Ions, vitamins and minerals Flashcards
what is 1 mole
Molar = one mole per litre
Millimolar (mM) 10-3 Micromolar (µM) 10-6 Nanomolar (nM) 10-9 Picomolar (pM) 10-12 Femtomolar (fM) 10-15
What is Diffusion
The process whereby atoms or molecules intermingle because of their random thermal motion.
Why do we have cell membrane?
The cell membrane acts as a diffusion barrier, enabling cells to maintain cytoplasmic concentrations of substances different from their extracellular concentrations.
what are the 2 types of transport?
Paracellular Transport
through tight junctions and lateral intercellular spaces.
Transcellular Transport through the epithelial cells.
what are the 2 types of transport proteins involved and which is faster?
1) Channel proteins form aqueous pores allowing specific solutes to pass across the membrane.
2) Carrier proteins bind to the solute and undergo a conformational change to transport it across the membrane.
Channel proteins allow much faster transport than carrier proteins.
By what process can you transport?
Active transport requires energy.
- Primary active transport is linked directly to cellular metabolism (uses ATP to power the transport).
e. g Na+/K+ ATPase, H+/K+ ATPase - Secondary active transport derives energy from the concentration gradient of another substance that is actively transported.
e. g SGLT-1 co-transport, HCO3-/Cl- counter
Facilitated transport /facilitated diffusion enhances the rate a substance can flow down its concentration gradient. This tends to equilibrate the substance across the membrane and does not require energy.
e.g GLUT-5, GLUT-2
Describe the absorption of carbohydrates
Absorption of glucose & galactose is by secondary active transport (carrier protein & electrochemical gradient). Carrier protein = SGLT-1 on apical membrane.
SGLT1 can transport glucose uphill against its concentration gradient (so effective when glucose at levels in the lumen are below those in the enterocyte).
Absorption of fructose is by 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.
Exit of glucose at the basolateral membrane is by facilitated diffusion. Carrier protein = GLUT-2, a high-capacity, low-affinity facilitative transporter.
Glucose between plasma and tissue/enterocyte generally equilibrated.
Describe how water is mainly absorbed and where this happens mostly?
99% of the H2O presented to the GI tract is absorbed.
The absorption of water is powered by the absorption of ions.
The greatest amount of water is absorbed in the small intestine, esp the jejunum, around 8 litres a day and large intestine absorbs 1.4 litres.
Many ions slowly absorbed by passive diffusion.
(Calcium and iron are incompletely absorbed, and this absorption is regulated)
Describe the standing gradient membrane
Sodium absorption changes as you move down the gut.
Driven by Na+
Transport of Na+ from lumen into enterocyte- complex and varies between species. Becomes more efficient as travel down intestine:
Counter-transport in exchange for H+ (proximal bowel)
Co-transport with amino acids, monosaccharides (jejunum)
Co-transport with Cl- (ileum)
Restricted movement through ion channels (colon)
Cl- co-transported with Na+ (ileum), exchanged with HCO3- (colon) into enterocytes. Both secondary active transport. This increases intracellular concentration of sodium.
K+ diffuses in via paracellular pathways (between cells) in small intestine, leaks out between cells in colon. Passive transport. K+ in the faeces.
Active transport of Na+ into the lateral intercellular spaces by Na+K+ATPase transport in the lateral plasma membrane, this removes the sodium from the enterocyte to the basement membrane in exchange for a K+. So Na+ concentration increases, changing the electrochemical gradient.
Cl- and HCO3- transported into the intercellular spaces due to electrical potential created by the Na+ transport.
High conc of ions in the intercellular spaces causes the fluid there to be hypertonic.
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 are carried away by the capillaries.
Where is calcium absorbed and what part of the intestine absorbs calcium?
- Most calcium is absorbed in the small intestine
- Duodenum and Ileum absorb Ca2+
- Ca2+ deficient diet increases gut’s ability to absorb.
- Vit D and parathyroid hormone stimulate absorption.
Describe the composition of the calcium?
- Low intracellular Ca2+
- High extracellular fluid-ca2+
How is calcium transported?
Ca2+ carried across apical membrane by:
i) Intestinal calcium-binding protein (IMcal)- facilitated diffusion.
ii) Ion channel
what are the implications for ca2+ transport across the cell?
Ca2+ acts as an intracellular signalling molecule.
-Need to transport Ca2+ while maintaining low intracellular concentrations.
Binds to calbindin in cytosol, preventing its action as an intracellular signal.
Describe the mechanism of calcium transport?
- Ca2+ carried across apical membrane by:
i) Intestinal calcium-binding protein (IMcal)- facilitated diffusion.
ii) Ion channel - Binds to calbindin in cytosol, preventing its action as an intracellular signal.
- Ca2+ pumped across basolateral membrane by plasma membrane Ca2+ ATPase (PMCA) against concentration gradient.
PMCA has a high affinity for Ca2+ (but low capacity).
Maintains the very low concentrations of calcium normally observed within a cell. - Ca2+ pumped across basolateral membrane by plasma membrane Na+/Ca2+ exchanger against concentration gradient.
The Na+/Ca2+ exchanger has a low affinity for Ca2+ but a high capacity. Requires larger concentrations of Ca2+ to be effective. Na+ inside cell in exchange for ca2+ against its conc gradient.
what is vitamin D important for?
- Essential for normal Ca2+ absorption
- Deficiency causes rickets, osteoporosis.
How does vitamin D work?
1, 25-dihydroxy D3 taken up by enterocytes:
- Enhances the transport of Ca2+ through the cytosol
- Increases the levels of calbindin
- Increases rate of extrusion across basolateral membrane by increasing the level of Ca2+ ATPase in the membrane.
why is iron biologically important?
What processes in the body is iron critical for?
Iron can act as an electron donor and an electron acceptor.
Oxygen transport (red blood cells)
Oxidative phosphorylation
(mitochondrial transport chain)
What happens when you have too much iron?
Iron is toxic in excess, but the body has no mechanism for actively excreting iron.
What are the implications for iron absorption?
- Need to be able to absorb it quickly
- Need to be able to limit the absorption, so despite having it in your diet you should be able to limit the ABSORPTION as it is toxic in excess
what forms can iron be present?
a) inorganic iron (Fe3+ ferric, Fe2+ ferrous)
b) as part of heme (haem) group
(haemoglobin, myoglobin and cytochromes).
What type of iron can the body absorb and describe what the other iron usually forms
Cannot absorb Fe3+, only Fe2+. Fe3+ insoluble salts with: hydroxide, phosphate, HCO3- Vit C reduces Fe3+ to Fe2+ Heme smaller part of diet, but more readily absorbed (20% of presented, rather than 5%).
Describe how heam is endocytosed
Dietary heme is highly bioavailable.
Heme is absorbed intact into the enterocyte.
Evidence that this occurs via heme carrier protein 1 (HCP-1), and via receptor-mediated endocystosis.
Fe2+ liberated by Heme oxygenase.
Describe iron uptake?
Duodenal cytochrome B (Dcytb) catalyzes the reduction of Fe3+ to Fe2+ in the process of iron absorption in the duodenum of mammals.
Fe2+ transported via divalent metal transporter 1 (DMT-1), a H+-coupled co-transporter, so a H+ also moves in with it.
Fe2+ binds to unknown factors, carried to basolateral membrane, moves via ferroportin ion channel into blood.
Which type of ion of iron is transported around the body?
Transport Fe3+.
Fe2+ moves across baslolateral membrane via ferroportin.
Hephaestin is a transmembrane copper-dependent ferroxidase that converts Fe2+ to Fe3+.
Fe3+ binds to apotransferrin, travels in blood as transferrin.
