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Small Intestine - Anatomy & Physiology (view source)
Revision as of 09:04, 16 July 2008
, 09:04, 16 July 2008→To secrete enzymes for the digestion of chyme and absorb the products of digestion
===To secrete enzymes for the digestion of chyme and absorb the products of digestion===
===To secrete enzymes for the digestion of chyme and absorb the products of digestion===
*In digestion, large, complex molecules are broken down into their constituents. They are then absorbed and used by the body for energy or used as building blocks for new complex molecules. The three main molecules that undergo digestion are carbohydrates, triacyglycerols and proteins.
*In digestion, large, complex molecules are broken down into their constituents. They are then absorbed and used by the body for energy or used as building blocks for new complex molecules. The three main molecules that undergo digestion are carbohydrates, triacyglycerols and proteins.
'''Carbohydrate Digestion and Absorption'''
*The main soluble carbohdrates found in food are starch, found mainly in plants, and glycogen, found mainly in animal meat.
*The main soluble carbohdrates found in food are starch, found mainly in plants, and glycogen, found mainly in animal meat.
**There are two types of starch, ''amylose'' which has α1-4 glycosidic links and, ''amylopectin'' which has α1-4 glycosidic links and α1-6 glycosidic links making it branched (branches every glucose 25 residues).
**There are two types of starch, ''amylose'' which has α1-4 glycosidic links and, ''amylopectin'' which has α1-4 glycosidic links and α1-6 glycosidic links making it branched (branches every glucose 25 residues).
**''Glycogen'' is synthesised in the liver and muscle and is similar to amylopectin as it has both α1-4 glycosidic links and α1-6 glycosidic links. However it is more highly branched with shorter branches (branches every 12-18 glucose residues).
**''Glycogen'' is synthesised in the liver and muscle and is similar to amylopectin as it has both α1-4 glycosidic links and α1-6 glycosidic links. However it is more highly branched with shorter branches (branches every 12-18 glucose residues).
*The first stage of carbohydrate digestion begins with α-amylase, which is an endoglycosidase. This means it breaks bonds in the middle of the polymer to produce di-, tri- and oligo-saccarides.
*The first stage of carbohydrate digestion begins with α-amylase, which is an endoglycosidase. This means it breaks bonds in the middle of the polymer to produce di-, tri- and oligo-saccarides.
*This begins in the [[Oral Cavity- Anatomy & Physiology|oral cavity]] where [[Oral Cavity - Salivary Glands - Anatomy & Physiology|saliva]] is produced that contains α-amylase. Salivary α-amylase is inactivated when it enters the stomach due to the acidic pH.
*This begins in the [[Oral Cavity Overview - Anatomy & Physiology|oral cavity]] where [[Oral Cavity - Salivary Glands - Anatomy & Physiology|saliva]] is produced that contains α-amylase. Salivary α-amylase is inactivated when it enters the stomach due to the acidic pH.
*Carbohydrate digestion continues in the lumen of the [[Small Intestine - Anatomy & Physiology|small intestine]] as pancreatic α-amylase enters the [[Duodenum - Anatomy & Physiology|duodenum]] in the pancreatic duct. This is the site of the majority of carbohydrate digestion.
*Carbohydrate digestion continues in the lumen of the [[Small Intestine - Anatomy & Physiology|small intestine]] as pancreatic α-amylase enters the [[Duodenum - Anatomy & Physiology|duodenum]] in the pancreatic duct. This is the site of the majority of carbohydrate digestion.
*The second stage is the digestion of di-, tri-, and oligo-saccharides to monosaccharides.
*The second stage is the digestion of di-, tri-, and oligo-saccharides to monosaccharides.
**The energy release is used to transport glucose and galactose up their concentration gradients into the enterocyte.
**The energy release is used to transport glucose and galactose up their concentration gradients into the enterocyte.
**Glucose and galactose can then diffuse into the blood (portal vein) by carrier mediated diffusion via a GLUT-5 transporter.
**Glucose and galactose can then diffuse into the blood (portal vein) by carrier mediated diffusion via a GLUT-5 transporter.
'''Triacylglycerol Digestion and Absorption'''
*Triacylglycerols (TAGs) are digested by lipases.
*TAG digestion begins in the [[Oral Cavity Overview - Anatomy & Physiology|oral cavity]], where lingual lipase is secreted in the saliva. It removes a fatty acid from the 3 position on the glycerol molecule producing 1,2-diacylglycerol and a free fatty acid.
*TAG digestion continues in the small intestine, with pancreatic lipase and bile from the liver. Bile is necessary for digestion to occur, as pancreatic lipase is water soluble and the TAG and 1,2-diacylglycerol (1,2-DAG) are lipid soluble. Bile creates an interface for the enzyme to digest the lipid molecules. Bile also emulsifies fats; it reduces the size of lipid droplets increasing the surface area available for digestion.
*Pancreatic lipase removes any further fatty acids from the 3 position and then from the 1 position to produce 2-monoacylglycerol (2-MAG) and a fatty acid.
**Pancreatic lipase is unable to remove the fatty acid from the 2 position, so an enzyme called '''isomerase''' transfers the fatty acid from the 2 postion to the 1 postion to produce 1-monoacylglycerol (1-MAG).
**Pancreatic lipase can then remove the fatty acid from the 1 position to produce a fatty acid and glycerol.
***''NB: Pancreatic lipase works quickly, whilst isomerase works slowly. Thus, 2-MAG often accumulates and is absorbed (70% of digested TAG are absorbed as 2-MAG). A small proportion is absorbed as 1-MAG (6%).''
*The products of TAG digestion diffuse passively into the enterocyte as they are lipid soluble. They are then recombined to produce TAG.
**Fatty acids are converted to fatty acyl CoA by the addition of CoA. Fatty acyl CoAs are then added successively to 2-MAG to produce a TAG.
**In the golgi apparatus, TAG are then packaged with proteins, phospholipid and cholesterol into lipoproteins called '''chylomicrons'''.
***Chylomicrons are too large to enter the capillaries but instead enter the lymph to eventually join the blood via the thoracic duct.
***This enables the lipid soluble TAG to be transported in the blood.
==Regulation & Control==
==Regulation & Control==