Osmosis is the movement of water molecules down a water potential gradient, through a semipermeable membrane (also termed a partially permeable membrane). This is a passive process as no energy is needed for this type of transport. To understand this definition, we first need to know what water potential means.
The passive forms of transport include simple diffusion, facilitated diffusion, and 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 plant cells
Plant cells contain rigid cell walls made of cellulose. This structure helps with maintaining a characteristic called turgidity - this describes the cellular state of being firm and upright with water. This is highly important in stabilizing plant tissue and preventing wilting. Turgidity is only achieved when plant cells are placed in a hypotonic solution as water diffuses into the cell via osmosis. As water molecules enter the cell, they exert pressure against the cell membrane which is then forced to press against the cell wall. This is called turgor pressure or hydrostatic pressure. The plant cell will appear swollen and firm under these conditions.
When placed in a hypertonic solution, plant cells will undergo plasmolysis. This is the process by which water leaves the cell via osmosis, causing the cytoplasm to shrink away from the cell wall. The plant cell's appearance is said to be flaccid.
When placed in an isotonic solution, plant cells are also said to be flaccid. There is no net movement of water molecules, thus the cells are neither turgid nor have undergone plasmolysis.
As a result of osmosis, plant cells perform best in hypotonic environments due to their turgidity.
Figure 1
Plant root hair cells
The uptake of water in plant root hair cells relies on osmosis. The cytoplasm and vacuole of plant root hair cells contain many dissolved solutes, meaning it has a lower water potential than the soil. Due to this water potential gradient, water molecules move into the plant root hair cell from the soil. This water potential gradient is maintained as the water moves into neighbouring cells, down a water potential gradient through each cell's semipermeable membrane.
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.
Figure 2
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
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
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
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
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 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.
Figure 3
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.
Investigating osmosis in plant cells
We have looked at how osmosis works so we can now perform an experiment to investigate the process. We are going to look at how we can work out the water potential inside potato cells using dilutions of sucrose solution.
For this experiment, you'll need to know what isotonic solutions are because this is what we are going to calculate! If you haven't already, read the section titled 'what is tonicity?' and then check back here.
A potato
A cork borer
Set of weighing scales
Distilled water
Sucrose solution
Boiling tubes
Water bath
Using the cork borer, cut 6 uniform potato pieces of similar sizes and surface area. Use a paper towel to dry each piece.
Weight each potato piece.
Make serial dilutions from a 1M sucrose solution using distilled water. Use concentrations of 1.0, 0.8, 0.6, 0.4, 0.2 and 0.0M.
Add 10cm³ of each sucrose concentration solution to a boiling tube and label each tube.
Add a potato piece to each boiling tube, making sure each piece is fully immersed in the sucrose solution.
Place the boiling tubes in a water bath and leave for 20 minutes.
After 20 minutes, extract the potato pieces from each boiling tube. Use paper towels to blot the excess solution from each potato piece.
Weight each potato piece.
Calculate the percentage change in mass for each potato piece and its corresponding sucrose solution.
The percentage change in mass will tell you which potato pieces have gained or lost water as a result of osmosis. From the data recorded, we can create a calibration curve to work out the water potential of the potato pieces. Calibration curves are used to determine an unknown concentration by comparing the unknown with known standard concentrations (standard curves).
To create your calibration curve, follow these steps:
Plot percentage change in mass (Y-axis) against sucrose concentration (X-axis) on a graph.
Draw a line of best fit.
The plot at which the line of best fit crosses your X-axis (X-intercept) indicates the water potential of your potato pieces. At this point, there is no change in mass because the sucrose concentration is considered isotonic to the water potential inside the potato pieces.
Osmosis - Key takeaways
- Osmosis is the movement of water molecules down a water potential gradient, through a semipermeable membrane. This is a passive process. as no energy is needed.
- Hypertonic solutions have a higher water potential than the inside of cells. Isotonic solutions have the same water potential as the inside of cells. Hypotonic solutions have a lower water potential than the inside of cells.
- Plant cells function best in hypotonic solutions whereas animal cells function best in isotonic solutions.
- The main factors that affect the rate of osmosis are water potential gradient, surface area, temperature and the presence of aquaporins.
- The water potential of plant cells, such as potato cells can be calculated using a calibration curve.