Session 2.1: Water Movement Across Membranes Flashcards
Why can water quite easily move across hydrophobic membrane bilayer?
Small Uncharged (despite dipole of H-O)
How does water move through the membrane bilayer
Diffusion
Driven by osmotic gradient of solutes
How is water distributed ina. 70kg man
42L - total body water
2/3 of which, 28L - intracellular fluid - mostly in tissues, 2% in blood cells
1/3 of which, 14L - extracellular fluid, 75% of which is in interstitial water, 10.5L, 25% in plasma
Blood volume = blood cells + plasma, 2+3.5=5.5L
What is the ionic distribution like between intra cellular fluid and extracellular fluid
Intracellular - Na+, Cl- (lower in ICF), K+ (higher in ICF)
Comparison to ECF
Sodium has concentration gradient inwards (higher on outside), 145mM to 12mM
Potassium has concentration gradient outwards (lower on outside), 4mM to 140mM
Calcium has concentration gradient inwards (higher on outside), 1.5mM to x10-7mM
Chloride has concentration going inwards (higher on outside), 123mM to 4.2mM
What is isotonic
Equal solute to equal water
What is hypotonic
Less solute, more water
What is hypertonic
More solute. Less water
How will water move dependent on tonicity
Water moves from where there is the most water (hypotonic) to isotonic to where there is less water (hypertonic)
What happens if animal cells placed in hypotonic solution
More water on outside, so move into cell
Gain water
Swell
Burst (cytolysis)
What happens if animal cells placed in hypertonic solution
More water on inside so leaves Lose water Shrivel Shrink Lyse
Why is there osmotic equilibrium among different fluid compartments
Because water can move freely through the cell membrane and blood capillary wall
The osmolality of plasma, interstitial fluid, and intracellular fluid is roughly the same, around 280-310mOsm/kg or mmol/L
What is osmolality determined by
In ECF - Na and Cl (80%)
How can serum osmolality be estimated in clinical practice
By doubling serum sodium
Concentration x 2 (as 2 ions) x osmotic coefficient
How can osmotically effective concentration be determined
Osmotic coefficient x number of ions formed by dissociation(eg: NaCL =2) x molar concentration of solute
What’s the definition of osmole
Measure of a solution’s ability to create osmotic pressure and thus affect the movement of water
What is osmole proportional to and give 3 examples
Proportional to the number of osmotic particles in solution
1mol of glucose - 1osmolar solution in 1L water
1 mol of NaCl - 2 osmolar solution in 1L water
1 mol of CaCl2 - 3 osmolar solution in 1L water
What is osmolality
When the concentration of a solution is expressed in osmoles per kilogram of water, the osmolar concentration of a solution is referred to as its osmolality
1osmole/Kg of water = 1osmole/L = 1000milliosmoles/L = 1000mOSM = 1000mmol/L
What is osmosis
The diffusion of water from a hypotonic solution to a hypertonic solution across a semi permeable membrane
How can the movement of water be prevented
Applying pressure to the piston against direction of flow = osmotic pressure of solution, pressure required to stop flow of water.
If cell inflating = pressure increases, if cell shrivelling = pressure decreases
How else is water able to move across lipid bilayer
Aquaporin channels
Not ion channels, integral membrane proteins
Permit rapid water diffusion (must faster that passive diffusion)
There are 13 or more distinct isoforms - different functions and roles
How does water move through aquaporin channels
Depends on the solute concentration gradient
3 billion water molecules per second pass through channel
Single file passage
Positively charged residues within the pore which prevent ions from moving through the channel, only water. So Hydrogen ion gradients not disrupted and neither is pH
Aquaporin family members are
Widely distributed
Lots in kidney
