Osmosis and Osmolarity MCAT Biology & Biochemistry > Cellular Anatomy & Physiology > Cellular Anatomy & Physiology

OSMOSIS AND OSMOLARITY

OSMOSIS

Osmosis: The diffusion of water across a semi-permeable membrane due to differences in solute concentrations A way to think about it...

Let's imagine a way to think about the direction of water movement. You have two glasses filled with marbles but one is filled with a lot more marbles than the other. If you added water to both glasses to reach a certain height, then the glass with more marbles would require less water to reach the height. There is a lower concentration of water in this glass than the other glass.

However, if the two glasses were connected by a semi-permeable membrane, osmosis would cause water to flow from the glass of high water concentration to the glass with low water concentration. It is important to note that this is not the actual mechanism of osmosis, just an easy way to think about it.

OSMOLARITY

Osmolarity (osmotic concentration)

• Measure of solute concentration (osmoles of solute per liter)

Ionic compounds

• Often dissociate in solution (NaCl becomes Na+ and Cl-)

• 2 moles of NaCl therefore become 4 osmoles of solute (2 Na+ and 2 Cl-)

Nonionic compounds

• Don't dissociate in solution

• 2 moles of glucose therefore become 2 osmoles of solute

SEMI-PERMEABLE NATURE OF CELLULAR MEMBRANES

Isotonic solution

• Solution has same solute concentration as the inside of the cell

1 / 4 Hypertonic solution

• Solution has greater solute concentration than the inside of the cell

Hypotonic solution

• Solution has less solute concentration than the inside of the cell

If a red blood cell is placed in a:

• Isotonic solution - water freely diffuses in and out of the cell at equilibrium

• Hypertonic solution - water diffuses out of the cell and the cell shrivels up

• Hypotonic solution - water diffuses into the cell and the cell lyses

FULL-LENGTH TEXT

• Here we will overview the concepts of osmosis and osmolarity.

- These concepts, in addition to the concept of diffusion that we learned about earlier, are important when we later discuss the movement of molecules across a cell membrane.

• First, start a table to define some key terms.

• Denote that osmosis is the diffusion of water across a semi-permeable membrane due to differences in solute concentrations.

- We will explain this in detail shortly.

• Denote that osmolarity (also known as osmotic concentration) is a measure of solute concentration (i.e. the amount of osmoles of solute per liter).

Begin with osmosis.

• Draw two U-shaped pipes.

• Use a porous membrane to separate both sides of the pipes.

- Indicate that it is a semi-permeable membrane, which means it allows some molecules to pass through it, but not others.

2 / 4 • Indicate that water is present in the left pipe and it is at equal levels on both sides.

• Add some solute molecules to the left side of the pipe; they are too large to fit through the membrane.

• Now add a greater number of solute molecules to the right side of the pipe.

• Indicate that the left side of the pipe has a low solute concentration and the right side of the pipe has a high solute concentration.

- It is this difference in solute concentration across a semi-permeable membrane that causes osmosis to occur.

• In the pipe on the right, indicate that water has moved from the left side, which now has a lower level of water, to the right side, which now has a higher level.

- Indicate that because the amount of solute remained the same on each side and it was only the water that moved, the concentration of solute is now the same on both sides of the membrane.

Let's imagine another way to think about the direction of water movement.

• You have two glasses filled with marbles but one is filled with a lot more marbles than the other.

- If you added water to both glasses to reach a certain height, then the glass with more marbles would require less water to reach the height.

- There is a lower concentration of water in this glass than the other glass.

- However, if the two glasses were connected by a semi-permeable membrane, osmosis would cause water to flow from the glass of high water concentration to the glass with low water concentration.

- It is important to note that this is not the actual mechanism of osmosis, just an easy way to think about it.

Now, indicate that we will discuss the concept of osmolarity, which is the solute concentration measured as the amount of osmoles of solute per liter.

• Draw one box for ionic compounds in solution.

• And another for nonionic compounds in solution.

• Indicate that we will be adding 2 moles of sodium chloride to the left box, using two combined shapes to distinguish the two ions.

- Because ionic compounds dissociate within solution, within the box, draw the two shapes that represent the sodium

3 / 4 ions and the two shapes that represent the chloride ions.

- There should be four separate shapes in the box.

• Now indicate that our solution of sodium chloride has four osmoles of solute.

• Above the right box, indicate that we will be adding two moles of glucose to the solution, using a single shape to represent the glucose molecule.

- Because glucose does not dissociate in solution, draw two shapes in the box to represent the two moles of glucose added.

• Finally, indicate that our solution of glucose has two osmoles of solute.

- Thus, osmolarity is similar to the concept of molarity, but with the key difference that one mole of a substance can actually have more than one osmoles in solution (as we've seen with ionic compounds in solution).

To demonstrate the semi-permeable nature of a cell's plasma membrane (allowing water to freely flow while restricting the movement of other molecules), let's explore what would happen if you drop a red blood cell into solutions with three different solute concentrations.

• Begin with a solution that is isotonic.

• Draw a normal red blood cell in the isotonic solution.

- Show that the solution has the same solute concentration as the interior of the cell.

- Show that water freely diffuses in and out of the cell at an equilibrium.

• Now, show a hypotonic solution.

- It has a lower solute concentration than the intracellular environment.

- Indicate that if we place our red blood cell in a hypotonic environment (in which the red blood cell has a higher solute concentration than the solution), water would diffuse into the cell and cause it to lyse.

• Lastly, show a hypertonic solution.

- It has a greater solute concentration than the intracellular environment.

- Show that in a hypertonic environment (in which the red blood cell has a lower solute concentration than the solution), our cell would shrivel up.

- Show that water would leave the cell.