Osmosis

What is water potential?

Water potential is a measure of the potential energy of water molecules. Another way to describe it is the tendency of water molecules to move out of a solution. The unit given is kPa (Ψ) and this value is determined by the solutes dissolved in the solution.

Pure water contains no solutes. This gives pure water a water potential of 0kPa - this is the highest water potential value a solution can have. The water potential becomes more negative as more solutes are dissolved in the solution.

Another way to view it is by looking at dilute and concentrated solutions. Dilute solutions have a higher water potential than concentrated solutions. This is because dilute solutions contain fewer solutes than concentrated ones. Water will always flow from a higher water potential to a lower water potential - from a more dilute solution to a more concentrated solution.

What is tonicity?

To understand osmosis in living cells, we are first going to define three types of solution (or types of tonicity):

• Hypotonic solution

• Isotonic solution

• Hypertonic solution

A hypotonic solution has a higher water potential than inside the cell. Water molecules tend to move into the cell via osmosis, down a water potential gradient. This means the solution contains fewer solutes than the inside of the cell.

An isotonic solution has the same water potential as the inside of the cell. There is still the movement of water molecules but no net movement as the rate of osmosis is the same in both directions.

A hypertonic solution has a lower water potential than inside the cell. Water molecules tend to move out of the cell via osmosis. This means the solution contains more solutes than the inside of the cell.

Osmosis in animal cells

Unlike plant cells, animal cells paint a cell wall to withstand an increase in hydrostatic pressure.

When placed in a hypotonic solution, animal cells will undergo cytolysis. This is the process by which water molecules enter the cell via osmosis, causing the cell membrane to burst due to the elevated hydrostatic pressure.

On the flip side, animal cells placed in a hypertonic solution become crenated. This describes the state in which the cell shrinks and appears wrinkled due to water molecules leaving the cell.

When placed in an isotonic solution, the cell will remain the same as there is no net movement of water molecules. This is the most ideal condition as you do not want your animal cell, for example, a red blood cell, to lose or gain any water. Luckily, our blood is considered isotonic relative to red blood cells.

Fig. 2 - Structure of red blood cells in different solution types

Reabsorption of water in the nephrons

The reabsorption of water takes place in the nephrons, which are tiny structures in the kidneys. At the proximally convoluted tubule, which is a structure within the nephrons, minerals, ions and solutes are actively pumped out, meaning the inside of the tubule has a higher water potential than the tissue fluid. This causes water to move into the tissue fluid, down a water potential gradient via osmosis.

At the descending limb (another tubular structure in the nephrons) the water potential is still higher than the tissue fluid. Again, this causes water to move into the tissue fluid, down a water potential gradient.

What factors affect the rate of osmosis?

Similar to the rate of diffusion, the rate of osmosis can be affected by several factors, which include:

• Water potential gradient

• Surface area

• Temperature

• Presence of aquaporins

Water potential gradient and rate of osmosis

The greater the water potential gradient, the faster the rate of osmosis. For example, the rate of osmosis is greater between two solutions that are -50kPa and -10kPa compared to -15kPa and -10kPa.

Surface area and rate of osmosis

The greater the surface area, the faster the rate of osmosis. This is provided by a large semipermeable membrane as this is the structure that water molecules move through.

Temperature and rate of osmosis

The higher the temperature, the faster the rate of osmosis. This is because higher temperatures provide water molecules with greater kinetic energy which allows them to move faster.

Presence of aquaporins and rate of osmosis

Aquaporins are channel proteins that are selective for water molecules. The greater the number of aquaporins found in the cell membrane, the faster the rate of diffusion. Aquaporins and their function are explained more thoroughly in the following section.

Aquaporins in Osmosis

Aquaporins are channel proteins that span the length of the cell membrane. They are highly selective for water molecules and therefore allow the passage of water molecules through the cell membrane without the need for energy. Although water molecules can move freely through the cell membrane by themselves due to their small size and polarity, aquaporins are designed to facilitate rapid osmosis.

Fig. 3 - Structure of aquaporins

This is highly important, as osmosis that takes place without aquaporins in living cells is too slow. Their main function is to increase the rate of osmosis.

For example, the cells lining the collecting duct of the kidneys contain many aquaporins in their cell membranes. This is to speed up the rate of water reabsorption into the blood.