PHYS - Digestion & Absorption Processes Flashcards
Where are enzymes that digest carbs secreted from?
Saliva
Pancreas
Small Intestine
Where are enzymes that digest lipids secreted from?
Saliva
Stomach
Pancreas
Where are enzymes that digest proteins secreted from?
Stomach
Pancreas
Small intestine
What are the two types digestive activity?
Cavital (luminal) - from enzymes secreted into GI lumen from salivary glands, stomach, pancreas
Membrane (contact) - from enzymes synthesized and expressed by epithelial cells
What are the three cell types seen in the intestinal epithelium?
Enterocytes - epithelial cells (digest, absorb, secrete)
Goblet cells - mucous-secreting cells (protect intestinal lining)
Paneth Cells - mucosal defenses against infection (secrete agents that kill bacteria and induce inflammatory response)
What are the methods enterocytes absorb material from the gi tract?
Pinocytosis - takes place at base of microvilli; vesciles of GI luminal fluid are absorbed into enterocytes (major mechanism of uptake of proteins)
Passive diffusion - particles move through pores in apical membrane or through intercellular spaces
Facilitated Diffusion
Active transport
What layers must materials travel through in order to go from the GI lumen to the blood stream?
Unstirred Layer of GI luminal fluid Glycocalyx Apical Membrane Cytoplasm Basolateral Membrane Basement Membrane Blood/Lymph Capillary Wall
GI tract can adapt to various changes in its structure to ensure proper digestion and absorption, but there are limitations to this, such as what?
If the distal ileum is resected, Vitamin B12 and bile salts can no longer be absorbed from the GI lumen
Certain genetic abnormalities cannot be compensated for either, such as Lactase enzyme deficiency.
What are the three end products of carb digestion?
Three monosaccharides
Glucose
Galactose
Fructose
What does alpha amylase do?
Breaks down starch into alpha-Dextrins, Maltose, and Maltotriose
What does alpha-Dextrinase do?
Breaks down alpha-Dextrose into glucose molecules
What does maltase do?
Breaks down maltose into glucose moleucles
What does sucrase do?
Breaks down Maltotriose into glucose molecules and breaks apart sucrose (disaccharide) into glucose and fructose
What does Trehalase do?
Breaks down trehalose into glucose molecules
What does Lactase do?
Breaks down lactose (disaccharide) into glucose and galactose at the brushborder
What forms of transportation are utilized in the absorption of monosaccharides?
facilitated diffusion and cotransporters
What is SGLT1?
A cotransporter found on apical membrane of enterocytes.
Moves glucose and Na+ into the cell from GI lumen OR it moves galactose and Na+ into the cell
What is GLUT 5?
Fructose transporter, found on apical membrane of enterocytes
Moves Fructose into the cell from the GI lumen
What is GLUT 2?
Sugar transporter found on the basolateral membrane
Moves glucose, galactose, and fructose from inside the enterocyte out into the blood
What happens to lactose in an individual without lactase at the brushborder?
Lactose remains undigested and causes osmotic diarrhea.
It also ferments in the gut causing excess gas
What is the difference between an endopeptidase and an exopeptidase and give exaples of each.
Endopeptidase are enzymes that break peptide bonds of non-terminal amino acids
- Pepsin
- Trypsin
- Chymotrypsin
- Elastase
Exopeptidases are eznymes only capable of breaking peptide bonds of terminal amino acids
- Carboxypeptidase A
- Carboxypeptidase B
What is enterokinase?
Enterokinase is a brushborder enzyme in the small intestine that is responsible for converting trypsinogen into trypsin
Trypsin is endopeptidase that is also repsonsible for activating several other peptidases, including trypsinogen
What is an olgiopeptide?
A protein chain consisting a small number of amino acids (more than 3)
How are proteins moved from the GI lumen into the enterocyte?
Co-transport
On the apical membrane, there are 4 variations of Amino Acid - Na+ cotransporters; one each for neutral, acidic, basic, and imino AA’s
Additionally, there is a H+/Di- and Tripeptide costransporter
How are proteins moved from enterocytes into the blood?
On the basolateral membrane, there are 4 variations of channels that allow amino acids to diffuse into the blood; one for each neutral, acidic, basic, and imino AA
The di- and tripeptides either diffuse through the membrane and into the blood, or intracellular peptidases break them down into single amino acids where they each individually utilize one of the 4 channels above
What is Congenital trypsin absence?
Disorder that severely reduces the ability of the GI tract to breakdown proteins.
Trypsin is needed to activate all the pancreatic peptidases
How does cystic fibrosis affect protein digestion?
Cystic fibrosis can cause mucous to buildup in the pancreatic ducts, preventing pancreatic peptidases from entering the duodenum
How does cystinuria affect protein digestion?
A genetic disorder that causes a defect or absence in the Na+-aa transporter for di-basic amino acids (including cysteine)
Prevents absorption of di-basic aa’s
These amino acids instead are secreted in urine, and can crystallize forming stones
What is Hartnup Disease?
Genetic disorder that disrupts ability to uptake neutral amino acids
Symptoms:
Diarrhea, mood changes, photosensitivty, skin rash, neurologic problems (muscle tone), short stature, uncoordinated movments
Urine has abnormally high levels of tryptophan and its byproducts, such as serotonin
How are triglycerides broken down?
Triglycerides are broken down into a monoglyceride an two fatty acids
This done via gastric, lingual, and pancreatic lipases
How are cholesterol esters broken down?
Cholesterol ester hydrolase enzymes breaks down cholesterol esters into one molecule of cholesterol and one fatty acid
How are phospholipids broken down?
Phospholipase A2 breaks down phospholipids into Lycolecithin and one fatty acid
How are lipids absorbed into enterocytes?
Lipids are hydrophobic, they have to be collected in micelles in order to be removed from the GI lumen.
How are lipids digested in the stomach?
10% of triglycerides are digested in the stomach
Lingual and gastric lipase enzymes break down lipids.
The lipids are emulsified into micelle lipid droplets via dietary proteins (no bile is in the stomach)
When lipids are detected in the small intestine, CCK is secreted which slows down gastric motility, providing more time for lipids to be digested
What is pancreatic lipase?
Pancreatic enzyme that breaks down lipids
It is secreted from the pancreas in its active form, but once it interacts with bile, it becomes inactivated.
The enzyme Colipase is required to displace the bile molecule, reactivating pancreatic lipase
What is Colipase?
It is an enzyme that interacts with pancreatic lipase when it is inactivated by bile.
Colipase is secreted in its inactive form (Procolipase)
Procolipase is activated via trypsin
What is Cholesterol ester hydrolase?
It is an enzyme responsible for breaking down cholesterol esters into cholesterol and fatty acid.
Also hydrolizes triglcyerides into glycerol
What is Phospholipase A2?
Enzyme that breaks down phosphilipids into lycolecithin and fatty acid
It is secreted as a proenzyme, and is activated by trypsin
What happens to the digested lipids once they enter the enterocyte?
The monoglyceride, cholesterol, and lycolecithin are re-esterified with free fatty acids, forming triglyceride, cholesterol ester, and phospholipid
These three molecules are then collected in the enterocyte to form a chylomicron
The chylomicron is exocytosed across the basolateral membrane into a lymph vessel
What is ApoB?
It is a molecule that is essential for absorption of dietary lipids into the lymph.
Without ApoB, you have a condition called abetalipoproteinemia, which blocks absorption of dietary lipids
What is Zollinger-Ellison syndrome?
Gastrin secreting tumor in the pancreas
Increases HCl secretion from parietal cells in the stomach
Causes an overload of acid in the duodenum, can cause duodenal ulcer
What are Pancreatic Insufficiency and Pancreatitis?
Pancreatic Insufficiency: pancreas not secreting adequate quantities of digestive enzymes
Pancreatitis: Infection or inflammation of pancreas, impairs HCO3- and enzyme secretion
How does an Ileal resection affect lipid metabolism?
Interupts body’s ability to recycle bile salts, which are normally reabsorbed in the distal ileum.
Causes a decrease in bile salt pool, bile is excreted
What is SIBO?
Small Intestine Bacterial Overgrowth
Bacteria deconjugate bile salts, which impairs micelle formation, reducing fat absorption
Caused by decrease gastric acid secretion and decrease in motility of small intestine
What is Tropical Sprue and how does it affect lipid absorption?
Tropical Sprue is a condition that invovles infection and causes a reduction in intestinal epithelial cells, reducing the ability to absorb nutrients
What is Celiac Sprue?
Autoimmune disorder
Antibodies develop against gluten component
Leads to destruction of villi and hyperplasia of intestinal crypts
Causes deficiency in folte, iron, calcium, and Vitamins A, B12, and D absorption
Managed with gluten free diet
What is pernicious anemia?
Reduced number of circulating RBCs due to vitamin B12 deficiency
Caused by either Atrophic Gastritis (chronic inflammation of stomach, loss of parietal cells)
OR
Autoimmune Metaplastic Atrophic Gastritis (immune system attacks Intrinsic Factor protein or parietal cells)
How is Vitamin B12 absorbed in the GI tract?
Requires complexing with proteins
In the stomach, B12 binds with R-Protein
In small intestine, pancreatic proteases break apart R-Protein and B12 then binds to Intrinsic Factor.
In the distal Ileum, Intrinsic Factor-B12 complex is absorbed via Intrinsic factor receptor
Once B12 enters the small bowel mucosa, it binds with Transcobalamin II, which carries it into the blood
How does a gastrectomy affect B12 absorption?
Loss of parietal cells, source of Intrinsic Factor
