SI epithelium, glucose and sodium must be exported from the cell into the blood. Glucose diffuses down the concentration gradient (facilitated diffusion) into the capillary blood in the villus Na+ diffuses into the blood via sodium-potassium pumps (this requires ATP) Other monosaccharides such as fructose are not absorbed by active transport This only requires facilitated diffusion BiologyMad.com Absorption of Amino Acids Similar to that of glucose (active transport) However sodium-dependent amino acid transporters are required Absorption of amino acids is dependent on the gradient of the sodium ions BiologyMad.com So far… Absorption of glucose and amino acids is dependent on the sodium ion gradient. Active transport from lumen into epithelial cell Sodium is pumped out of cell into blood to help maintain concentration gradient Facilitated diffusion of glucose and amino acids from epithelium into the blood Blood travels from the gut to the liver via the hepatic portal vein BiologyMad.com Absorption of Fats BiologyMad.com Emulsification of Fats Bile salts play a critical role in lipid assimilation by promoting emulsification Bile salts have hydrophobic domain that intercalate into the lipid. They also have a hydrophilic domain that remains at the surface This aids in the breakdown of large aggregates or droplets into smaller and smaller droplets BiologyMad.com Hydrolysis of Fats Hydrolysis of triglyceride into monoglyceride and free fatty acids is accomplished predominantly by pancreatic lipase. Lipase is a water-soluble enzyme, and with a little imagination, it's easy to understand why emulsification is a necessary prelude to its efficient activity. BiologyMad.com Formation of Micelles As monoglycerides and fatty acids are liberated through the action of lipase, they retain their association with bile salts and complex with other lipids to form structures called micelles. Micelles are essentially small aggregates of mixed lipids and bile salts suspended within the ingesta. As the ingesta is mixed, micelles bump into the brush border and the lipids, including monoglyceride and fatty acids, are absorbed. BiologyMad.com Absorption of hydrolysed lipids Lipids are absorbed by a mechanism distinctly different from what we've seen for monosaccharides and amino acids. The major products of lipid digestion - fatty acids and monoglycerides - enter the SI epithelium by simple diffusion across the plasma membrane Fatty acids and monoglyceride are transported into the endoplasmic reticulum, where they are used to synthesise triglyeride! The vesicles formed are then transported (via exocytosis) to the lacteal in the villus, which ends up being transported via the lymphatic vessel BiologyMad.com Absorption of Nutrients Proteins and carbohydrates: active transport Lipids: broken down and reassembled Water: osmosis Vitamins and minerals: assorted means BiologyMad.com Large Intestine BiologyMad.com Large Intestine: Structure and Function Functions: absorbs nutrients and water, and eliminates waste Structure: Cecum (blind pouch), appendix Colon: ascending, transverse, descending, sigmoid Rectum, anus BiologyMad.com Large Intestine Water and electrolytes (i.e. sodium and chloride) needs to be recovered by absorption Na+ transported from lumen by sodium pumps. Water follows by osmosis Bicarbonate is secreted to neutralise acids generated from microbes. This also aids the absorption of chloride ions Goblet cells secrete mucus Normal faeces are roughly 75% water and 25% solids BiologyMad.com Endocrine and Nervous Systems Regulation of Digestion Regulation dependent on volume and content of food Nervous system: stretch receptors in stomach Hormones: Gastrin: stimulates release of gastric juice Secretin: stimulates pancreas to secrete water and bicarbonate Cholecystokinin (CCK): signals pancreas to secrete digestive enzymes BiologyMad.com Nutrients: Utilised or Stored Until Needed BiologyMad.com