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Wize University Biology Textbook > The Plasma Membrane

Osmosis

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Osmosis and Osmolarity

Simply put, osmosis is the diffusion of water.
  • Water is a polar molecule that diffuses into the cell through aquaporins (channels).
  • There must exist a concentration gradient. Example: Solute is added to intracellular or extracellular space.
  • Degree to which the concentration of water is decreased depends on the number of particles of solute.
  • Osmolarity is the number of particles a solute dissociates into when in solution (per liter).


Photo by Rice University / CC BY

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Osmolarity and Cell Volume

Water can always diffuse through the membrane to establish diffusion equilibrium (intracellular = extracellular osmolarity).
  • In an isotonic solution: the movement of water is even across the membrane in either direction (cell volume maintained).
  • In a hypertonic solution: a gradient created by a molecule results in a net movement of water out of the cell resulting in shriveling. Example: a cell is put in a solution with a lot of salt.
  • In a hypotonic solution: a gradient created by a molecule results in a net movement of water into the cell resulting in swelling. Example: a cell is put in a solution with no salt.



Practice: Shrink or Not to Shrink?

Sea urchin eggs are isotonic to seawater. Consider these conditions:

a) Eggs are in seawater.
b) Eggs are in 65% seawater (35% distilled water).
c) Eggs are in pure distilled water.

True or False?

i. Under condition b, the egg will not change size.

ii. Under condition a, the egg will not shrink.

iii. Under condition c, the egg will increase in size.

Practice: Change in Cell Volume

When a cell (osmolarity = 300 mOsm) is placed in a solution containing 300 mOsm KCl and 200 mOsm NaCl (osmolarity = 500 mOsm). Which of the following are true?
Extra Practice
Practice: Membrane Transport 2
Sea urchin eggs are isotonic to seawater. Consider these conditions:
a) Eggs are in seawater.
b) Eggs are in 65% seawater (35% distilled water).
Practice: Membrane Transport 2
Sea urchin eggs are isotonic to seawater. Consider these conditions:
a) Eggs are in seawater.
b) Eggs are in 65% seawater (35% distilled water).
Practice: Membrane Transport 2
Sea urchin eggs are isotonic to seawater. Consider these conditions:
a) Eggs are in seawater.
b) Eggs are in 65% seawater (35% distilled water).
Practice: Membrane Transport 2
Sea urchin eggs are isotonic to seawater. Consider these conditions:
a) Eggs are in seawater.
b) Eggs are in 65% seawater (35% distilled water).
An animal cell is placed in an isosmotic solution, yet you observe the cell to become shrivelled. What can be said about the solution?
Practice: Transport
How would each of the following molecules be transported into the cell? (aimple diffusion, facilitated diffusion, primary active transport, or secondary active transport).
1.
2.
Practice: Membrane Transport 2
Sea urchin eggs are isotonic to seawater. Consider these conditions:
a) Eggs are in seawater.
b) Eggs are in 65% seawater (35% distilled water).
Practice: Membrane Transport 1
By what method would you predict that these molecules would enter the cell?
a) Oxygen:
b) Potassium:
A plant cell placed in an isotonic solution would appear to be ...
An animal cell is placed in an isosmotic solution, yet you observe the cell to become shrivelled. What can be said about the solution?