Diffusion, Osmosis, and Cellular Tonicity

Osmosis and Semipermeable Membranes

While diffusion applies to many types of particles, osmosis refers specifically to the movement of a solvent (usually water) across a semipermeable membrane.

A semipermeable membrane is a barrier that allows certain molecules (the solvent) to pass through while blocking others (the solute). In osmosis, water molecules move from a side with a lower solute concentration to a side with a higher solute concentration.

Osmotic Pressure

As water moves across the membrane to dilute a concentrated solution, it exerts a force against the membrane. This force is known as osmotic pressure. The magnitude of this pressure is directly related to the concentration difference: the greater the difference in solute concentration between the two sides, the higher the osmotic pressure.

Cellular Tonicity: How Solutions Affect Cells

In biological systems, the concentration of solutes in the fluid surrounding a cell (such as blood plasma) is vital for maintaining cell integrity. We describe the effect of these solutions on cells using the term tonicity.

Depending on the concentration of the surrounding solution relative to the inside of the cell, three different scenarios can occur:

1. Isotonic Solutions

An isotonic solution has the same solute concentration as the inside of the cell. Because there is no net movement of water, the cell maintains its normal shape and function.

2. Hypertonic Solutions

A hypertonic solution has a higher solute concentration than the inside of the cell. Because the concentration of solute is higher outside, osmosis causes water to flow out of the cell. This loss of water causes the cell to shrink, a process known as crenation.

3. Hypotonic Solutions

A hypotonic solution has a lower solute concentration than the inside of the cell. In this case, water flows into the cell to try and dilute the higher internal concentration. This causes the cell to swell and, if the pressure becomes too great, it may burst—a process called hemolysis.

Study Tip: Summary of Tonicity

• Isotonic: Concentration is equal $\rightarrow$ No net movement $\rightarrow$ Cell stays the same.
• Hypertonic: Outside is "more" concentrated $\rightarrow$ Water leaves $\rightarrow$ Cell shrinks (Crenation).
• Hypotonic: Outside is "less" concentrated $\rightarrow$ Water enters $\rightarrow$ Cell bursts (Hemolysis).

Worked Example: Calculating Concentration

In medical settings, understanding the exact concentration of a solution is crucial. Concentration is often expressed as Molarity (M), which is the number of moles of solute per liter of solution.

Problem: A medical technician needs to prepare a solution. If 1.26 moles of sulfuric acid ($\text{H}_2\text{SO}_4$) are dissolved in enough water to make 2.50 L of solution, what is the molarity?

Solution: Using the molarity formula:

$$M = \frac{\text{moles of solute}}{\text{liters of solution}}$$

Substitute the given values:

$$M = \frac{1.26 \text{ mol}}{2.50 \text{ L}} = 0.504 \text{ M}$$