SM_143b: Absorption of Carbohydrates and Proteins Flashcards
Carbohydrates must be ____ before absorption
Carbohydrates must be hydrolyzed before absorption
Starch is hydrolyzed by ____ of salivary and pancreatic secretions, which attacks interior 1,6-bonds
Starch is hydrolyzed by alpha amylase of salivary and pancreatic secretions, which attacks interior 1,6-bonds
Simple sugars are broken down by ____ into monosaccharides on the brush border
Simple sugars are broken down by disaccharidases into monosaccharides on the brush border (contact digestion)
Disaccharidases are located in the ____, have a short ____, and come from ____
Disaccharidases are located in the glycocalyx, have a short half life, and come from enterocytes
(short half life because destroyed by digestive proteins)
Disaccharidases are subject to ____, which prevents the buildup of osmotically active materoals in the lumen
Disaccharidases are subject to end product feedback inhibition, which prevents the buildup of osmotically active materoals in the lumen
Lactase converts ____ to ____ and ____
Lactase converts lactose to glucose and galactose
Maltase breaks ____ into ____
Maltase breaks terminal 1,4 bonds of saccharides into glucose
Isomaltase breaks ____ into ____
Isomaltase breaks terminal 1,6 bonds of saccharides into glucose
Sucrase converts
____ to ____
____ to ____
Sucrase converts
- Sucrose to glucose and fructose
- Maltose to glucose and glucose
Lactose intolerance results from ____
Lactose intolerance results from lactase deficiency
Describe transport of glucose and galactose
Transport of glucose and galactose
- Transport requires simultaneous transport of Na+, which is moving down the electrochemical gradient established by basolateral Na+/K+ ATPase (2 sodium / 1 glucose)
- Glucose and galactose leave the cell by facilitated diffusion at the basolateral membrane and diffuse into capillaries
- Glucose and galactose absorption is negligible in absence of Na+ transport
- Glucose absorption involves substantial water and electrolye absorption (useful for rehydration)
GLUT1, GLUT2, GLUT3, GLUT4, and GLUT5 transport hexoses ____
GLUT1, GLUT2, GLUT3, GLUT4, and GLUT5 transport hexoses down a concentration gradient
SGLUT1 transports hexoses ____
SGLUT1 transports hexoses down a concentration gradient using energy provided by an electrochemical gradient of sodium, which is co-transported
Describe the affinities of the transporters for glucose and other hexoses
Affinities of the transporters for glucose and other hexoses
- GLUT1, GLUT3, GLUT4 have high affinity for glucose and function at maximal rate under physiological glucose
- GLUT2 has low affinity for glucose, allowing alteration of transport rate in proportion to increased glucose after meals
- GLUT5 has high affinity for fructose
Fructose absorption occurs via ____
Fructose absorption occurs via facilitated diffusion
(goes down concentration gradient but carrier is specific for fructose)
Hydrolysis of proteins is initiated by ____ followed by ____
Hydrolysis of proteins is initiated by gastric pepsin followed by pancreatic proteases
(yielding amino acids and small polypeptides)
Pepsinogen becomes ____
Pepsinogen becomes pepsin
(via H+ and pepsin itself)
Trypsinogen becomes ____
Trypsinogen becomes trypsin
(via enterokinase and trypsin itself)
Chymotrypsinogen becomes ____
Chymotrypsinogen becomes chymotrypsin
(via trypsin)
Final hydrolysis of proteins is by ____
Final hydrolysis of proteins is by brush border peptidases
- Yields tripeptides, dipeptides, and amino acids
- Peptides hydrolyzed to amino acids in enterocytes
- Peptidases are subject to end product inhibition
Amino acids are absorbed by ____ and ____ which include ____, ____, and ____
Amino acids are absorbed by Na+-co-transport and 3 carrier systems, which include neutral system I, basic system, and neutral system II
- Neutral system I: for neutral amino acids
- Basic system: for basic amino acids (e.g. arginine, ornithine, and lysine)
- Neutral system II (imino system): for proline, hydroxyproline, sarcosine, and other N-methyl substituted glycines
___ is released by the presence of fat and protein in the small intestine and helps decrease gastric emptying
CCK is released by the presence of fat and protein in the small intestine and helps decrease gastric emptying
The gastric zymogen pepsinogen is converted to the active enzyme pepsin by ___
The gastric zymogen pepsinogen is converted to the active enzyme pepsin by gastric HCl
Describe general mechanisms of digestion and absorption
General mechanisms of digestion and absorption
Describe carbohydrate digestion and absorption
Carbohydrate digestion and absorption
Describe digestion in the mouth
Digestion in the mouth
- Mastication and high water content of saliva increase surface area and solubility of food articles to enhance the ability of digestive enzymes to work
- Starch digestion begins through action of salivary a-amlyase
Alpha amylase converts starch to ____, ____, and ____
Alpha amylase converts starch to maltose, maltotriose, and alpha dextrins
(hydrolyzes internal a-1,4-linkages)
Describe the pancreatic / intestinal phase of carbohydrate digestion
Pancreatic / intestinal phase of carbohydrate digestion
- Stomach contents reach small intestine
- Acid is neutralized by bicarbonate secreted by pancreas: pancreatic alpha amylase has same specificity as salivary alpha amylase but total activity of pancreatic enzyme is much greater
- Further digestion of oligosaccharides occurs by enzymes located on the brush border membrane of epithelial cells lining the small intestine: activity is highest in upper jejunum and gradually declines through the rest of the small intestine
____ and ____ have the highest capacity to absorb sugars
Duodenum and upper jejunum have the highest capacity to absorb sugars
Monosaccharides are absorbed by ____ and ____
Monosaccharides are absorbed by active transporters (major mechanism) and diffusion
- Only glucose, galactose, and fructose
Describe the mechanism of digestion of carbohydrates to monosaccharides and absorption
Mechanism of digestion of carbohydrates to monosaccharides and absorption
SGLUT1 is expressed in ____
SGLUT1 is expressed in the intestinal mucosa
GLUT2 is found in ____
GLUT2 is found in pancreatic beta cells
GLUT4 is ____
GLUT4 is insulin-responsive
Phases of protein digestion include ____, ____, and ____
Phases of protein digestion include gastric, pancreatic, and intestinal
- Essentially all ingested prrotein is digested and absorbed
- Ingested protein is almost completely absorbed by the time the meal has traversed the jejunum
Describe the phases of digestion and absorption of protein and its degradative products
Phases of digestion and absorption of protein and its degradative products
In the gastric phase, pepsinogen secreted by stomach mucosal cells is converted to ___ by ___
In the gastric phase, pepsinogen secreted by stomach mucosal cells is converted to active enzyme pepsin by acidic pH
- Pepsin hydrolyzes 15% of dietary protein
- Pepsin (endopeptidase): cleaves polypeptide chains of proteins into large peptide fragments
Describe the pancreatic phase of protein digestion
Pancreatic phase of protein digestion
- CCK and secretin released from intestinal endocrine cells
- Stimulate secretion of propeptidases from pancreas into small intestine and release of NaHCO3 to neutralize stomach acid
- Inactive propeptidases converted to active peptidases in intestine
Describe process of enteropeptidase
Enteropeptidase
- CCK stimulation
- Enteropeptidase secreted by intestinal mucosal epithelial cells into lumen of small intestine
- Converts trypsinogen to trypsin
- Trypsin converts other trypsinogen molecules and other inactive proenzymes into the active forms (endopeptidases, exopeptidases)
- Endopeptidseases break down large peptide fragments to smaller peptides
- Carboxypeptidases remove amino acids sequentially from carboxy terminal
- Free amino acids and small peptides produces
In the small intestinal phase of protein digestion, ____ on the luminal surface of intestinal epithelial cells remove amino acids sequentially from the NH2 terminal end of peptides, producing free amino acids and di- and tri-peptides
In the small intestinal phase of protein digestion, aminopeptidases on the luminal surface of intestinal epithelial cells remove amino acids sequentially from the NH2 terminal end of peptides, producing free amino acids and di- and tri-peptides
Describe protein digestion and absorption
Protein digestion and absorption
