Biochem - Protein Digestion

Question 1

1. What cell type secretes HCl?
2. How does H+ get into the lumen?
3. How does Cl- get into the lumen?

Answer 1

1. Parietal cells
2. H+ comes from the CO₂ + H₂O -> HCO₃^- in the parietal cell
   H+ is secreted into the lumen via H+/K+ antiport
3. HCO₃^- is secreted into the blood via HCO₃^-/Cl^- antiport
   Cl^- is then actively pumped into the lumen

Question 2

1. What cell type secretes Pepsinogen and how is it secreted?
2. What stimulates pepsinogen secretion?
3. How is pepsinogen activated?
4. What is pepsin’s main function in the stomach?

Answer 2

1. Chief cells secrete via exocytosis
2. Ach, secretin (S cells), CCK (I cells), histamine (from ECL cells) and gastrin (from G cells) stimulate pepsinogen secretion
3. Autoactivation at pH < 5 & activation by pepsin
4. Begins proteolysis in stomach

Question 3

Describe how Parietal cells are stimulated

Answer 3

Gastrin, Ach, and Histamine can stimulate parietal cells

Gastrin and Ach can stimulate parietal cells directly

Gastrin and Ach can also stimulate ECL (enterochromaffin like cells) to secrete Histamine which can stimulate parietal cells

Question 4

1. What are the stimuli for secretion from intestinal endocrine cells?
2. What do I Cells and S Cells secrete?

Answer 4

1. amino acids, oligopeptides, and polypeptides in the duodenum (acid chyme)
2. I cells - CCK
   S cells - secretin

Question 5

Secretin

1. What cell secretes secretin?
2. What does secretin stimulate?

Answer 5

1. S cells
2. stimulates pancreas -> alkaline secretion rich in HCO3

Question 6

CCK

1. What cell type secretes CCK?
2. What does CCK stimulate

Answer 6

1. I cells release CCK
2. CCK causes contraction of gallbladder and relaxation of sphincter of Oddi
   CCK stimulates pancreatic acinar cell and intestinal mucosal epithelial cells