Me. mbrane Transport: Osmosis and Tonicity - Comprehensive Notes

The number of channels and carriers in the plasma membrane determines the maximum rate at which a substance can be transported.
A cell can alter the transport rate of a given substance down its concentration gradient by changing either the number of channels or carrier proteins in the plasma membrane.
A greater rate occurs with increased numbers of these transport proteins and a lesser rate with decreased numbers.

Osmosis is unlike other types of passive membrane transport because it involves water movement and does not involve the movement of solutes.
Osmosis is the passive movement of water through a semi permeable or selectively permeable membrane in response to a difference in relative concentration of water on either side of a membrane.
The plasma membrane is selectively permeable; the phospholipid bilayer allows water passage but prevents movement of most solutes.
Water movement across the plasma membrane occurs in one of two ways:
- Slip between the phospholipid bilayer molecules.
- Through integral protein water channels called aquaporins.
Cells can alter the amount of water crossing the plasma membrane by changing the number of aquaporins.

The phospholipid bilayer is nonpermeable to most solutes.
In osmosis, solutes are classified into two categories based on whether their passage across the plasma membrane is prevented by the bilayer:
- Permeable solutes: e.g., small and nonpolar solutes such as O2,\, CO2,\, ext{and}\, ext{urea} pass through the bilayer.
- Nonpermeable solutes: charged polar or large solutes such as ext{iron},\ ext{glucose},\ ext{and proteins} are prevented from crossing the bilayer.
The term solutes in this discussion on osmosis refers to nonpermeable solutes.

A difference in solute concentration can exist between the cytosol and the surrounding fluid because solutes do not freely cross the phospholipid bilayer.
When a solute concentration exists, a water concentration also exists.
A solution with a greater concentration of solutes contains a lower concentration of water. For example:
- A solution containing 3\% solutes has 97\% water.
- A solution containing 1\% solutes has 99\% water.
The solute percentage reflects the collective percentage of all solutes (e.g., glucose, proteins, sodium).

The net movement of water by osmosis is dependent upon the concentration gradient between the cell's cytosol and the surrounding solution.
Water moves down its concentration gradient from the side with higher water concentration to the side with lower water concentration (i.e., toward the solution with greater solute concentration).
Water continues to move until equilibrium is reached, at which point the water concentration inside the cell equals the water concentration in the surrounding fluid.
In other words, water moves toward the solution with the lower water concentration.
Figure 4.11 (conceptual) shows water moving across the plasma membrane by osmosis from an area of high water concentration to an area of low water concentration.

Osmotic pressure is the pressure exerted by the movement of water across a semipermeable membrane due to a difference in water concentration.
The steeper the gradient, the greater the amount of water moved by osmosis and the higher the osmotic pressure.
Figure 4.12 is used to visualize the movement of water via osmosis in a U-shaped tube with a semipermeable membrane that allows water passage but restricts solutes.
In such a setup, initially side A has more solutes and less water than side B. Water moves from side B into side A by osmosis, down the gradient, until the two sides have equal water concentrations.
Osmotic pressure can be measured indirectly: placing a stopper on side A and exerting force to return the fluid to its original level increases hydrostatic pressure.
The osmotic pressure in this setup is equal to the hydrostatic pressure required to restore the fluid to its original level. This relationship can be summarized as
\pi = \Delta P_{\text{hydrostatic}}.

When water crosses the plasma membrane by osmosis, the cell gains or loses water, leading to a change in cell volume. This property is called tonicity.
Tonicity describes the relative concentration of water (and solutes) and the resulting net movement of water when cells are immersed in solutions.
Three specific terms describe relative water concentration in solutions: isotonic, hypotonic, and hypertonic.
The following discussions reference illustrations showing erythrocytes (red blood cells) in different solutions.

An isotonic solution has the same relative concentration of solutes and water as the cell's cytosol; therefore, there is no net water movement.
Isotonic erythrocytes maintain their normal biconcave shape in isotonic solutions.
Example: intravenous normal saline (normally termed as normal saline) contains 0.9\%\ NaCl and ~99.1\%\ water, which maintains a patient’s fluid balance and keeps erythrocytes from changing shape.

A hypotonic solution has a lower concentration of solutes and a higher concentration of water than the cell's cytosol.
Under these conditions, water moves into the cell down its concentration gradient, from higher outside water concentration to lower inside water concentration.
Entry of water into the cell increases both cell volume and internal pressure; erythrocytes swell under hypotonic solutions (see Fig. 4.13b).
Hemolysis is the specific term for the rupture of erythrocytes and can be fatal if IV solutions of pure water are administered, as the water would osmotically lyse cells.
Pure water (0% solutes, 100% water) is the most extreme example of a hypotonic solution to cellular cytosol.

A hypertonic solution has a higher concentration of solutes and thus a lower concentration of water than the cell's cytosol.
Water moves out of the cell into the surrounding fluid, where water concentration is lower.
This leads to a decrease in cell volume and pressure; erythrocytes shrink (crenation) in hypertonic solutions (see Fig. 4.13c).

It is important not to replace fluids with pure water intravenously, as this can cause cell lysis.
Seawater or very high-salt solutions (e.g., ~3% NaCl) create hypertonic conditions relative to blood cells and can dehydrate cells or cause crenation if not managed carefully.
Understanding tonicity helps predict red blood cell morphology and guides clinical fluid therapy decisions.

Isotonic solution: same relative concentration of water as the cell's cytosol; no net water movement.
Hypotonic solution: more water outside than inside; water moves into the cell; cells swell (possible lysis).
Hypertonic solution: less water outside than inside; water moves out of the cell; cells shrink (crenation).