Exam #5: Digestion & Absorption Flashcards

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1
Q

What types of carbohydrates are absorbed in the digestive system?

A

ONLY monosaccharides

**Oligo & poly-saccharides are digsested into monosaccharides

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2
Q

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

A

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

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3
Q

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

A

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
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4
Q

What are the sites of protein digestion?

A

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

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5
Q

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

A

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

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6
Q

What are the key protein transporters in the gut?

A

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

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7
Q

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

A

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
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8
Q

Describe the process of lipid absorption.

A

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)

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9
Q

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

A
  • Lipids are packaged with proteins called, “lipoproteins”
  • Chylomicrons of lipids and lipoproteins make it into the circulation
  • Chylomicrons are taken up by the liver
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10
Q

What hormone control lipid digestion?

A

CCK

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11
Q

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

A

Pancreatic lipase is the actual digestive enzyme

Co-lipase anchors pancreatic lipase to the fatty droplets

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12
Q

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

A

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

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13
Q

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

A

NPC1L1 transporter (Neimann Pick C1 Like1 transporter )

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14
Q

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

A

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

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15
Q

What is the mechanism for the absoprtion of folic acid?

A

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

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16
Q

What is the site for the absorption of Vitamin B12?

A

Ileum, the location of the intrinsic factor receptor

17
Q

What is the mechanism for the absoprtion of Vitamin B12?

A

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

18
Q

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

A

Duodenum

19
Q

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

A
  • 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
20
Q

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

A

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.

21
Q

What is the mechanism for the absorption of iron?

A

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

22
Q

Describe the pathophysiology and symptoms of lactose intolerance.

A
  • 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

23
Q

Describe the pathophysiology and symptoms of celiac spure

A
  • 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
24
Q

Describe the pathophysiology and symptoms of pernicious anemia.

A

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”