Mammalian extracellular fluid and cytoplasm each have osmolarities of roughly 30

Question

Mammalian extracellular fluid and cytoplasm each have osmolarities of roughly 300 mOsm. Explain why a solution of 1.3% NaCl (w/v) is hypertonic relative to the cytoplasm of most mammalian cells; is the solution hyer-, iso-, or hypoosmotic relative to most cells? Explain why a solution of 0.9% NaCl and 300 mOsm urea, although considered hyperosmotic to the cytosol, is in fact roughly isotonic (to most mammalian cells)?

Explain why a solution of 0.9% NaCl and 300 mOsm urea, although considered hyperosmotic to the cytosol, is in fact roughly isotonic (to most mammalian cells)

Explanation / Answer

Mammalian extracellular fluid and cytoplasm each have osmolarities of roughly 300 mOsm. Explain why a solution of 1.3% NaCl (w/v) is hypertonic relative to the cytoplasm of most mammalian cells; is the solution hyer-, iso-, or hypoosmotic relative to most cells?

Answer:

Osmolarity (Osm/L) is the total concentration of all solutes in the solution

1.3% NaCl = 1.3g in 100 mL

This Means 13g/L

molarity of 1.3% NaCl is

(Mol weight of NaCl = 58.44)

= 13/58.55

=0.222 M

=222 mM

Since NaCl ionizes into Na+ and Cl- (two components) its osmolarity will be

=222 x 2

=444 mOsmoles

Since the osmolarity of 1.3% NaCl is more than that of the physiological osmolarity, it is considered hypertonic.

Explain why a solution of 0.9% NaCl and 300 mOsm urea, although considered hyperosmotic to the cytosol, is in fact roughly isotonic (to most mammalian cells)?

Answer:

Mammalian extracellular fluid and cytoplasm each have osmolarities of roughly 300 mOsm.

Since 0.9% NaCl yields 307 mOsmoles and the given osmolarity of urea is 300 mOsm, these two solutions are isotonic with the cytosol.

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