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M04.03.02 Osmosis
Osmosis is a fundamental process that regulates the movement of water across cell membranes, playing a vital role in maintaining fluid balance in biological systems. This process is driven by the osmotic properties of solutions and is crucial for proper cellular function.
What is Osmosis?
Definition
Osmosis is the diffusion of water molecules across a semipermeable or selectively permeable membrane. Water moves from a region of higher water concentration to a region of lower water concentration.
Water Concentration and Solutes
The concentration of water in a solution is inversely related to the concentration of solutes. The higher the solute concentration, the lower the water concentration, which leads to osmotic movement.
Osmotic Properties
The two primary measures of osmotic properties are osmolarity and osmolality:
| Osmotic Property | Definition |
|---|---|
| Osmolarity | Concentration of particles per liter of solution (mOsm/L) |
| Osmolality | Concentration of particles per kilogram of solvent (mOsm/kg) |
Significance of Osmotic Properties
- Osmolarity measures the total concentration of solute particles in a solution.
- Osmolality measures the number of solute particles about the weight of the solvent, which is particularly important in physiological contexts.
Osmotic Movement Example
Consider two compartments, A and B, separated by a membrane that is permeable to water but not to solutes:
- Side A: Lower solute concentration, higher water concentration.
- Side B: Higher solute concentration, lower water concentration.
As a result, water moves from Side A to Side B, causing a rise in water level in Side B and a decrease in Side A.
| Compartment | Solute Concentration | Water Concentration | Osmotic Movement |
|---|---|---|---|
| A | Low | High | Water moves from A to B |
| B | High | Low | Water enters B |
Osmosis and Effective Osmoles
- Effective osmoles: Solutes that do not easily cross the membrane create an osmotic force for water movement.
- Example: Plasma proteins act as effective osmoles for the vascular compartment, as they cannot easily cross the capillary membrane.
- Sodium is an effective osmole for the extracellular compartment since it cannot penetrate the cell membrane easily but crosses the capillary membrane.
Points to Remember
- Osmosis involves water movement from regions of higher to lower water concentration.
- Osmolarity and osmolality measure solute concentrations, which determine osmotic movement.
- Solutes that do not easily cross membranes act as effective osmoles and generate osmotic pressure.
Bibliography
- Lodish, H., Berk, A., Zipursky, S. L., et al. (2000). Molecular Cell Biology. 4th edition. New York: W. H. Freeman.
- Silverthorn, D. U. (2015). Human Physiology: An Integrated Approach. 7th edition. Pearson.