Physiology of the small intestine

Question 1

Which carbohydrates are absorbed by the small intestine?

Answer 1

Glucose
Galactose
Fructose

Question 2

Where are carbohydrates digested?

Answer 2

Initiated in mouth by Salivary Amylase

Majority of digestion occurs in the stomach and duodenum by Pancreatic Amylase

Question 3

How does Pancreatic Amylase digest carbohydrates?

Answer 3

Digests alpha 1,4 glycosidic bonds to break down starch into disaccharides (lactose, maltose and sucrose) which are then converted into glucose, galactose and fructose by brush-border enzymes (lactase, maltase and sucrase)

Question 4

How are Glucose and Galactose absorbed in the small intestine?

Answer 4

Absorbed across apical membrane by secondary active transport with Na+ through Sodium-Glucose Cotransporters (SGLT1).
They then leave the cell via GLUT2 receptors across the basolateral membrane into the blood.

Question 5

How is Fructose absorbed in the small intestine?

Answer 5

Enters cell by facilitated diffusion via GLUT5 receptors and transported into the blood via GLUT2 receptors

Question 6

How are proteins digested?

Answer 6

Begins in stomach (via action of Pepsin which breaks proteins into amino acids and oligopeptides)
Process completed in small intestine with brush border and pancreatic enzymes which split oligopeptides into amino acids, dipeptides and tripeptides

Question 7

How are amino acids absorbed in the small intestine?

Answer 7

Absorbed into cell via a Sodium Cotransporter before being transported across basolateral membrane into blood via facilitated diffusion

Question 8

How are dipeptides and tripeptides absorbed in the small intestine?

Answer 8

Absorbed via H+ dependent cotransporters into cell where they are hydrolysed into amino acids

Question 9

How are lipids digested?

Answer 9

Digestion initiated by lingual and gastric lipases and remainder digested in small intestine where bile aids digestion by emulsifying fat goblets into smaller chunks (micelles) which have a larger surface area.
Micelles digested by Pancreatic Lipase, Phospholipase A2 and Cholesterol Ester Hydrolase which break them down into fatty acids, monoglycerides, cholesterol and lysolecithin

Question 10

How are lipids absorbed?

Answer 10

Diffuse into enterocytes where they are re-esterified to form the original lipids, triglycerides, cholesterol and phospholipids which are then packaged inside apoproteins to form Chylomicron (which enter lymphatic system via lacteals)

Question 11

What is Steatorrhea?

Answer 11

Fatty faeces caused by disruption to normal absorption of lipids
Can be caused by pancreatitis (preventing secretion of Pancreatic Lipase), gallstones (preventing bile from entering duodenum) or IBDs