Flashcards in Exam #5: Digestion & Absorption Deck (24):
What types of carbohydrates are absorbed in the digestive system?
****Oligo & poly-saccharides are digsested into monosaccharides
What are the sites of carbohydrate digestion? What enzymes facilitate digestion at these different sites?
1) Salivary amylase in the mouth
2) Pancreatic amylase in the lumen of the intestines
Oligosacchardie digestion to monosaccharide occurs at the brush border
What are the transporters used for the absorption of carbohydrates? What is the major site of carbohydrate absorption?
The gross anatomical structure responsible for carbohydrate digestion is the small intestine; however, the majority occurs in the duodenum of the small intestine.
- Apical membrane (lumen)=
1) Glucose & galactose= SGLT1 (Na+/Glucose Cotransporter)
2) Fructose= GLUT5 glucose transporter
- Basolateral membrane
GLUT2= transport of glucose, galactose, and fructose
What are the sites of protein digestion?
1) Stomach= gastric peptidase (pepsin) initiates digestion
2) Duodenum= pancreatic peptidase (trypsin....etc.) continue digestion to oligopeptides
3) Brush Border of Enterocytes=
- Absorption of oligopeptides
- Further digestion to amino acids & absorption
What are the mechanisms of protein digestion? How does protein digestion & absorption compare to carbohydrate?
1) Gastric peptidsase (pepsin)
2) Pancreatic peptidase (trypsin...etc.)
3) Brush border enzymes
*****Proteins can be absorbed as oligopeptides vs. carbohydrates that CANNOT be absorbed as oligosaccharides
What are the key protein transporters in the gut?
1) PEPT1= oligopeptide & H+ cotransporter on the luminal membrane
2) Amino Acid Transporter B= Na+ & amino acid cotransporter on the luminal membrane
*****Note that like carbohydrates, most proteins are absorbed in the duodenum of the small intestine
What are the sites of lipid digestion? What is the primary site lipid absorption?
1) Lumen of the stomach
- Lingual lipase
- Gastric lipase
2) Lumen of the duodenum=
- Pancreatic lipase
- Bile is mixed with emulsified lipids to form a micelle
*****Note that the stomach plays a mechanical role i.e. physically decreasing the size of the lipid and increasing the SA for pancreatic lipase
- Duodenum is site of absorption
Describe the process of lipid absorption.
1) Emulsification in the stomach
2) Micelle formation in the duodenum w/ bile salts
3) At the apical membrane, the lipid content to the micelle is absorbed & the bile salt is recycled
- Some contents enter via diffusion
- Cholesterol used the Neimann Pick C1 Like1 transporter (NPC1L1)
What happens to lipids after they are absorbed across the apical membrane of the small intestine?
- Lipids are packaged with proteins called, "lipoproteins"
- Chylomicrons of lipids and lipoproteins make it into the circulation
- Chylomicrons are taken up by the liver
What hormone control lipid digestion?
How do the functions of pancreatic lipase and co-lipase differ?
Pancreatic lipase is the actual digestive enzyme
Co-lipase anchors pancreatic lipase to the fatty droplets
What happens to the absorption of fats in the absence of bile acids and salts?
Decreased absorption of fats, which leads to a deficiency in the fat soluble vitamins that are absorbed in the duodenum including Vitamins:
A, D, E, & K
What is the transporter used for cholesterol absorption in the duodenum?
NPC1L1 transporter (Neimann Pick C1 Like1 transporter )
What is the function of folic acid? Where is folic acid absorbed?
Folic acid is essential for DNA synthesis & cell division; it is absorbed in the duodenum
What is the mechanism for the absoprtion of folic acid?
Uptake of folic acid is mediated by an exchange protein on the surface of enterocytes
What is the site for the absorption of Vitamin B12?
Ileum, the location of the intrinsic factor receptor
What is the mechanism for the absoprtion of Vitamin B12?
1) Intrinsic factor is secreted by parietal cells in the stomach
2) Vitamin B12 & IF form a complex in the duodenum
3) B12+ IF Complex binds intrinsic factor receptor in the ileum
4) In the cell B12 binds to a carrier protein, transcolbalmin II
5) Transcobalmin II + Vitamin B12 Complex is secreted into the blood and taken up by the liver for storage
What is the site for the absorption of Ca++?
What is the mechanism for the absorption of Ca++? How is this regulated?
- Ca++ moves down its concentration gradient in the duodenum via a Ca++ channel
- Vitamin D causes the expression of the Ca++ channel in the duodenum
What is the site for the absorption of iron? What form of iron is absorbed? Why?
Note, iron comes in two forms, FerrOUS (2+) & FerIC (3+). FerrIC (3+) iron will precipitate in a pH greater than 3 & cannot be absorbed. Thus, a Vitamin C dependent reaction reduces 3+ to 2+ FerrOUS iron for absorption.
What is the mechanism for the absorption of iron?
1) In the duodenum, soluble FerrOUS (2+) iron complexes with "transferrin"
2) FerrOUS (2+) iron + Transferrin Complex binds a receptor and is taken up into the duodenum
3) FerrOUS iron is secreted into the blood where it binds with Transferrin in the blood & is transporter to the liver
*****Note that Iron can ALSO be absorbed via the "Divalent Cation Transporter 1" (DCT1), to be secreted into the blood
Describe the pathophysiology and symptoms of lactose intolerance.
- Lactose= disaccharide that is broken down into glucose & galactose via Lactase enzyme
- Normally, glucose & galactose are absorbed via SGLT1
- Without "Lactase," Lactose remains in the lumen, resulting in osmotic diarrhea
*****Additional symptoms include distention and cramping from bacterial breakdown of lactose-->CO2
Describe the pathophysiology and symptoms of celiac spure
- This is the same thing a gluten-insensitivity
- Loss of mature villous epithelium in the duodenum & jejunum
- Caused by immune response to gluten (protein component in cereal grains), leading to malabsorption & inflammation