Diffusion. Protein Mediated Diffusion integral membrane proteins transport proteins (transporters) diffusion through hydrophilic channels facilitated diffusion. Hydrophilic Channels. Fig. 3.8.
Protein Mediated Diffusion
Channels have specificity; they are (more or less) specific for certain ions, e.g., Na+ channel, K+ channel, Ca++ channel, Cl- channel.
Leak channels: Some are always open.
Gated channels: usually closed, opened by a specific stimulus
Water moves down its concentration gradient,
from an area of higher water concentration
to an area of lower water concentration.
Therefore, the net movement of water is
from an area of lower solute concentration
to an area of higher solute concentration.
No net movement because
hydrostatic pressure = osmotic pressure
units of measurement for osmotic pressure: mm Hg or osmolality
osmolality = osmoles / (Kg solvent)
for a dilute solution: osmolality osmolarity = osmoles / (L solution)
Osmolality is more accurate than osmolarity, because volume varies with temperature, but weight does not. But, you are more familiar with calculating molarities than molalities. Therefore,
know how to calculate osmolarity.
[For the purposes of this course assume complete dissociation of any solutes.]
(1 mole Na+ + 1 mole Cl-)
therefore, osmolality = 290 mosmolal
number to memorize:
body osmolarity = 300 mosmolar
normal saline = 0.9% NaCl
molecular weights: Na = 23, Cl = 35.5: NaCl = 58.5 g
0.9% NaCl = 0.9g NaCl x 1000 mL x 1 mole NaCl x 2 osmoles
100 mL sol’n 1 L sol’n 58.5g NaCl mole NaCl
= 0.308 osmoles NaCl x 1000 mosmoles NaCl
1 L sol’n1 osmole NaCl
= 308 mosmoles NaCl
= 308 mosM
Behavior of Animal Cells in Solutions
of Different Osmolarities
osmolarity lower than plasma
same osmolarity as plasma
osmolarity higher than plasma
assumptions: Solute is impermeant and is not metabolized.