Physiology 2130 Lecture Notes - Lecture 64: Pepsin, Autocatalysis, Endopeptidase
1 May 2018
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Lecture 064: Small Intestine
Protein digestion in the stomach
● Starts within the stomach
● Pepsinogen
○ Made by chief cells
○ Large protein
○ Inactive
○ Needs to be cleaved into pepsin
○ Is autocatalytic in an acidic environment
■ Folds and becomes activated
■ Activated pepsinogen can cleave itself and other pepsinogen into
pepsin
● Pepsin
○ Protein digestion enzyme (protease)
○ Recognize a specific sequence and binds to it
■ Cleaves inside that sequence
● Endopeptidase
○ Digest collagen very well because of the sequence it contains
● Smaller peptide fragments are generated
○ However, smaller peptide still unabsorbable
○ Needs to be cut into single amino acids
○ This does not happen in the stomach, but rather by other proteases in the small
intestine
● Pepsin initiates digests but does not complete it!
Protein digestion in the small intestine
● Duodenum
○ 10 inches in length, first part of the small intestine
○ Location of enzymes mixing with chyme, most chemical digestion occurs here
○ Motility pattern are carefully controlled to optimize chemical digestion
■ Small intestine receive feedback to decide if the food has finished
digestion
● Jejunum
○ Slightly longer tube and maximized surface area for optimal nutrient absorption
■ Lots of villi (finger like projection from the mucosa wall)
○ Have lots of absorptive cells
■ Have transporters for the nutrients
● To transport single amino acids, monosaccharides,
macronutrients
● Ileum
○ Capable of nutrient absorption
■ However it has less villi
○ Absorbs different things than the jejunum
find more resources at oneclass.com
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■ Absorb bile
○ Backup system for nutrient absorption increase the jejunum is removed or if the
villi are destroyed
Nutrient Absorption Efficiency
● Carbohydrates
○ 100%
● Protein and lipids
○ 95-98%
● Only in healthy states, efficiency decreases otherwise
Small intestine anatomy
● Mucosa
○ Comes into contact with lumen content
○ Has villi
■ Increase surface area for absorption
○ Different epithelial cells
■ Absorptive epithelial cells
● ONLY IN THE SMALL INTESTINE (NOT THE STOMACH)
● Submucosa
○ Thin-connective tissue layer
■ Connects mucosa to the muscularis externa
○ Have glands that dip into the submucosa
■ Submusa gland
● Secretes an alkaline mucus
● Neutralizes the acidic chyme
● Also lubricates and protects the delicate small intestine cells from
abrasion
● Muscularis externa
○ Only have longitudinal and circular muscle
■ Increase/decrease diameter
■ shorter/length tube
○ Regular tube-like shape
● Serosa
○ Continuous with the mesentery
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Activated pepsinogen can cleave itself and other pepsinogen into. Recognize a specific sequence and binds to it. Digest collagen very well because of the sequence it contains. Needs to be cut into single amino acids. This does not happen in the stomach, but rather by other proteases in the small intestine. Pepsin initiates digests but does not complete it! 10 inches in length, first part of the small intestine. Location of enzymes mixing with chyme, most chemical digestion occurs here. Motility pattern are carefully controlled to optimize chemical digestion. Small intestine receive feedback to decide if the food has finished digestion. Slightly longer tube and maximized surface area for optimal nutrient absorption. Lots of villi (finger like projection from the mucosa wall) To transport single amino acids, monosaccharides, macronutrients. Backup system for nutrient absorption increase the jejunum is removed or if the villi are destroyed. Only in healthy states, efficiency decreases otherwise. Have glands that dip into the submucosa.
