What 3 locations does Digestion occur?
1. Luminal digestion
- In the GI lumen
- Mediated by enzymes in salivary glands, stomach, and pancreas
2. Membrane digestion
- Brush border enzymes
- a-dextrinase, maltase, sucrose, trehalase, and sucrose
- Synthesized by enterocytes that are inserted into the membrane
Brush border on the luminal surfaces of the cells is permeable to both sodium ions and water.
3. Intracellular digestion
- Mediated by cytoplasmic enzymes within enterocytes
Where does Carbohydrates get absorbed?
1. Proximal Small Intestines
Where does Bile and Vitamin B12 get absorbed?
1. Distal Ileum
What does Lactose breakdown into?
1. Glucose and Galactose
2. Due to Lactase making it into a monosaccharide for it to be absorbed inot the intestinal mucosa
What does Sucrose breakdown into?
1. Glucose and Fructose
2. Breaksdown by sucrase
How are carbohydrates (starch) digested?
1. Begins in the mouth by salivary a-amylase
2. Most digestion occurs in the small intestines by Pancreateic amylase
- Breaksdown into disaccharides
- Disacchrides are broken down by Intestinal Brush Border Enzymes to monosaccharides
- a-dextrinase, maltase, sucrose, trehalase, and sucrose
How is Glucose and Galactose Absorbed?
*Comes from Lactose
1. Transported from intestinal lumen into the cells by SGLT 1 (Secondary Active transport-Cotransport)
2. From cell to blood Glucose and Galactose transported by Facilated diffusion (GLUT 2)
- Oral Rehydration Solution
- Contains Na and Glucose for reuptake
- Restores body fluid volume
- Ex: Cholera
- NA and glucose are rapidly reabsorbed by intestinal epithelia via SGLT 1
How is Fructose Absorbed?
1. Transports by Facililated Diffusion (GLUT 5) across the brush-border membrane
- Patient can't express Functional SGLT-1
- Unable to resabsorb glucose
- Alternate route
- GLUT-5 converts fructose to glucose in the Liver
What is Lactose Intolerance?
1. Absent Brush-Border Enzyme Lactase
- Lactase splits Lactose into Glucose and Galactose
2. Lactose can't be brokendown without lactase and remains undigested in the intestinal lumen
- Holds water in the lumen causing Osmotic Diarrhea
- Lactose gets fermented by colonic bacteria
- Produces gases (hydrogen and carbon dioxide) and Lactic Acid
How are proteins Digested?
1. Begins in the stomach due to acidic pH and Pepsin
2. Most digestion occurs in the Small Intestines by Pancreatic and Brush-border proteases
*Enterocytes cytoplasm site of peptide degradation which allows amino acids to enter portal blood
How does Stomach Protein Digestion occur?
1. Chief cells secrete Pepsinogen
2. Parietal cells secrete HCl
3. HCl converts Pepsinogen to Pepsin in Low pH
- Pepsin is denatured in the duodenum b/e pH is greater than 5
*Pepsin is not essential for protein digestion
- Pancreatic and brush-border proteases can digest proteins and pepsin is not necessary
What is Cystinuria?
1. Disorder in the transport of amino acids
- Cystine, Lysine, Arginine, and Ornithine ("COAL")
- Absent in Small Intestines and Kidney
2. No reabsorption of amino acids causing Excess CYSTINE EXCRETION
What is Cystic Fibrosis?
1. Mutation in CFTR (cystic fibrosis transmembrane conductance regulator)
2. Thick mucus blocks ducts that prevent transport of enzymes to intestines for digestion
3. Pancreatic enzyme deficiency
- leads to intestinal malabsorption
How are Lipids digested in the Stomach?
1. Lipid digestion begins in the stomach
- By Lingual and Gastric Lipases
2. Lingual and Gastric lipases hydrolyze triglycerides into Glycerol and 2 fatty acids
How are Lipids digested in the Small Intestines?
1. Digestion of lipid is completed in the Small Intestines
- By Pancreatic lipase, choleterol ester hydrolase, and phospholipase A2
2. Pancreatic lipase Requires COLIPASE
- Hydrolyze fats
- Displaces bile salt to create binding site for pancreatic lipase
What are Chylomicrons?
1. Reesterfied lipids packed with Apoproteins
- Triglycerids and Cholesterol at CORE
- Apoproteins on OUTSIDE
3. Packaged in vesicles on the Golgi apparatus
- Chylomicrons too large to enter blood, which they enter the LACTEAL of the lymph
*Apoproteins synthesized by Enterocytes for absorption
What is Abetalipoproteinemia?
1. Failure in synthesizing Apo B (Beta-lipoproteins)
- Malabsorption of chylomicrons
- Dietary lipids
- Fat soluble vitamins
How is Calcium Absorbed?
1. Calcium enter enterocytes by calcium channels
2. Binds to CALBINDIN
3. Calcium in enterocytes transport by Ca2+-ATPase and 3Na+/Ca2+ exchanger
4. Vitamin D induces Calbindin D-28 K in enterocytes
- Vitamin D-dependent Ca2+ binding protein
- Increase calcium absorption
5. Calcium absorption depends on VITAMIN D3
How is Iron Absorbed?
1. Free iron bind to Apoferritin
2. In Circulation, iron binds to transferrin
- Transports iron from the small intestines to the liver for storage (Stored as Ferritin)
3. Iron is transported from the Liver to Bone Marrow
- Released in bone marrow for Synthesis of Hemoglobin
How are Electrolytes transported in the Jejunun?
1. Na+ absorption occurs via (mutation in transport causes diarrhea)
- Na+-glucose cotransport
- Na+-amino acid cotransport
- Na+- H+ exchanger
2. Carbonic anhydrase
- Converts CO2 and H2O into H+ and HCO3-
- HCO3- absorbed into blood
3. Jujunum, Net absorption of NaHCO3
How are Electrolytes transported in the Ileum?
1. Contain all the same transporters as the Jejunum plus
- Cl--HCO3 exchanger in apical membrane
- Cl-transporter in basolateral membrane
2. HCO3 is secreted instead of being absorbed into the blood as in the Jejunum
3. Net movement of NaCl into the cell for absorption
How are electrolytes transported in the Colon?
- Na+-K+ channels (Stimulated by Aldosterone)
- Na absorption into cell
- K secretion into lumen