GI digestion and absoption

Question 1

1. Explain where carbohydrate digestion occurs, what enzymes are required and which sugars they target.

Answer 1

Amylase in saliva secreted by salivary glands and in small bowel secreted by pancreas starts to break down starches (alpha-1,4 and akpha-1,6 linkages), but free glucose is never the product of amylase digestion.
Beta-1,4 linkages in dietary “fiber” cannot be broken down.
Only simple monomeric sugars can be absorbed, so small polysaccharides are converted to monomers by enterocyte surface enzymes.

Question 2

2. Predict the small intestine and colonic consequence of a deficiency in the enzyme lactase following the consumption of dairy products.

Answer 2

The brush border enzyme lactase converts lactose to glucose and galactose.
Without lactase, gas and diarrhea (osmotic) due to colonic bacterial digestion of lactose.

Question 3

3. Compare and contrast the carbohydrate uptake mechanisms in terms of location, ions involved and specificity.

Answer 3

fructose is glut 5
SGLT1 requires sodium as cotransporter
glut 2 moves stuff across basolateral side

Question 4

4. List the four mechanisms of protein uptake

Answer 4

1. sodium dependent co transporters that utilize the Na+/K+ ATPASE gradient are the major route for the different classes of amino acids. water follows
2. sodium independent transporters of amino acids
3. specific carriers for small peptides linked to H+ uptake co transporter like PEPT1
4. Pinocytosis of small peptides by enterocytes

Question 5

5. Explain the roles of pancreatic lipase, colipase, and micelles. Discuss how fat soluble vitamins are absorbed and the consequences of fat malabsorption (steatorrhea) on their uptake.

Answer 5

More complicated since fats are insoluble in water.
Fat droplets are emulsified by bile salts and lecithin to form small particles, increasing the surface area for subsequent digestion by lipase and colipase.
Products of lipase ingestion are solubilized in bile-salt micelles and are able to diffuse passively through the enterocyte membrane.
Inside the enterocytes, triglycerides are resynthesized and packages into chylomicrons.
These are released into the interstitial space by exocytosis and taken up into the lacteals (too large for capillaries).
Fat-soluble vitamins are absorbed by the same route.
Fat malabsorption disorders include liver disease (bile salt deficiency), pancreatic insufficiency (lack fat-digesting enzymes), and weight-loss meds.

Question 6

6. Describe the composition and formation of chylomicrons, their movement across the enterocyte basolateral membrane, and the route of entry into the cardiovascular system.

Answer 6

Chylomicrons enter the lymphatic system and through azygous vein, reach the circulatory system.

Question 7

7. Describe the absorption of water-soluble vitamins, including the role of intrinsic factor in the absorption of vitamin B12.

Answer 7

Water soluble vitamins either enter the enterocyte by simple diffusion (biotin, folic acid) or via specific transporters (e.g. Vit B12)

Question 8

8. Explain the physiological significance of the regulation of luminal water content and daily fluid balance. Understand the role of the intestinal epithelia in regulating fluid movement along with the pathways of secretion and absorption of major ions in the small and large intestine.

Answer 8

Important points are that water readily moves across the intestinal epithelium and that water absorption follows the absorption of solutes and is therefore absorbed isotonically.
About 9 L of fluid is put into the gut each day and only about 100-200 mls are lost

After the stomach, the small intestinal contents become iso-osmotic with respect to the blood
Water and ions can move paracellularly and/or transcellularly, depending on location
There is a net fluid secretion from cells in the intestinal crypts and a net fluid absorption from enterocytes on the villi. Villi surface area > crypt surface area
Water absorption is linked to sodium absorption.

Question 9

9. List the different classes of diarrhea and the mechanisms by which oral rehydration fluids are able to counter the loss of water and electrolytes.

Answer 9

Osmotic diarrhea is caused by impaired digestive or defects in absorption. 
Lactase deficiency
Ileal resection (bile salts not resorbed)
Celiac Disease

Secretory diarrhea is due to increased cAMP levels in cells and activation of CF chloride channel.
Vibrio cholera
Motility disorders

Question 10

where are bile salts reabsorbed

Answer 10

distal ileum by enterohepatic circulation

Question 11

what are the fat soluble vitamins

Answer 11

ADEK

Question 12

pottssium absorption

Answer 12

paracellular in jejunum, transcellular in colon

Question 13

calsium and magnesium compete for reabsorbtion

Answer 13

1

Question 14

vit d and calcium reabsorbtion

Answer 14

vit D fascilitates the process

Question 15

iron absorption

Answer 15

picked up as heme or fe2+

• binds to apoferritin to form ferritin that stays in the cell and is lost when the cell dies
• binds to tranferrtin leaves cell and goes into blood