Discover the microscopic world where every cell functions as a miniature hydroelectric plant, capable of regulating water and substance flows.
Imagine a microscopic world where each cell is a miniature hydroelectric plant, capable of controlling water and substance flows. This is how osmotic pressure functions—one of the most important yet mysterious phenomena in biology2 . For high school students studying biology at a specialized level, understanding osmotic phenomena becomes a key competency that opens doors to understanding the fundamental principles of life1 .
Osmotic phenomena are not just an abstract concept from a textbook, but a mechanism underlying the vital activity of every plant and animal cell. With its help, plants lift water from roots to leaves, our cells maintain optimal volume, and organisms adapt to various environmental conditions2 . Today we will uncover the secrets of osmotic pressure and explore how pedagogical approaches help master this complex but incredibly interesting topic.
Osmosis is a physical phenomenon in which water molecules pass through a semi-permeable membrane from an area with lower concentration of dissolved substances to an area with higher concentration. In a living cell, the role of such a membrane is performed by the cytoplasmic membrane along with the cytoplasm2 .
Osmotic pressure is the force that causes water to move through the membrane. It arises precisely when two solutions with different concentrations are separated by a semi-permeable partition. In plant cells, the osmotic pressure of cell sap is a regulator of water movement through the plant and its distribution between individual organs2 .
Several types of substance transport into and out of cells are distinguished:
Osmotic processes perform vitally important functions for plants:
| Plant Type | Habitat | Osmotic Pressure (kPa) |
|---|---|---|
| Mesophytes | Moderate moisture | 500-1000 |
| Xerophytes | Arid areas or saline soils | 6000-10000 |
| Hydrophytes | Freshwater bodies | 100-300 |
Source: experimental data from studying osmotic pressure using the plasmolysis method2
One of the most illustrative experiments for studying osmotic pressure is the plasmolysis method. It is based on the semi-permeability property of cytoplasm. If a cell is placed in a solution whose concentration exceeds that of the cell sap, the cytoplasm begins to lose water, decreases in volume, and detaches from the cell walls—this phenomenon is called plasmolysis2 .
Experiment goal: learn to determine the osmotic pressure of cell sap in onion skin cells and elodea algae using the plasmolysis method2 .
From the original 1M sugar or sodium chloride solution, prepare solutions with concentrations: 0.9M; 0.7M; 0.5M; 0.3M; 0.1M and pure water. Pour 10ml of each into jars2 .
From the convex side of the colored onion scale, make 10 thin sections sized 5mm² each2 .
Place 2 sections into each jar with solution, starting with the highest concentration. After the sections have been in the solutions for 20 minutes, examine them under a microscope. Prepare preparations in a drop of the corresponding solution2 .
Record observations in a table, noting the presence or absence of plasmolysis in each solution.
| Solution Concentration (M) | Amount of 1M Salt Solution (ml) | Amount of Water (ml) |
|---|---|---|
| Water | — | 10 |
| 0.1 | 1 | 9 |
| 0.3 | 3 | 7 |
| 0.5 | 5 | 5 |
| 0.7 | 7 | 3 |
| 0.9 | 9 | 1 |
Source: instructions for laboratory work on determining osmotic pressure2
In cells of sections that were in solutions with higher concentration than the cell sap concentration, plasmolysis is observed. In solutions with lower concentration, plasmolysis does not occur. In sections immersed in an isotonic solution (where solution concentration approximately equals cell sap concentration), the beginning of plasmolysis is observed2 .
For example: if plasmolysis began in 0.3 M sodium chloride solution (cytoplasm detached only in cell corners), and there was no plasmolysis in 0.1 M solution, then the osmotic pressure of 0.3 M solution is higher than the osmotic pressure of cell sap, and the osmotic pressure of 0.1 M solution is lower2 .
| Solution Concentration | Plasmolysis Presence | Cell Condition |
|---|---|---|
| Water | Absent | Cells are turgid, cytoplasm adheres to walls |
| 0.1 M | Absent | Cells are turgid, cytoplasm adheres to walls |
| 0.3 M | Initial | Cytoplasm detaches only in cell corners |
| 0.5 M | Moderate | Cytoplasm detached from side walls |
| 0.7 M | Significant | Cytoplasm significantly detached from walls |
| 0.9 M | Deep | Cytoplasm completely separated from walls |
Conducting experiments to study osmotic phenomena requires the use of specific materials and reagents:
For cellular observation
Different concentrations
Jars and containers
Pipettes and thermometers
Studying osmotic phenomena in a specialized biology course is not just a school curriculum requirement, but a powerful tool for developing scientific thinking. As modern research demonstrates, the right approach to teaching this topic enables students to develop the ability to consciously use terminology, understand fundamental biological principles, and apply acquired knowledge in non-standard situations1 .
Experimental work on determining osmotic pressure using the plasmolysis method not only reveals a specific physiological mechanism but also develops skills in formulating hypotheses, analyzing results, and drawing conclusions—those key competencies that form the basis of the scientific method1 .
Osmotic phenomena are a bridge between abstract physical-chemical principles and the dynamic reality of living organisms. By understanding them, we approach solving one of nature's deepest mysteries—the mystery of life itself.