Exam #5: Digestion & Absorption

Question 1

What types of carbohydrates are absorbed in the digestive system?

Answer 1

ONLY monosaccharides

**Oligo & poly-saccharides are digsested into monosaccharides

Question 2

What are the sites of carbohydrate digestion? What enzymes facilitate digestion at these different sites?

Answer 2

Polysaccharide digestion=

1) Salivary amylase in the mouth
2) Pancreatic amylase in the lumen of the intestines

Oligosacchardie digestion to monosaccharide occurs at the brush border

1) Lactase
2) Maltase
3) Isomaltase

Question 3

What are the transporters used for the absorption of carbohydrates? What is the major site of carbohydrate absorption?

Answer 3

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

Question 4

What are the sites of protein digestion?

Answer 4

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

Question 5

What are the mechanisms of protein digestion? How does protein digestion & absorption compare to carbohydrate?

Answer 5

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

Question 6

What are the key protein transporters in the gut?

Answer 6

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

Question 7

What are the sites of lipid digestion? What is the primary site lipid absorption?

Answer 7

1) Lumen of the stomach
- Lingual lipase
- Gastric lipase
- Emulsification

2) Lumen of the duodenum=
- Pancreatic lipase
- Co-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

Question 8

Describe the process of lipid absorption.

Answer 8

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)

Question 9

What happens to lipids after they are absorbed across the apical membrane of the small intestine?

Answer 9

• Lipids are packaged with proteins called, “lipoproteins”
• Chylomicrons of lipids and lipoproteins make it into the circulation
• Chylomicrons are taken up by the liver

Question 10

What hormone control lipid digestion?

Answer 10

CCK

Question 11

How do the functions of pancreatic lipase and co-lipase differ?

Answer 11

Pancreatic lipase is the actual digestive enzyme

Co-lipase anchors pancreatic lipase to the fatty droplets

Question 12

What happens to the absorption of fats in the absence of bile acids and salts?

Answer 12

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

Question 13

What is the transporter used for cholesterol absorption in the duodenum?

Answer 13

NPC1L1 transporter (Neimann Pick C1 Like1 transporter )

Question 14

What is the function of folic acid? Where is folic acid absorbed?

Answer 14

Folic acid is essential for DNA synthesis & cell division; it is absorbed in the duodenum

Question 15

What is the mechanism for the absoprtion of folic acid?

Answer 15

Uptake of folic acid is mediated by an exchange protein on the surface of enterocytes

Question 16

What is the site for the absorption of Vitamin B12?

Answer 16

Ileum, the location of the intrinsic factor receptor

Question 17

What is the mechanism for the absoprtion of Vitamin B12?

Answer 17

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

Question 18

What is the site for the absorption of Ca++?

Answer 18

Duodenum

Question 19

What is the mechanism for the absorption of Ca++? How is this regulated?

Answer 19

• 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

Question 20

What is the site for the absorption of iron? What form of iron is absorbed? Why?

Answer 20

Duodenum

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.

Question 21

What is the mechanism for the absorption of iron?

Answer 21

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

Question 22

Describe the pathophysiology and symptoms of lactose intolerance.

Answer 22

• 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

Question 23

Describe the pathophysiology and symptoms of celiac spure

Answer 23

• 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

Question 24

Describe the pathophysiology and symptoms of pernicious anemia.

Answer 24

Pernicious anemia= Vitamin B23 deficiency that leads to “megaloblast anemia” i.e. too few blood cells, with the remaining cells too big

• Caused by atrophy or destruction of gastric mucosa, which decreases parietal cell production of IF
• Destruction or the ileum leading to less IF +Vitamin B12 Complex receptors
• Vitamin B12 & folic acid are needed for cell growth
• Lack of these–>decrease in cell growth esp. RBC
• Specifically, RBCs increase in size but don’t divide–> “megaloblastic”