Transport of Water Flashcards
Two Pathways for Transmembrane Flux of Water
- 10% occurs via an interlipid pathway by passive diffusion (drivining force is the chemical potential energy gradient)
- 90% occurs through “aquaporins” specific for H20
* Occurs by facilitated diffusion
Selectivity of Aquaporin
Specific amino acids within the central pore region of the polypeptides regulate the selectivity of the aquaporins for the passage of water only
- dependent upon the dipole porteries of the water molecule andi its changing orientation as it passes trhough the narrowest part of the pore
- also dependent on positively charged aa in the polypeptide chain with in the central pore
- prevents passage of H3O+
Osmosis
A displacement of volume due to the net movement of water down its concentration gradient across a semi-permeable membrane barriers
The impermeable solute particles are referred to as osmolytes (and we say they are “osmotically active”
Net Flow of Water
- High H20 concentration –> low H20 concentration
- low concentration of osmolytes –> high concentration of osmoyltes
Osmosis vs. Simple Diffusion of H20
- Osmosis is movement of water down its concentraiton gradient that is accompanied by a reduction in volume of the respective compartment from which it flows
- Simple diffusion of water is movement of water down its concentration that occurs between two solutions containing different concentrations of a permeamble solute
- No volume change will occur b/c both the solute and water molecules will diffuse down their respective gradients until a complete equilibrium is achieved
Osmotic Pressure
Dependent upon the relative number of particles of solute and solvent per unit volume of solution
And degree of dissociated of solute particles from each other
Cell Membrane
Due to its fragility and lack of rigidity, the cell plasmalemmal membrane cannt support a measureable transmembrane hydrostatic pressure different between cell cytoplasm and ECF
Osmotic Pressure Difference Eqn for an ideal semi-permeable membrane
dPi= RTn(Co-Ci)= RTndC
RTphindC for non-ideal solution
Keep in mind that in order to maintain normal cell volume, the cell cytoplasm must have the same osmolal concentraiton of osmolytes as the ECF and blood plasma
Tonicity
Tonicity of a physiological salt solution refers to its effect on the volume of a cell place in it.
Isotonic extracellular solution
A solution containing a total osmolyte concentration just sufficient to maintain a cell at its normal in vivo colume
A 154 mM NaCl solution is isotonic (and isosmolal) for mamallian cells since at this concentration the cells will maintain their normal in vivo volume.
Hypotonic
An extracellular solution with an osmoylte concentration less than that of th cytoplasm
- H20 concentration of the ECFF is greater than that of the cytoplasm
- Water will move down its concentration gradient into the cell with consequent cell volume expansion (cell lysis)
- Cell lysis can be reversible if the cell is place back into an isotonic solution with a short time interval
Hypertonic
An ECF with an osmoylte concentration that is greater than that of the cytoplasm
- H20 concentration of cytoplasm is greater than that of the ECF
- Water will move down gradient out of the cell with consequent cell volume contraction -> crenation
Osmolyte concentration difference
Generates the osmotic pressure difference between the plasma and the interstitial fluid and is due to the capillary wall-impermeable proteins
Inhibition of the Na+/K+ membrane pump (by ouabain or a cardiac glycoside)
Cause a cell volume increae due to:
- continued passive leak ratio of 3Na+ inward to 2K+ outward leading to:
- inside gain of one + ion
- small depolorization
- cell volume expansion
- continued outward leak of K+ will lead to
- further depolarization and net inward flux of Cl-
The result is a net intracellular gain of one cation and one anion resulting in an inward osmotic gradient and consequent cell swelling
