Osmosis
Water also moves across the membrane following its own gradient, although this gradient is not measured by the concentration of water, but by that of the solutes dissolved in it. Understanding osmosis explains why a red blood cell can burst in distilled water and why salt preserves food, as in traditional salted bacon or a jar of jam.
What is osmosis?
In Biology class you cut chips from a potato for a practical. After two hours in distilled water they are stiff and heavier, but in strong salt solution they go soft and lose mass: what moves water across living membranes?
Osmosis is different from the diffusion of solutes: only water crosses the membrane, because the solutes cannot. Osmosis also matters for plant health: when cells are turgid, they push against their walls and support the plant, but if the soil is dry or salty the cells lose water, turgor falls and the plant wilts.
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
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.
The effects of osmosis depending on the type of cell
Investigating the rate of osmosis
Three factors affect the rate of osmosis: the temperature (higher temperature means faster movement of water molecules, so faster osmosis), the concentration gradient (the bigger the difference in solute concentration across the membrane, the faster the net flow of water) and the surface area of membrane exposed (more area means more water can cross at once).
A typical practical uses potato cylinders. Cut identical cylinders with a cork borer, blot them dry, measure their mass and length, and place one in each test tube of distilled water or sucrose solution of different concentrations. After a set time (for example 1 hour at the same temperature), blot and re-measure. To make it a fair test, change only one factor and keep the others the same. Calculate the percentage change in mass. Cylinders in a hypotonic solution gain mass and become stiff, while those in a hypertonic solution lose mass and go soft. Plot the percentage change in mass against the solution concentration and draw a line of best fit. The point where the line crosses zero (no change in mass) gives the concentration that is isotonic to the potato cells.
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.
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 behaviour 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: preservation and extreme organisms
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.
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.