GI Digestion & Absorption

Question 1

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

Answer 1

Amylase catalyzes the hydrolysis of internal alpha-1,4 linkages to generate maltose, maltotriose, and alpha-limit dextrin.

Free glucose is never the product of amylase digestion

Question 2

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

Answer 2

Poop ahoy!

Lactose intolerance is caused by the absence of the brush border enzyme lactase. Unabsorbed lactose draws water into the intestinal lumen, producing an osmotic diarrhea. The diarrhea results from reduced small intestinal Na+ and fluid absorption as well as fluid secretion secondary to the osmotic effects of non-absorbed monosaccharide.

Also, gut bacterial flora metabolize the unabsorbed lactose, forming stinky cheese farts. Well, they may not actually smell like stinky cheese, but they are stinky and caused by cheese. I’m going to bed now.

Question 3

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

Answer 3

x

Question 4

List the four mechanisms of protein uptake

Answer 4

1. Sodium dependent co-transporters that utilize the N+/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 (di- and tri-) linked to H+ uptake (co-transporter; example is PEP T1)
4. Pinocytosis of small peptides by enterocytes (infants)

Question 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

Lipase: Digests triglycerides
Colipase: helps anchor lipase to surface of emulsified fate droplets
Micelles: required to transport the products of fat digestion through the unstirred water layer near the surface of enterocytes

Fat-soluble vitamins are digested and absorbed in a process similar to that of other dietary fats, so they will be malabsorbed in a patient with steatorrhea

Question 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 are found in the cytosol of enterocytes. They contain resynthesized triglycerides, phospholipids, cholesterol and apolipoproteins. The chylomicrons are incorporated into vesicles in the Golgi and exocytosed from the basolateral membrane. The chylomicrons then enter the lacteals (lymphatic system) for entry into the cardiovascular system.

Question 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 include B vitamins, vitamin C, niacin, folic acid, pantothenic acid and biotin. With the exception of vitamin B12, these vitamins are absorbed via Na-cotransport or passive diffusion in the upper small intestine. Vitamin B12 forms a complex with intrinsic factor (released from parietal cells in the stomach). The brush border of enterocytes in the ileum has receptors for intrinsic factor. The intrinsic-factor/vitamin B12 complex binds this receptor and is subsequently absorbed.

Question 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

• About 9 L of fluid is put into the gut each day and of this, only about 100-200 mls are lost
• Movement of water follows the movement of solutes (that requires a solute gradient or active transport)
• 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

Question 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

1. Motility Disorder
2. Osmotic Diarrhea: Caused by impaired digestion or defects in absorption (ie Lactase deficiency, ileal resection b/c bile salts are not absorbed, and in Celiac disease)
3. Secondary Diarrhea: May be caused by Vibrio cholerae. Increases cAMP levels in cells and this in turn activates the CF chloride channel (and thus water) on the lumenal surface.