Ions, Vitamins And Minerals Flashcards
What does molar mean?
One mole per litre
How can molecules cross the epithelium to enter the blood stream?
Para cellular transport - through tight junctions and lateral intercellular spaces
Trans cellular transport - through the epithelial cells
What are the ways solutes can cross a membrane?
Simple diffusion
Facilitated transport
Active transport (primary -ATP, or secondary -energy from another substances conc gradient that is actively transported)
Transport proteins include: channel proteins (can be openable), carrier proteins
Transport through channel proteins is in general quicker
What are the types of carrier mediated transport?
Uniport - only requires transported molecule
Symport- transported molecule and co transported ion go in the same direction
Antiport- transported molecule and co transported ion go in opposite direction
These last two are forms of coupled transport
How is the absorption of carbohydrates done (glucose and galactose)?
Absorption of glucose and galactose
Secondary active transport
Carrier protein = SGLT-1 on apical membrane
SGLT-1 can transport glucose against its conc gradient (useful if glucose in the lumen is low)
Absorption on basolateral membrane is by facilitated diffusion
Carrier proteins = GLUT-2, high capacity, low affinity
Concentration between plasma and enterocytes is generally equilibreated
How is absorption of carbohydrates done (fructose)?
Fructose
Facilitated diffusion
Carrier protein = GLUT-5 on apical membrane
Effective at relatively low concentrations of fructose in the lumen as tissue and plasma levels are low
What are some facts about water and electrolytes in terms of absorption?
99% of water in the GI tract is absorbed
The absorption of water is powered by the absorption of ions
Greatest amount of water is absorbed in the small intestine (esp jejunum)
Many ions are slowly absorbed by simple diffusion
Calcium and iron are incompletely absorbed
Approx 8L ow water are absorbed a day in the small bowel, and 1.4L in the large bowel
Where does the water we absorb come from?
We invest about 2L a day
Saliva produced gives about 1.2L
Gastric secretions produce about 2L
Bile 0.7L
Pancreatic juice 1.2L
Intestinal 2.4L
The absorption of water is through a standing gradient osmosis, what does this entail?
Driven by Na+
Transport of sodium ions from the lumen into Enterocyte. Becomes more efficient as you travel down the intestine:
Proximal bowel - counter transport in exchange for H+
Jejunum- co transported with amino acids and monosaccharides
Ileum - cotransport with Cl-
Colon - restricted movement through ion channels
This sodium is then transported into the lateral intercellular spaced by the sodium potassium ATPase
What is the absorption of Cl- and K+ like (separate)?
Cl-:
Co transported with Na+ (ileum)
Exchanged with HCO3- (colon)
(Both secondary active transport)
K+:
Defuses in via paracellular pathways in small intestine, leaks out between cells in colon
Passive transport
How does the standing gradient osmosis effect water?
Cl and bicarbonate are Transported into the intercellular spaces due to potential created by the sodium transport
The high concentration of ions in the intercellular spaces causes the fluid there to be hypertonic ( so water drawn into intercellular spaces)
Osmotic flow of water from the gut lumen via adjacent cells, tight junctions into the intercellular space
Water distends the intercellular channels and causes increased hydrostatic pressure
Ions and water move across the basement membrane of the epithelium and I carried away by the capillaries
What is an overview of the absorption of calcium?
Duodenum and ileum absorb Ca2+
A calcium deficient diet increases the guts ability to absorb it
Vitamin D and parathyroid hormone stimulate absorption
Diet 1-6 g/day, secretions 0.6g. Absorb 0.7g
How can calcium ions be absorbed into the blood?
Using vitamin D via the Paracellular tight junctions (diffusion)
Facilitated diffusion Into the cells via a channel protein.
Then binds to calbindin (prevents its action as an intracellular signal)
Calbindin can either be presented to a channel protein
Or exchanged for sodium via another channel
Vitamin D has a strong role in the transcription of the calcium transporters and absorption of calcium
How is calcium absorbed into blood (better slide)?
Cancium ions carried across apical membrane by:
Intestinal calcium binding protein (IMcal) - facilitated diffusion
Ion channel
Ca2+ is then pumped across basolateral membrane by plasma membrane calcium ATPase (PMCA) against conc gradient
PMCA has a high affinity for Ca2+ but low capacity
Maintains very low conc of calcium usually in cell
OR
Calcium pumped across basolateral membrane by plasma membrane Na+/Ca2+ exchanger against conc gradient
The Na*/Ca2+ exchanger has a low affinity for calcium but a high capacity. It requires large concentrations of calcium to be effective
What is the role of vitamin D in calcium absorption?
Essential for normal calcium absorption
Deficiency causes Rickets or osteoporosis
1,25-dihydroxy cholecalciferol taken up by enterocytes:
Enhances the transport of calcium through the cytosol
Increases the level of calbindin
Increases the rate of extrusion across Bazel lateral membrane by increasing the level of calcium ATPase in the membrane
What is an overview of iron absorption?
Acts as an electron donor and acceptor (useful in oxygen transportation and oxidative phosphorylation)
Iron is toxic in excess but the body has no mechanism for actively excreting iron
Absorption is inhibited by hepcidin
Ingestion is about 15-20mg/day, but absorption is only 0.5-1.5 mg/day
Iron is present in the diet as ferric (3+) and ferrous (2+)
And as part of haem group (haemoglobin, myoglobin and cytochromes)
We can only absor ferrous (2+) iron
Vitamin C reduces 3+ to 2+
Haem is a smaller part of diet but better absorbed
How does iron uptake occur?
Duodenal cytochrome b catalyses the reduction of FE3+ to FE2+ in the process of iron absorption in the duodenum of mammals
Ferrous (2+) iron can be transported via divalent metal transporter 1 (DMT-1) and H+ coupled co transporter
Fe2+ binds to unknown factors, carried to Basolateral membrane, moves via ferroportin Ion Channel into blood
Hephaestin is a transmembrane copper dependant ferroxidase that converted Fe2+ back to Fe3+
Fe3+ binds to apotransferrin, travels in blood as transferrin
Hepcidin suppresses ferroportin function to reduce iron absorption when stored of ferritin are full
OR
Binds to apoferritin in cytosol to form ferritin Micelle
Ferritin is globular protein complex. 2+ is oxidised to 3* which crystallises within protein shell
A single ferritin can store up to 4000 iton ions
What happens to iron bound to ferritin?
Irreversible
Iron/ferritin is not available for transport into plasma
It is lost in the intestinal lumen and excreted in the faeces then the epithelial cell is shed
Increase Of iron concentration in the cytosol increases ferritin synthesis
How are vitamins absorbed?
Fat soluble (A,D,E,K) are transported to brush border in micelles. k is taken up by active transport
Specific transport mechanism for vitamin c (ascorbic acid), folate, vitamin B1 (thiamine) and vitamin B12
What is the role of vitamin B12?
Liver contains large stores 2-5mg
Impaired absorption retards tha maturation of red blood cells - pernicious anaemia
Most B12 in food is bound to proteins
How is vitamin B12 absorbed?
Normally dietary intake is with B12 bound to a protein
In the stomach the protein is broken down
The B12 then binds to haptocorrin (R protein) which prevents the denaturation of B12 by HCL
B12 binding protein is secreted by parietal cells
VitB12/intrinsic factor is resistant to digestion
B12/IF complex binds to cubilin receptor, then taken up in distal ileum
Once in the cell the complex breaks down
B12 binds to protein transcobalamin II, crosses basolateral membrane
Travels to liver bound to TCII
TCII receptor on cells allows them to take up complex
Proteolysis then breaks down TCII inside cell
