Digestion

Lipid absorption

-Absorption in epithelial cell. --Free fatty acids ---Simple diffusion into epithelial cell --Mixed micelles ---Fatty acid transporter protein -Lipids that are endogenous (originate from liver): --in epithelial cell, resynthesis of triglyceride. --Packaging in chylomicrons. --Enter lacteal and lymphatics, then blood -Lipoprotein lipase --Digestion of endogenous triglycerides back to free fatty acids and monoglycerides

Monosaccharides absorption

-Carbohydrates are broken down into smaller polysaccharides in the mouth by salivary amylase. Once in the small intestine, pancreatic lipase further breaks the polysaccharides into disaccharides (maltose, sucrose, lactose). These disaccharides are broken down into monosaccharides by the digestive enzymes found at the brush border of the enterocytes. -Maltose is obtained via the digestion of starch by amylase. It consists of two glucose monomers, the bond between these two glucose is broken down by maltase. Sucrose consists of glucose and fructose, bond broken down by sucrase. Lactose consists of glucose and galactose, bond broken down by lactase. -The cells of the small intestine, called enterocytes, can only absorb the simplest of sugars in their monomeric form. Therefore absorb glucose, galactose, and fructose. -Glucose and galactose enter via a Na+ linked secondary active transport. ATP is used to establish an electrochemical gradient in which there is a higher concentration of Na+ on the lumen side. As Na+ travels into the cell bia a membrane protein, it pulls glucose/galactose with it (Na+ symport). -Fructose however enters the cell via passive transport. While inside the cell, majority converted into glucose. -Fructose, galactose, and glucose leave cell via passive transport on the basolateral side and enter the blood and travel to the liver. (Rest of info from ppt.) -Transported into epithelial cells by symport with Na+. facilitated diffusion into blood capillaries. Transport to liver, hepatic portal system

What are the four layers of the GI tract? Make sure you know the contents of each one, and the function for digestion.

-Mucosa (innermost) --Lines lumen of GI tract --Epithelium (mucous membrane) --Consists of simple columnar epithelium supported by lamina propria (thin layer of areolar CT containing numerous lymph nodules) --External to lamina propria is thin layer of smooth muscle called muscularis mucosae; responsible for numerous small folds in certain portions of GI tract. Folds greatly increase absorptive surface area. --Specialized goblet cells secrete mucus throughout most of GI tract --Absorptive and major secretory layer -Submucosa --Thick, highly vascular layer of CT that serves the mucosa --Dense irregular CT --Absorbed molecules that pass through columnar epithelial cells of mucosa enter into blood and lymphatic vessels of submucosa --Contains glands and nerve plexus. Submucosal plexus (Meissner's plexus) provides a nerve supply to the muscularis mucosae of small and large intestine. Glands secrete substances and enzymes Nutrients absorbed into lymph or blood here Nourishes surrounding tissues; brings oxygenated blood to the muscles in the muscularis layer Muscularis (muscularis externa) Responsible for segmental contractions and peristaltic movement Smooth muscle is responsible for this, smooth muscle does motility Contains inner (muscularis interna) and outer (muscularis externa) longitudinal layer of smooth muscle. Contractions of these layers move food through tract and physically pulverize and mix food with digestive enzymes Myenteric plexus (Auerbach's plexus) located between the two muscle layers, provides the major nerve supply to entire GI tract Includes fibers and ganglia from both SNS and PSNS Serosa (adventitia) Completes wall of GI tract Binding and protective layer consisting of areolar CT covered with a layer of simple squamous epithelium (serosa covered by visceral peritoneum) Cells secrete serous fluid to lubricate organs from sticking to each other

Amino acid absorption

-Absorbed to the hepatic portal vein -The mechanism by which amino acids are absorbed is conceptually identical to that of monosaccharides. The lumenal plasma membrane of the absorptive cell bears at least four sodium-dependent amino acid transporters - one each for acidic, basic, neutral and amino acids. These transporters bind amino acids only after binding sodium. The fully loaded transporter then undergoes a conformational change that dumps sodium and the amino acid into the cytoplasm, followed by its reorientation back to the original form. -Thus, absorption of amino acids is also absolutely dependent on the electrochemical gradient of sodium across the epithelium. Further, absorption of amino acids, like that of monosaccharides, contributes to generating the osmotic gradient that drives water absorption. -Free amino acid transported into epithelial cell by a symport with Na+. facilitated diffusion into blood capillaries. Transport to liver, hepatic portal system.

What is the ultimate goal of digestion?

Reduce ingested food to monomers of the four major organic compounds for absorption. -Nucleic acids--nucleotides -Proteins--amino acids -Carbohydrates--monosaccharides -Lipids--glycerol and fatty acids