GI Lecture 5 Flashcards
Epithelial cells on villi
The enterocytes that are the cells that take up and deliver into blood virtually all nutrients form the diet.
Lacteal
the lymphatic vessels of the small intestine that absorb digested fats.
Goblet cells of villus
Cell that secretes the main component of mucus
Crypt
A small tubular gland or pit in the small intestinal wall
Villus
Absorptive protrusions of small intestines; increase surface area.
Describe how different molecules are transported across the intestinal epithelium into the blood.
Transcellular rout via carries for large organic molecules (ie amino acids, glucose)
Paracellular route through tight junctions for smaller molecules.
Some molecules transported via both routes.
The number and type of carrier molecules and the structure of the tight junctions varies in different parts of the gut.
What are some dietary sources of carbohydrates
- Sucrose (cane sugar) - a disaccharide of glucose and fructose.
- Lactose (milk sugar) - a disaccharide of glucose and galactose.
- Starches - large polysaccharides in almost all non-animal food
- Glycogen, amylose, alcohol, lactic acid, pectins, dextrins
- Large amounts of cellulose (not digested, fiber)
Describe how amylase breaks down starches
Salivary and pancreatic a‐ amylases partially break down glucose polymers (a-1,4 bonds of amylose and amylopectin) and yield the products: maltose, maltotriose, glucose oligomers and a-limit dextrin a-1,6 bonds and terminal bonds do no break down by amylases.
Is amylase necessary
Salivary amylase is not necessary for healthy humans, but important in infants and pancreatic insufficiency
How is salivary amylase inactivated
By acidic pH levels in the stomach
Isomaltase
The only enzyme that can break down a-1,6 bonds of a-limit dextrins. Yields glucosamylase, sucrase, and isomaltase.
Lactase
A brush border enzyme. Breaks lactose into glucose and galactose before it enters the SGLT-1 channel to transport into cytosol. Especially important for infants
Lactose intolerance
Inability to digest lactose
Where fructose is found
High fructose corn syrups for processed foods
Excess fructose effect
May overwhelm GLUT5 transporter
Sucrase-isomaltase
A brush border enzyme that binds sucrose. Breaks down sucrose into glucose (SGLT-1 transporter) and fructose (GLUT5 transporter) to be sent into cytosol
SGLT1
Na+-glucose transporter - ACTIVE transport - into cytosol
GLUT5
Fructose transporter - into cytosol
GLUT2
Transports glucose, galactose and fructose by FACILITATED diffusion - out of cytosol through basolateral membrane
What happens to sugar molecules that leave the epithelial cell via basolateral membrane
Goes into blood circulation via PORTAL VEIN
In what form are pepsins secreted from the chief cells in the stomach?
Inactive precursor pepsinogens which are inactive in the duodenum at a pH above 5. Pepsins cleave proteins into polypeptides at pH 2-3. 15% of dietary proteins digested this way (but are not necessary).
Where does activation of pancreatic proteases occur
The small intestine brush border, where the enzymes break down proteins into dipeptides or tripeptides (usually not amino acids).
How are peptides absorbed in the small intestine?
By the peptide transporter 1 (PEPT1) together with a proton supplied by an apical Na+-H+ exchanger (NHE).
How are absorbed peptides digested?
By cytosolic proteases, and amino acids are transported into bloodstream by a series of basolateral transport proteins
How are dietary proteins digested into polypeptides and amino acids?
Pepsin
How are polypeptides digested into oligopeptides?
Trypsin, Chymotrypsin, Elastas, Carboxypeptidase A or carbboxypeptidase B
What happens to oligopeptides and amino acids?
Oligopeptides go through brush-border peptidases and become dipeptides and tripeptides; along with amino acids, are finally absorbed.
What are the important dietary lipids?
Neutral fats (triglycerides), phospholipids, plant sterols, and cholesterol
What are some fat soluble vitamins?
Vitamin A (retinoic acid) - regulates gene transcription Vitamin D - important in Ca2+ absorption Vitamin E (tocopherol) - important antioxidant Vitamin K - important in blood clotting