GI Physiology - Digestion and Absorption in the Small Intestine

Question 1

What are the three carbohydrate prosucts absorbed by the small intestine?

Answer 1

Glucose, galactose, fructose.

Question 2

How does amylase digest starch?

Answer 2

Amylase digests the alpha 1-4 glycosidic bonds of starch to yield dissacharides.

Question 3

What is the main enzyme in the digestion of starch?

Answer 3

Pancreatic amylase.

Question 4

How are dissacharides converted to glucose?

Answer 4

By brush border enzymes.

Question 5

How are glucose and galactose absorbed across the apical membrane?

Answer 5

By secondary active transport (along with Na+) through the Sodium-Glucose cotransporter (SGLT1).

Question 6

How do glucose and galactose enter the blood across the basolateral membrane?

Answer 6

Glucose and galactose exit the cell via GLUT2 receptors across the basolateral membrane into the blood.

Question 7

How is fructose absorped across the apical membrane?

Answer 7

Fructose enters the cell by facilitated diffusion via GLUT5.

Question 8

How does fructose enter the blood across the basolateral membrane?

Answer 8

Fructose exits the cell via GLUT2 receptors across the basolateral membrane into the blood.

Question 9

Where does protein digestion take place?

Answer 9

• Protein digestion begins in the stomach with the action of pepsin, which breaks protein into amino acids and oligopeptides.
• The process of digestion is completed in the small intestine with brush border and pancreatic enzymes. They split the oligopeptides into amino acids, dipeptides and tripeptides.

Question 10

How are amino acids absorbed from the small intestine?

Answer 10

Amino acids are absorbed via a Sodium cotransporter. They are then transported across the basolateral membrane via facilitated diffusion.

Question 11

How are di and tripeptides absorbed from the small intestine?

Answer 11

Via separate H+ dependent cotransporters and once inside the cell are hydrolysed to amino acids.

Question 12

Where does digestion of lipids begin?

Answer 12

Lipid digestion is started by lingual and gastric lipases, but this only digests 10% of ingested lipids.

Question 13

How does bile aid in lipid digestion?

Answer 13

Emulsifies the fat goblets into smaller chunks, called micelles, which have a much larger surface area.

Question 14

What are the three major enzymes which hydrolyse micelles?

Answer 14

Pancreatic lipase, phospholipase A2 and cholesterol ester hydrolase.

Question 15

What are micelles broken down into?

Answer 15

Fatty acids, monoglycerides, cholesterol and lysolecithin.

Question 16

Describe the absorption of lipids.

Answer 16

• The products from digestion are released at the apical membrane and diffuse into the enterocyte.
• Inside the cell, the products are re-esterified to form the original lipids, triglycerides, cholesterol and phospholipids.
• The lipids are then packaged inside apoproteins to form a chylomicron.
• The chylomicrons are too large to enter circulation, so they enter lymphatic system via lacteals.