Diffusion
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Diffusion. Protein Mediated Diffusion integral membrane proteins transport proteins (transporters) diffusion through hydrophilic channels facilitated diffusion. Hydrophilic Channels. Fig. 3.8.

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Diffusion

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Diffusion

Diffusion

Protein Mediated Diffusion

  • integral membrane proteins

  • transport proteins

    (transporters)

    • diffusion through hydrophilic channels

    • facilitated diffusion


Diffusion

Hydrophilic

Channels

Fig. 3.8

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


Osmosis

Osmosis

  • Water transport

    • diffusion across the phospholipid bilayer

    • diffusion through hydrophilic channels (aquaporins)

  • Osmosis

    • diffusion of water across a semipermeable membrane

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.

Fig. 3.15


Diffusion

Osmosis

No net movement because

hydrostatic pressure = osmotic pressure

Fig. 3.15

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.


Osmolarity

Osmolarity

  • Osmolality/osmolarity is a measure of the number of particles in one liter of solution.

    [For the purposes of this course assume complete dissociation of any solutes.]

    • e.g.,1 mole of glucose  1 osmole of glucose

    • e.g., 1 mole of NaCl  2 osmoles NaCl

      (1 mole Na+ + 1 mole Cl-)


Diffusion

  • Osmolarity is a colligative property.

  • Therefore, osmolarity can be measured by freezing point depression or boiling point elevation.

  • freezing point of plasma = - 0.54 °C

    therefore, osmolality = 290 mosmolal

    number to memorize:

    body osmolarity = 300 mosmolar


Diffusion

  • What is the osmolarity of “normal” saline?

    normal saline = 0.9% NaCl

    molecular weights: Na = 23, Cl = 35.5: NaCl = 58.5 g

    mole NaCl

    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

    L sol’n

    = 308 mosM


Diffusion

Behavior of Animal Cells in Solutions

of Different Osmolarities

Fig. 3.16

osmolarity lower than plasma

same osmolarity as plasma

osmolarity higher than plasma

assumptions: Solute is impermeant and is not metabolized.


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