Protein and Carbohydrate digestion Flashcards
What is digestion?
the breakdown of nutrients so they can be absorbed across the intestinal wall
Where do the two phases of digestion occur?
within the lumen of the gut and within the brush border of enterocytes
What is the intestinal brush border covered with?
a secreted layer of glycocalyx
How does HCl begin the process of protein digestion?
it partially hydrolyzes some peptide bonds
How is pepsin created?
HCl cleaves inactive pepsinogen that is secreted by the chief cells of fundic glands which creates pepsin
What does pepsin do?
cleaves peptide bonds next to hydrophobic amino acids with aromatic side chains
What is rennin secreted by?
chief cells
What does rennin do?
cleaves between phe and met, especially in casein for neonates
What happens in the lumenal phase of protein digestion?
peptides activate the receptors of enteroendocrine cells which then secrete CCK, CCK goes to the blood which enters the pancreas which leads to increased pancreatic enzyme secretion
Where are digestive enzymed produced by enterocytes generally attaached to?
the apical membrane and they stick out of the glycocalyx of the brush border
What do intestinal brush border peptidases do?
they take small peptides to mono, di, or tri amino acids within the brush border
How many facilitated carriers are known for a single AA?
4
What is always transported with the AA across the apical membrane and why?
Na to provide a driving force to move the large AA across the membrane
What do di and tripeptides require to cross the apical membrane?
ATP - active transport
What pathways can amino acids follow for absorption?
neutral, basic, acidic, and proline pathways
Once inside the cell, what happens to di or tri-peptides that have crossed the apical membrane?
they are rapidly broken down to constituent amino acids via intracellular cytosolic peptidases
How do amino acids cross the basolateral membrane?
via facilitated diffusion down a concentration gradient - Na is not needed here
Once amino acids cross the basolateral membrane, what are they picked up by?
portal circulation
What is an example of an intact protein that is absorbed across the digestive tract?
colostral antibody
In monogastrics, what begins the breakdown of starch?
salivary amylase
Where does the majority of monogastric CHO digestion occur?
in the small intestine
What occurs during the luminal phase of CHO digestion?
pancreatic amylase is secreted and breaks down 1-4 alpha links between glucose molecules in CHO and takes it down to the level of disaccharide maltose and trisaccharides
What are the di and tri-saccharide digesting enzymes assocciated with brush border membranes?
sucrase, dextrinase, maltase, lactase, intestinal cell amulase, trehalase
In typical diets, what do 80% of carbohydrates end up as?
glucose
In typical diets, what do 20% of carbohydrates end up as?
fructose and galactose
Why do monosaccharides have no problem reaching the glycocalyx but have issues crossing the hydrophobic cell membrane?
because they are hydrophilic but they are too large to enter the cells by simple diffusion
How does the liver utilize carbohydrates?
it uses glucose to produce glycogen
How does muscle utilize carbohydrates?
uses glucose to produce glycogen for storage(fed state) or breaks down glycogen (fasted state)
What does adipose use carbohydrates for?
it uses glucose and changes it to acetyl CoA or glycerol for triglyceride synthesis
What does the nervous system use carbohydrates for?
it uses glucose except during extreme fasts, then it uses ketones
What does the reproductive tract use carbohydrates for?
it uses glucose and AA for the fetus for energy or changes glucose into lactose for milk
What do red blood cells use carbohydrates for?
it oxidizes glucose into lactate which is then returned to the liver for gluconeogenesis
