Digestion & Absorption Flashcards
The pH along the GI tract is mostly alkalytic, except it is acidic where?
Stomach
Besides villi on the mucosa, what other features increase SA?
Microvilli on the villi
What other things are located within the villi in the mucosa?
Blood vessels, Lacteals, Nerves (submucosal plexus)
How do the size of the villi change as we move through the GI tract?
Villi get shorter as we move through. SA is highest in proximal part of SI.
Is there villi in the LI?
NO
What structure is characteristic for BOTH SI and LI?
Crypts
What role do the crypts play?
Important role in regenerating epithelium. Contain stem cells
T/F. Turnover of epithelial cells is slow in GI.
FALSE. 17-70 billion cells are replaced every day
Entire GI tract is replaced in how many days?
5
Stem cells in the crypts can end up which fates?
- Absorptive cells
2. Secretory cells, either goblet or endocrine
What is an effect of impairment of stem cells in crypts?
Impaired absorption
What are the MONOMERS/absorbable pieces of CHO?
glucose, galactose, fructose
What are the MONOMERS/absorbable pieces of proteins?
AA, But can also absorb di and tri peptides
What are the MONOMERS/absorbable pieces of fats?
FA + monoglycerides + cholesterol, NOT TGs!
How do fats get through the PM?
Soluble in membrane - don’t need transporters
Even though we don’t need to worry about transport of fats, what else needs to be dealt with?
They have tendency to aggregate into big globs so need to be be able to get in there to break it down into individual components.
What are examples of POLYSACCHARIDES?
Starch, Cellulose (AKA FIBER)
In the MOUTH, STARCH + ________ =
Salivary Amylase
= Maltose + Glucose
In the DUODENUM, STARCH + ________ =
Pancreatic Amylase (from Pancreas) =Maltose + Glucose
What enzymes break down DISACCHARIDES? and where?
Hydrolytic Enzymes in the ILEUM (BRUSH BORDER). Located in microvilli itself –> GLYCOCALYX
What are the DISACCHARIDES and what are their monomers?
Sucrose –> Glucose + Fructose, Lactose –> Glucose + Galactose, Maltose –> Glucose x 2
Which PROTEIN ENZYMES are present in the STOMACH?
Pepsin
What PROTEIN ENZYMES are present in the DUODENUM?
Trypsin, Chymotrypsin, Carboxypeptidase
What PROTEIN ENZYMES are present in the ILEUM?
Aminopeptidase
What ENZYME is involved in FAT breakdown?
Pancreatic Lipase
Where is fat broken down into Monoglycerides and FAs?
Duodenum
T/F. Fiber can be broken down by enzymes
FALSE (will pass through undigested)
How does the epithelial cell get GLUCOSE INTO the cell?
- Gradient established by Na/K pump on BL side
2. Sodium-Glucose CO TRANSPORTER brings them in together
Do all the other MONOSACCHARIDES get into the cell the same way as glucose?
No. Fructose has to use GLUT5 facilitated transport
How do the MONOSACCHARIDES get OUT of the cell and into the BLOOD?
Facilitated diffused via GLUT2 transporters on BL membrane
What happens with LACTASE production as we age?
Decreases
What are 2 ways that lactase is inhibited?
- by glucose - “End product inhibition”
2. Intolerance
T/F. There is no problem created if we fail to absorb all the glucose from the lumen in the small intestines.
FALSE. need the nutrients!
Once peptides are broken down in stomach and duodenum, then are BROKEN DOWN FURTHER into ________, by: _________.
AMINO ACIDS by PROTEASES in duodenum and ileum.
Where do protein PROTEASES come from?
Inactive/Zymogen from PANCREAS, then activated in intestinal LUMEN
What is the first enzyme that becomes activated and then cleaves all the other enzymes into activity?
TRYPSIN
How does trypsin get activated?
Enterokinase in the GLYCOCAYLX (this first cleavage gets it all going)
How do we get the free AMINO ACIDS INTO the epithelial cell?
Co-transport with Sodium
How do we get the PEPTIDES into the epithelial cell?
Secondary active transport coupled with PROTON gradient
What has to happen to these peptides before they leave the epithelial cell?
Need to be broken down into AMINO ACIDS by PEPTIDASE before leaving via facilitated carrier
Why is it important for protein digestion and absorption to take place in the FIRST part of the SI?
Don’t want any of them to get to LI or else will become gas and carries fluid with it
Which protein exceptions are not broken down into AA and can be endocytosed and exocytosed in their complete form?
Mother’s antibodies in an infant (IgA)
What happens if run out of protons to couple the transport of peptides?
generate more H via CARBONIC ANHYDRASE
Which of the transporters are ESSENTIAL in luminal membrane for protein absorption?
a. Na-AA symporter
b. Na-H antiporter
c. Proton-peptide symporter
d. all of the above
D - all!
What is the problem once the FA gets into the cell?
Due to water content, all the fat wants to GLOB TOGETHER - so will need to EMULSIFY it for lipase to be able to ACCESS it
Most fat in a typical diet is in the form of:
TG
What are the products of pancreatic lipase activity?
2 Free FA
1 monoglyceride
T/F. There is no need for a transporter for free FA.
TRUE - can easily diffuse across membrane and into cell
Emulsification of fat requires which 2 processes?
- Mechanical disruption
2. Emulsification
Which organs do mechanical disruption of big fat globule?
Contractile activity of stomach and SI by segmentation and peristalsis
What prevents the fat droplets from re-aggregating after mechanical disruption?
Bile salts and phospholipids in BILE
What is bile composed of?
Bile salts - 50%, Phospholipids - 40%, Cholesterol - 4%, Bile pigments - 2%
Where is bile stored?
GALLBLADDER
What are MICELLES made up of?
FA, Monoglycerides –> NOT TG!, phospholipids, bile salts, VITAMINS
How is bile transporter to the duodenum?
COMMON BILE DUCT
What percent of bile is lost in feces and therefore replaced in the liver?
5%
What is bile composed of?
Bile salts - 50%
Phospholipids - 40%
Cholesterol - 4%
Bile pigments - 2%
What is the fat structure called after it leaves the Smooth ER and what else is added to it?
CHYLOMICRON - cholesterol, ApoB, vitamins too (NOT FREE FA)
How does the chylomicron get OUT?
Processed by golgi and eventually fuse with PM releasing (EXOCYTOSE) the chylomicron into the Interstial fluid to enter the LACTEALS
What do Micelles have that emulsion droplets DO NOT?
Bile salts and phospholipids
Beside B6 (which is absorbed via diffusion), which vitamin does NOT get absorbed via Mediated transport?
B12 - too large and too charged a molecule. But bind to INTRINSIC factor before being endocytosed.
What are the fat-soluble vitamins?
A D E K
What is an implication of missing pieces of your gut in regards to vitamin absorption?
There are DIFFERENT SITES of absorption for the water-soluble vits
Where does INTRINSIC FACTOR come from?
secreted by gastric PARIETAL Cells
Beside B6 (which is absorbed via diffusion), which vitamin does get absorbed via Mediated transport?
B12 - too large and too charged a molecule. But bind to INTRINSIC factor before being endocytosed.
Inside the enterocyte, B12 binds to what to enter portal blood:
Transcolbalamin II
Fat-soluble vitamins NEED CARRIERS in circulation, what are they?
A: in chylomicorn
D: binding protein before conversion in liver
E: lipoproteins and RBCs
K: VLDL
Where in the SI does B12 + IF get endocytosed?
LOWER portion of ILEUM
Where is CALCIUM absorption REGULATED?
Duodenum and jejunum
How is IRON absorption REGULATED?
intracellular binding with PROTEIN and FERRITIN
If want MORE Ca to come across, then what happens?
Increase BINDING PROTEINS inside cell = CALBINDIN
What happens when we have LOW IRON content in BLOOD?
Opposite process, DECREASE FERRITIN –> lets iron go, and IRON MOVES OUT into blood
How is the amount calbindin determined?
by Vitamin D
What happens when we have HIGH IRON content OUTSIDE in the blood?
Will stimulate production of FERRITIN inside cell which BINDS IRON and doesn’t move it out –> FECES
