Digestion and Absorption Flashcards Preview

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

1. Proximal Small Intestines

1. Distal Ileum 

1. Glucose and Galactose

2. Due to Lactase making it into a monosaccharide for it to be absorbed inot the intestinal mucosa

1. Glucose and Fructose

2. Breaksdown by sucrase 

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

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)

3. Clinical

• 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

1. Transports by Facililated Diffusion (GLUT 5) across the brush-border membrane

2. Clinical

• Patient can't express Functional SGLT-1
  • Unable to resabsorb glucose
• Alternate route
  • GLUT-5 converts fructose to glucose in the Liver

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 

• Nonabsorbable
• Holds water in the lumen causing Osmotic Diarrhea
• Lactose gets fermented by colonic bacteria
  • Produces gases (hydrogen and carbon dioxide) and Lactic Acid

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

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

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

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

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

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

1. Reesterfied lipids packed with Apoproteins

2. Contains

• 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