Osmosis (Biology)
Osmosis is the passive movement of water molecules across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). This net movement continues until equilibrium is reached or until an opposing pressure prevents further flow. Osmosis is fundamental to cell function, driving turgor pressure in plant cells, reabsorption of water in kidney tubules, and the movement of water into roots from soil.
Key Formula
osmotic pressure = i × M × R × T
LaTeX: \Pi = iMRT
| Symbol | Meaning | Unit |
|---|---|---|
| Π | Osmotic pressure | atm |
| i | van 't Hoff factor (number of solute particles) | dimensionless |
| M | Molar concentration of solute | mol/L |
| R | Ideal gas constant | L·atm/(mol·K) |
| T | Absolute temperature | K |
Worked Example
Problem
A cell contains a 0.3 M NaCl solution. It is placed in pure water at 25°C. Calculate the osmotic pressure driving water into the cell. (NaCl fully dissociates, so i = 2; R = 0.0821 L·atm/mol·K)
Solution
Step 1: Identify values — i = 2, M = 0.3 mol/L, R = 0.0821 L·atm/(mol·K), T = 298 K Step 2: Apply the formula: Π = iMRT Step 3: Π = 2 × 0.3 × 0.0821 × 298 Step 4: Π = 2 × 0.3 × 24.47 Step 5: Π = 2 × 7.34 = 14.68 atm
Answer
The osmotic pressure is approximately 14.7 atm
Effect of Osmosis on Animal and Plant Cells in Different Solutions
| Solution Type | Solute Concentration | Effect on Animal Cell | Effect on Plant Cell | Term |
|---|---|---|---|---|
| Hypotonic | Lower than cell | Swells, may lyse (cytolysis) | Swells, becomes turgid | Plasmolysis reversed |
| Isotonic | Equal to cell | No net change | No net change (flaccid) | Equilibrium |
| Hypertonic | Higher than cell | Shrinks (crenation) | Shrinks (plasmolysis) | Plasmolysis |
Interactive Tools
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Related Terms
Diffusion (Biology)
Diffusion is the passive net movement of molecules or ions from a region of higher concentration to a region of lower concentration, driven by the concentration gradient until equilibrium is reached. In biology, simple diffusion across cell membranes occurs for small, nonpolar molecules such as oxygen, carbon dioxide, and ethanol, which can pass directly through the phospholipid bilayer. Facilitated diffusion involves channel or carrier proteins to transport polar or charged molecules down their concentration gradient without requiring energy.
Active Transport
Active transport is the movement of molecules or ions across a cell membrane against their concentration gradient (from low to high concentration), requiring the expenditure of cellular energy in the form of ATP. Primary active transport uses ATP directly to power transport proteins called pumps, while secondary active transport uses the electrochemical gradient established by primary active transport to drive the movement of another solute. Active transport is essential for maintaining cellular ion balances, nutrient uptake, and nerve impulse transmission.
Cell Wall
The cell wall is a rigid or semi-rigid layer located outside the plasma membrane in plant cells, fungi, bacteria, and some algae, providing structural support and protection against mechanical stress and osmotic lysis. In plants, the primary cell wall is composed mainly of cellulose microfibrils embedded in a matrix of hemicellulose and pectin, while the secondary cell wall may also contain lignin for added rigidity. The cell wall is absent in animal cells, which instead rely on the extracellular matrix for structural support.
From Greek "osmos" (a push, impulse), derived from "othein" (to push). The term was introduced by French physiologist Henri Dutrochet in the 1820s, who first described the phenomenon of water movement across membranes.