Digestion and Absorption in the GI Tract

Question 1

• Digestion begins in _ and is completed in the SI
• Absorption takes place primarily in _

Answer 1

• Stomach (and mouth )
• Small Intestine

Question 2

• Two main pathways for absorption

Answer 2

• Cellular
  
  • ​Lumen-apical membrane-intestinal epithelial cell-basolateral membrane-blood
  • Transportes in membrane
• Paracellular
  
  • ​Intestinal epithelial cell-lateral intercellular space-intestinal epithelial cell
  • Tight Junctions

Question 3

• What carbohydrate digestive enzymes are located in:
  
  • ​Saliva
  • Pancreas
  • Intestinal Mucosa

Answer 3

• Saliva
  
  • ​Amlyases
• Pancreas
  
  • ​Amylases
• Intestinal Mucosa
  
  • ​Sucrase
  • Maltase
  • Lactase
  • Trehalase
  • Alpha Dextrinase

Question 4

• What protein digestive enzymes are secreted by the:
  
  • ​Stomach
  • Pancreas
  • Intestinal Mucosa

Answer 4

• Stomach
  
  • Pepsin
• Pancreas
  
  • Trypsin
  • Chymotrypsin
  • Carboxypeptidase
  • Elastase
• Intestinal Mucosa
  
  • Amino-oligopeptidase
  • Dipeptidase
  • Enterokinase

Question 5

• What lipid digestive enzymes are secreted by the :
  
  • ​Saliva
  • Stomach
  • Pancreas

Answer 5

• Saliva
  
  • ​Lingual Lipase
• Stomach
  
  • ​Gastric Lipase
• Pancreas
  
  • ​Lipase-colipase
  • Phospholipase A2
  • Cholesterol ester hydrolase

Question 6

• What are the two types of digestive activity?

Answer 6

• Cavital (luminal)
• Membrane (contact)

Question 7

• Cavital (luminal) digestion

Answer 7

• Digestion from action of enzymes secreted by salivary glands, stomach and pancreas

Question 8

• Membrane (contact) digestion

Answer 8

Hydrolysis by enzymes synthesized by epithelial cells/enterocytes

Question 9

• Villi are longest in the _ and shortest in the _ of the SI

Answer 9

• Duodenum
• Terminal Ileum

Question 10

• What cell types are pesent in the intestinal epithelium and what are their functions?

Answer 10

• Enterocytes-digestion, absorption, and secretion (replaced 3-6 days); susceptible to chemo
• Goblet Cells-mucus secreting cells
• Paneth Cells-mucosal defenses against infection

Question 11

• Where does pinocytosis occur?
• What macromolecule is taken up via pinocytosis?

Answer 11

• Base of microvilli
• Proteins

Question 12

• Besides pinocytosis, what other methods are used by enterocytes to absorb nutrients into the blood?

Answer 12

• Passive diffusion
• Facilitated diffusion
• Active transport

Question 13

• What barriers/layers must the solute moving from lumen to blood cross?

Answer 13

• Unstirred layer of fluid
• Glycocalyx
• Apical Membrane
• Cytoplasm of Cell
• Basolateral Membrane
• Basement Membrane
• Wall of Blood Capillary or Wall of Capillary of Lymphatic Vessel

Question 14

• In what clinical scenarios can the intestine adapt to maintain homeostasis in digestion and absorption?
• In what clinical scenarios is it limited?
• In what GENETIC ABNORMALITY is it lost?

Answer 14

• Small bowel resection and bypass
• Terminal ileum recetion-absorption of B12 and bile salts is abolished
• Lactase deficiency

Question 15

• What are the three end products of carbohydrate digestion?

Answer 15

• Glucose
• Fructose
• Galactose

Question 16

• What enzyme converts trehalose to glucose?
• What enzyme converts lactose to glucose and galactose?
• What enzyme converts sucrose to glucose and fructose?
• What enzyme converts alpha dextrin to glucose?
• What enzyme converts maltose to glucose?
• What enzyme converts maltotriose to glucose?

Answer 16

• Trehalase
• Lactase
• Sucrase
• Alpha dextrinase
• Maltase
• Sucrase

Question 17

_ and _ diffusion are key transport mechanisms in the absorption of carbohydrates

What transporters are present in the small intestine that aid in absorption of carbohydrates?

Answer 17

• Co-transport and facilitated diffusion
• APICAL
  
  • ​SGLT1 (Transports Na+/Glucose or Galactose)
  • GLUT5 (Transports Fructose)
• Basolateral
  
  • Na+/K+ ATPase
  • GLUT 2 (Transports Glucose, Galactose and Fructose)

Question 18

• Lactose intolerance can cause _ because of undigested lactose remaining in lumen and holding H20
• This also leads to unabsorbed lactose being fermented into methane and H+ gas

Answer 18

• Osmotic diarrhea

Question 19

• _ is a brush border enzyme that helps convert trypsinogen to trypsin in the small intestine
• Trypsin is an _ that helps convert various proteases in the SI into their active form

Answer 19

• Enterokinase
• Endopeptidase

Question 20

• What are the pancreatic endopeptidases?

Answer 20

• Trypsin
• Chymotrypsin
• Elastase

Question 21

• What are the pancreatic exopeptidases?

Answer 21

• Carboxypeptidase A
• Carboxypeptidase B

Question 22

• Pepsin in the stomach breaks down proteins into _ and _
• Luminal enzymes in the small intestine (Carboxypeptidase A and B, Trypsin, Chymotrypsin and Elastase) convert proteins into _, _, _and _
• _ in small intestine are further broken down by _ (BRUSH BORDER) to _, _, and _

Answer 22

• Amino acids
• Oligopeptides

Small Intestine:

• Amino acids
• Dipeptides
• Tripeptides
• Oligopeptides
• Oligopeptidases further broken down by peptidase brush border into amino acids and dipeptides, tripeptides

Question 23

• _ and _ diffiusion are key in absorption of proteins
• What transporters are present on enterocytes that aid in protein digestion?

Answer 23

• Co-transport and facilitated diffusion (just like carbohydrates)
• Apical
  
  • ​Na+/H+ ATPase
  • Na+/Amino Acid cotransporter (4 different types: neutral, acidic, basic, imino)
  • H+/Dipeptide or Tripeptide cotransporter
• Basolateral
  
  • ​Na+/K+ ATPase
  • Amino acid transporters (neutral, basic, imino, acidic)

Question 24

• Chronic pancreatitis and cystic fibrosis-how do these affect protein reabsorption?

Answer 24

• Deficiency of pancreatic enzymes/defect in transporters of enterocytes

Question 25

* How does ***congenital trypsin*** absence affect protein metabolism?

Answer 25

* All pancreatic enzymes are non-functional (need trypsin to become activated)

Question 26

* ***Cystinuria***

Answer 26

* **Defect/Absence in Na+/AA cotransporter on apical membrane of enterocytes and Kidney (specifically di-basic aa transporter (cystine, lysine, argignine, ornithine)** * **Cystine crystals in urine**

Question 27

* ***HARTNUP***

Answer 27

* **Cannot absorb neutral AAs** * **AR-SLC6A19 gene** * **Symptoms similar to NIACIN DEF (PALLEGRA)** * **​**Diarrhea * Mood swings (serotonin not being synthesized) * Neurological problems * Red, scaly skin rash * Photosensitivity * Short Stature * Uncoordinated Movements * Urine (high levels of neutral aas (ex: tryptophan) and by-products (serotonin)

Question 28

* \_% of protein digestion occurs in stomach * Since bile acids are not present _ perform the emulsifying action

Answer 28

* 10% * Dietary proteins

Question 29

* In order to allow sufficient time for the digestion of lipids, _ is secreted immediately when dietary lipids first appear in the small intestine * This functions to \_

Answer 29

* CCK * Decrease rate of gastric emptying

Question 30

* \_% of protein digestion occurs in the SI * _ emulsify fats * _ are secreted to aid in digestion (Name these)

Answer 30

* 90% * Bile salts * Pancreatic Enzymes * Pancreatic lipase * Colipase * Cholesterol ester hydrolase * Phospholipase A2

Question 31

***Pancreatic lipase*** ***Secreted as active or inactive?*** ***Inactivated by _ and resolved with \_***

Answer 31

* Active * Bile salts * Colipase

Question 32

* ***Colipase*** * ***​Secreted in active or inactive?*** * ***Function?***

Answer 32

* Secreted as procolipase (inactive) * Activated by trypsin * Binds to pancreatic lipase and displaces bile salts (to activate it)

Question 33

* ***Cholesterol ester hydrolase***

Answer 33

* Catalyzes production of cholesterol * Hydrolyzes triglycerides to produce glycerol

Question 34

* ***Phospholipase A2***

Answer 34

* Breaks down phospholipids into **lysolecithin and fatty acids**

Question 35

* ***What are the steps for digestion and absorption of lipids?***

Answer 35

1. Solubilization by micelles (lumen) 2. Diffusion of micellar content across apical membrane 3. Reesterification 4. Chylomicron formation (ApoB important here) 5. Exocytosis of chylomicron into lymph

Question 36

* **Abetalipoproteinemia is no absorption of lipids caused by lack of _ in chylomicrons**

Answer 36

* ApoB

Question 37

* Errors in lipid metabolism can lead to \_ * Possible mechanisms?

Answer 37

* Steatorrhea * Any abnormalities: * Pancreatic enzyme secretion * Bile acid secretion * Emulsification * Micelle formation * Diffusion into intestinal cells * Chylomicron formation * Transfer of chylomicrons into lymph

Question 38

* ***Factors that can cause a deficit in bile salt (and thus affect absorption of lipids)***

Answer 38

* Ileal resection * Small intestinal bacterial overgrowth

Question 39

* ***Ileal resection***

Answer 39

* Interrupts enterohepatic circulation of bile salts * Total pool of bile salts is reduced

Question 40

* ***SIBO (small intestinal bacterial overgrowth)*** * ***​What is it?*** * ***What are the two main causes?***

Answer 40

* Bacteria deconjugate bile salts-impairs micelle formation and leads to fat malabsorption * Bacterial overgrowth also damages intestinal mucosa * Causes: * Decreased acid secretion (remember acid helps destroy bacteria) * Small intestine dysmotility

Question 41

* ***Tropical Sprue***

Answer 41

**Decreased number of intestinal epithelial cells reduces surface area** **Fat malabsorption and steatorrhea** **Def of Folate and B12** Diarrhea Cramps, nausea, weight loss, gas, indigestion Tx: Tetracycline and folate (6 mo)

Question 42

***Celiac Sprue***

Answer 42

* Autoimmune disorder * **Abs against gluten-destruction against small intestinal villi and hyperplasia of intestinal crypts** * Malabsorption of **folate, iron, B12, Ca2+, Vitamins A, B12, and D** * Sx: Abdominal pain, constipation, diarrhea, unexplained weight loss, vomiting and nausea, **steatorrhea**, tingling, numbness in hands/feet, itchy skin w/rash, fatigue, seizures, easy bruising, bone fractures * Tx: Gluten free diet

Question 43

* How are fat soluble vitamins (A,D,E,K) absorbed?

Answer 43

* Same mechanism as lipids * Solubilization by micelles * Diffusion of micellar content across apical membrane * Reesterification * Chylomicron formation * Exocytosis of chlyomicron into lymph

Question 44

* What are the water soluble vitamins and how are they absorbed?

Answer 44

* Water soluble (B1, B2, B3, B12, C, Biotin, Folic Acid, Nicotinic Acid, Pantothenic Acid) * **Na+ Dependent cotransport mechanism in small bowel**

Question 45

* Vitamin B12 forms complexes with what proteins to aid in its absorption? * What is the function of B12? (What she was specific about?)

Answer 45

* IF,R proteins, and transcobalamin II * **Important in DNA synthesis in red blood cells**

Question 46

* ***What can lead to a disruption in the absorption of Vitamin B12?***

Answer 46

* Gastrectomy: Loss of parietal cells * Gastric bypass

Question 47

* ***Pernicious anemia***

Answer 47

* Causes: * Atrophic gatritis: chronic inflammation, loss of parietal cells * Autoimmune metaplastic atrophic gastritis-immune system attacks IF protein or gastral parietal cells * **remember that B12 is important for DNA synthesis of RBCs (if yopu don't have IF from parietal cells, can't properly absorb B12)**

Question 48

* ***Inadequate Ca2+ absorption can lead to _ in children and _ in adults*** * ***How is Ca2+ absorbed in the SI?***

Answer 48

* Ricket's * Osteomalacia * **Vitamin D dependent Ca2+ binding protein**

Question 49

* **How is iron absorbed in the SI?**

Answer 49

* Binds to **apoferritin** in intestinal cells * Binds to **transferrin** in blood

Question 50

* Summary of where things are absorbed in the GI tract

Answer 50

Question 51

* Summary of how things are absorbed in the GI tract

Answer 51