At the open-air market, the cucumber vendor sprays her cucumbers with water and they look firm, but a cucumber with salt and lime juice wrinkles up and releases liquid. Why does water enter some cells and leave others? And at the hospital, why is the IV fluid they give you "0.9% saline solution" and never pure water? Osmosis explains it all.
What osmosis is
If you put a strip of potato in distilled water, after a couple of hours it becomes rigid and weighs more. If you put it in water with a lot of salt, it becomes soft and loses weight. What moves water across living membranes?
Move the interactive control: the side with more solute gains water and the other loses it.
Why does it happen? On the concentrated side there are fewer free water molecules per unit of volume. So more water molecules pass from the dilute side to the concentrated side than in the opposite direction. The net result is that water "goes where there is more solute."
Practice★☆☆Not started
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.
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.
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.
Osmolarity counts how many dissolved particles there are per liter. The cytoplasm of an erythrocyte has about 300 mOsm/L. A 0.9% NaCl solution ($0.9\ \text{g}/100\ \text{mL}$) has a similar osmolarity because each NaCl splits into two ions ($\text{Na}^+$ and $\text{Cl}^-$): it is isotonic and the cell keeps its volume. By contrast, 0.2% NaCl has very few particles: it is hypotonic and the cell swells and undergoes hemolysis.
Tonicity is what actually happens to the cell, and it depends on the solutes that cannot cross the membrane.
Practice★★★Not started
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.
Osmosis works because living matter is mostly water with dissolved salts. Of the inorganic molecules, these two are key:
Water: it is the solvent of the cell. It is polar and forms hydrogen bonds, so it dissolves ions and polar molecules, and it is the medium where reactions take place. It is the "solvent" that moves by osmosis.
Mineral salts: dissolved, as ions ($\text{Na}^+$, $\text{K}^+$, $\text{Cl}^-$, $\text{Ca}^{2+}$), they determine the solute concentration and therefore the osmotic balance. Precipitated, they form structures such as bones and teeth (calcium phosphate).
Organic molecules (carbohydrates, lipids, proteins, and nucleic acids) are not ions: they contribute very little to osmolarity per unit of mass, but they do form the structure of the cell.
Practice★★★Not started
In the U-tube osmometer shown in the diagram, a perfect semipermeable membrane (permeable to water and impermeable to solutes) separates two aqueous glucose solutions of…
In the U-tube osmometer shown in the diagram, a perfect semipermeable membrane (permeable to water and impermeable to solutes) separates two aqueous glucose solutions of 0.1 M and 0.5 M. Initially, the liquid level is identical in both arms.
a) A student states that the net flow of water will continue until the glucose concentration in both arms equalises exactly at 0.3 M. Refute their statement with reasons, explaining what physical force halts osmosis and why, at the equilibrium indicated by "?", a concentration difference remains between both sides.
b) If in a new setup the 0.5 M glucose solution in the right arm is replaced by a 0.5 M NaCl solution while maintaining the 0.1 M glucose solution in the left arm, predict with justification whether the final level difference reached will be greater than, less than, or equal to that in the initial experiment.
Applied osmosis: food preservation and extreme organisms
Practice★★☆Not started
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 cucumber with salt releases liquid because the salt creates a hypertonic environment and water leaves its cells. Hospital IV fluid is 0.9% saline so that it is isotonic and does not damage red blood cells.
Summary
Let's practice! 3 medium exercises in this section