Exercises: Osmosis

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.

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.

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 solution undergoes plasmolysis, whereas in a hypotonic solution 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.

Bacteria also experience the effects of osmosis. Use this fact to explain why salting and sugaring are effective traditional methods for preserving food.

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 (hemolysis). 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.

Unit challenge. As a biomedical advisor, you must evaluate how osmotic stress affects cell integrity in order to design an isotonic drink.

1. Home experiment: Cut three potato strips of exactly $5.0\text{ cm}$. Submerge one in mineral water (medium A), another in water with $1.0\text{ g}$ of table salt per $100\text{ mL}$ (medium B), and another with $10.0\text{ g}/100\text{ mL}$ (medium C) for $2$ hours.

2. Data collection: Measure the final length of each strip with a millimeter ruler, calculate its percentage change ($\Delta L = \frac{L_f - L_i}{L_i} \times 100$), and record its tactile firmness in a data table.

3. Reasoned report: Create a graph with your measurements and write a brief report justifying:

4. The observed osmotic behavior (turgor versus plasmolysis) and the role of aquaporins.

5. The reason why the ultrastructure of the lipid bilayer prevents the free diffusion of $\text{Na}^+$ and $\text{Cl}^-$ ions, explaining what mediated transport (passive or active) they would require to cross it.

Evaluation criteria:

• Accuracy in empirical data collection and calculation of $\Delta L$.

• Correct biological justification of osmosis according to the tonicity of the medium.

• Proper integration of membrane ultrastructure and transport mechanisms.