Osmosis
When fish is packed in salt so it lasts without a refrigerator, or when a red blood cell is dropped into distilled water, water moves across the membrane. In this section you will see why it does so, what happens to the cell, and how you can check it yourself with a potato.
What osmosis is
Imagine you cut two identical strips of potato. You leave one in plain water for two hours and the other in water with a lot of salt. The first one ends up stiff and heavier. The second one ends up soft, limp, and lighter. In both cases the potato cells did not change: only the water outside changed. What moves water across living membranes? You will check it later in a lab.
A semipermeable membrane is selective: some substances cross it and others do not. The cell's plasma membrane is of this type. Watch how water moves in the model when one side has more solute than the other:
Why does water go toward where there is more solute? Because on that side there are fewer free water molecules per mL, since part of the water is tied up surrounding the solute particles. More water molecules pass from the dilute side to the concentrated side than in the opposite direction. The net result is that water "goes toward where there is more solute."
Define osmosis and state what property a membrane must have for this phenomenon to occur.
Define osmosis and state what property a membrane must have for this phenomenon to occur.
Hypotonic, isotonic, and hypertonic solutions
To predict which way water will move, you compare the outside medium with the inside of the cell.
The diagram shows three red blood cells in three containers. The dots are solute molecules.
In A there are few solutes outside and the red blood cell is swollen: water entered. In B the concentration is similar inside and outside, and the red blood cell keeps its normal disc shape with a dimpled center. In C there are many solutes outside and the red blood cell is shriveled: water left.
What does it mean if a solution is hypotonic, isotonic or hypertonic relative to a cell? Explain by indicating which way water tends to move in each case.
What does it mean if a solution is hypotonic, isotonic or hypertonic relative to a cell? Explain by indicating which way water tends to move in each case.
Three identical red blood cells are placed in three different solutions (A, B and C) and after a few minutes they look like the diagram. State whether the solution in…
Three identical red blood cells are placed in three different solutions (A, B and C) and after a few minutes they look like the diagram. State whether the solution in each container was hypotonic, isotonic or hypertonic relative to the red blood cell, and justify your answer.
Effects of osmosis depending on the cell type
What happens to the cell depends on whether or not it has a cell wall.
Change the amounts in the model: with little solute inside versus a lot outside, and the other way around.
Red blood cells are placed in distilled water. Applying the concept of osmosis, explain what will happen to these cells and why.
Red blood cells are placed in distilled water. Applying the concept of osmosis, explain what will happen to these cells and why.
A plant cell in a hypertonic medium undergoes plasmolysis, whereas in a hypotonic medium it becomes turgid but does not burst, as an animal cell would. Explain which…
A plant cell in a hypertonic medium undergoes plasmolysis, whereas in a hypotonic medium it becomes turgid but does not burst, as an animal cell would. Explain which structure of the plant cell prevents this and how it acts.
Lab: the potato in water and in salt water
Now you are going to check osmosis with your own hands, as in a lab report. This experiment lets you see whether your predictions are correct.
Question: what happens to a potato in a hypotonic medium and in a hypertonic one?
Hypothesis (write it before you start): the strip in tap water _ and the strip in salt water ___.
Materials: a potato, a knife, two clear cups, tap water, table salt, a spoon, a kitchen scale (or a ruler), paper towels, and a clock. Ask your teacher for help with cutting and work carefully.
Procedure:
-
Cut two identical strips of potato (about 5 cm long and 1 cm on each side), without the skin.
-
Measure and record the mass and the initial stiffness of each strip (does it bend or snap?).
-
In cup A put 200 mL of tap water. In cup B put 200 mL of water with about 4 spoonfuls of salt, well dissolved.
-
Place one strip in each cup and wait between 30 minutes and 2 hours.
-
Take the strips out, dry them with a paper towel, and measure the mass again and check the stiffness again.
Data record (example of typical results, yours may vary):
| Cup | Medium | Initial mass | Final mass | Appearance |
|---|---|---|---|---|
| A | tap water (hypotonic) | 5.0 g | 5.4 g | stiff, turgid |
| B | salt water (hypertonic) | 5.0 g | 4.4 g | soft, limp |
Analysis: in A water entered the cells by osmosis, the mass increased, and the wall kept them turgid. In B water left the cells toward the medium with more salt, the mass dropped, and the cells lost turgor (the membrane began to pull away from the wall: plasmolysis).
Conclusion: the result confirms that water moves toward the medium with more solutes. If your hypothesis was different, explain why you think it failed.
Solutes that cross the membrane
So far we assumed that the solute does not cross the membrane. What happens if it can cross it?
Example with red blood cells, whose inside is equivalent to about 300 mOsm/L:
| Medium | Approximate osmolarity | Effect on the red blood cell |
|---|---|---|
| 0.9 % NaCl | about 300 mOsm/L | isotonic: keeps its volume |
| 0.2 % NaCl | about 70 mOsm/L | hypotonic: swells and bursts |
| 0.3 M urea | 300 mOsm/L at the start | urea crosses the membrane, water enters, and hemolysis occurs |
0.9 % NaCl has about 0.15 mol/L of NaCl, which separates into two particles (Na⁺ and Cl⁻), and that is why it gives about 300 mOsm/L. 0.2 % NaCl has almost a quarter of that concentration.
Human red blood cells are placed in three test tubes: tube 1 contains $0.9\,\%$ $\text{NaCl}$ ($0.9\ \text{g/100\ mL}$); tube 2 contains $0.2\,\%$ $\text{NaCl}$; and…
Human red blood cells are placed in three test tubes: tube 1 contains $0.9\,\%$ $\text{NaCl}$ ($0.9\ \text{g/100\ mL}$); tube 2 contains $0.2\,\%$ $\text{NaCl}$; and tube 3 contains a $0.3\ \text{M}$ aqueous urea solution (whose initial osmolarity is about $300\ \text{mOsm/L}$, equivalent to that of the erythrocyte cytoplasm). Under the microscope, it is observed that in tube 1 the red blood cells maintain their volume, but in both tube 2 and tube 3 they undergo swelling and complete lysis (haemolysis). Justify the osmotic behavior of the erythrocytes in tubes 1 and 2, and explain the apparent contradiction in tube 3 knowing that the membrane is permeable to urea.
Applied osmosis: preserving food and living in salt water
On the coast, the salt obtained by evaporating seawater (as in the salt flats of Baní) has been used for generations to preserve food. This is how the effect looks on a bacterium:
Bacteria also experience the effects of osmosis. Use this fact to explain why salting and sugaring are effective traditional methods for preserving food.
Bacteria also experience the effects of osmosis. Use this fact to explain why salting and sugaring are effective traditional methods for preserving food.
The cellular response to salinity is analysed in two plant species: the halophyte Salicornia europaea* and the glycophyte Arabidopsis thaliana*. When the concentration…
The cellular response to salinity is analysed in two plant species: the halophyte Salicornia europaea and the glycophyte Arabidopsis thaliana. When the concentration of $\text{NaCl}$ in the soil is increased to $300\ \text{mM}$, Arabidopsis undergoes irreversible plasmolysis and dies, whereas Salicornia maintains turgor by actively sequestering $\text{Na}^+$ ions into its vacuoles and accumulating proline and other compatible solutes in its cytosol. Interpret the osmotic problem that soil salinity creates for roots, thermodynamically justify how the two adaptations of Salicornia allow it to continue absorbing water without altering the function of its cytoplasmic enzymes, and predict what would happen to turgor if active transport into the vacuole were inhibited.
Back to the beginning: the potato strip in plain water becomes stiff because the water is hypotonic and enters its cells, which become turgid. In very salty water the medium is hypertonic, water leaves, the cells lose turgor, and the potato becomes soft and weighs less. It is exactly what you measured in the lab.