Osmosis & Osmolarity

Osmosis

Passive Membrane Transport:

net diffusion of water across a membrane

- high to low

Osmolarity

solute concentration

- gC

- osmol/L or osM

Ionic compounds

splits up when dissolved

- dissolution

- 2 osmoles

Solution

solvent + solute

Osmosis Steps

- barrier permeable to water only

- equalized solute concentration after equilibrium

(Water follows solute)

Osmotic Pressure

pressure that must be applied to the solution to prevent net flow of water

- gC proportional to RT

g

# osmoles / mole

Concentration (C)

mmol/L

Sigma

reflection coefficant (0-1)

Penetrating solutions

freely diffuse w/o contributing to osmotic pressure

- reflection coeff = 0

Nonpenetrating solutions

- reflection coeff = 1

Tonicity

tendency of a solution to resist expansion of ICF volume

- non-penetrating solutes in ECF versus ICF

Hypertonic

Tonicity:

cell shrinks

- solute concentration increases ICF concentration

Isotonic

Tonicity:

no change in cell volume

Hypotonic

Tonicity:

cell swells/expands

- solution decreases concentration

Membrane Permeability

smaller & more polar molecules diffuse more rapidly across bilayer

Increase ECF osmolarity

- water moves out of cell

- cells shrink

- ICF decreases

- total body osmolarity higher than normal

K+ disruption

affects cardiac & muscle function (membrane potential)

Ca2+

associated with exocytosis, bone formation, blood clotting

H+ & HCO-3

determine body PH

Osmoles

# of particles (ions or intact molecules) dissolved in solution

- regardless of charge

- # free particles X molarity for ions

Osmolality

osmoles per kg water

Decrease ECF osmolarity

- water moves into cell

- cell swells

- ICF increases

- total body osmolarity lower than normal

Isosmotic

same # of dissolved particles