Osmosis
Definition of Osmosis:
The net movement of a solvent, usually water, through a semipermeable membrane toward the side where impermeant solutes lower the solvent potential. In simple cases this corresponds to movement from a solution with lower solute concentration to one with higher solute concentration.[1]
This is the common definition for Osmosis, other definitions can be discussed in the article
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Osmosis is generally described as a consequence of a difference in water potential, or equivalently in the chemical potential of water, across a selectively permeable membrane. Dissolved particles that cannot pass the membrane lower the water potential on their side. Water therefore moves toward this side until the resulting hydrostatic pressure difference balances the osmotic driving force. In dilute ideal solutions this balancing pressure is described by the van ’t Hoff relation for osmotic pressure.
Osmosis is a phenomenon of selective passage of different molecules through a semipermeable membrane, resulting in a net influx to the membrane side with the highest concentration of blocked molecules. This influx stops when a sufficient pressure difference has built up between the compartments on either side of the membrane. The principle, based on the interaction between the different molecules and the membrane, is explained in the accompanying figure[2]. Here, two compartments with a free surface are separated by a membrane. On one side there is a liquid with molecules that can cross the membrane (e.g. a solvent, such as water) and on the other side a solution containing molecules that cannot cross the membrane (e.g. solutes, such as salt ions dissolved in water). The solute molecules form a barrier, preventing the solvent molecules in the solution from generating a neutralizing counterpressure for the molecules passing the membrane from the other side. The resulting net inflow of solvent molecules through the membrane increases the pressure in the solution compartment until an equilibrium is reached where the net solvent inflow is eventually neutralized.
The fact that some types of molecules can pass a membrane while other types cannot depends on the characteristics of the membrane. In cells, the semipermeable membrane is the lipid bilayer together with its embedded membrane proteins. The lipid bilayer is readily crossed by small non-polar molecules, but charged ions such as Na+, K+ and Cl- cannot freely cross because they are strongly hydrated and cannot easily enter the hydrophobic membrane interior. Water can cross the membrane, especially through aquaporin channels. Osmosis therefore arises because water is much more permeable than many solutes that remain effectively confined to one side of the membrane.[3][4]
Osmosis is an essential process for the cells of living organisms because it affects cell water balance and cell volume. The uptake and excretion of salts, nutrients and waste products occur through additional passive or active transport mechanisms across the cell membrane. Osmosis therefore can induce a net uptake of ambient fluid or a net loss of cell fluid, depending on the tonicity of the ambient fluid. To prevent expansion or shrinkage of cells, cells use different mechanisms to maintain a balance between water and solutes in their bodies. This is called homeostasis.
Contents
Related definitions[5]
Osmolality
Osmolality is the concentration of osmotically active particles expressed in osmoles per kilogram of solvent.
Osmolyte
Osmolytes are dissolved substances that contribute to osmotic balance. In cells, many important compatible osmolytes are low-molecular-weight organic compounds such as amino acids, sugars and methylamines, which can be accumulated or released without strongly disturbing cellular biochemistry.
Isosmotic intracellular regulation
Cellular mechanisms that, upon a change in surrounding salinity, lead to the adjustment of the intracellular osmotic pressure to meet that of the environment, thus minimizing variations in cell hydration. Adjustment of the intracellular osmotic pressure usually occurs through the production of organic osmolytes.
Anisosmotic extracellular osmoregulation
Mechanisms acting to maintain body (extracellular) fluid volume, osmotic pressure and ionic composition despite environmental salinity changes.
Osmoregulators
Species that carry out anisosmotic extracellular regulation when exposed to extracellular osmolality changes. This is achieved through several mechanisms involving various permeability and salt transport properties within different ion-transporting epithelia. Osmoregulators finally restore the salt balance through excretion of excess salt or excess water.
Osmoconformers
Species that maintain their internal medium isosmotic to their environment, minimizing water fluxes across membranes. Osmoconformers that are adapted to varying salinity usually maintain equal osmotic pressure through the production of organic osmolytes.
Many marine invertebrates are osmoconformers, while many fish and crustaceans show stronger osmoregulation.
Tonicity
- Hypertonic solution: a solution with enough non-penetrating solutes to cause cells to lose water and shrink.
- Hypotonic solution: a solution with too few non-penetrating solutes, causing cells to gain water and swell.
- Isotonic solution: a solution that causes no net change in cell volume.
Appendix Molecular interpretation of osmosis and formula
According to kinetic gas theory, all molecules in a fluid are in random (Brownian) motion with the same kinetic energy [math] ½ m v^2[/math], where [math]m[/math] is the mass and [math]v^2[/math] the mean squared velocity of a molecule. This means that large heavy molecules move more slowly than small light molecules. Mutual hitting of molecules generates a pressure [math]p[/math] in the fluid, which is proportional to the kinetic energy of the molecules. According to the ideal gas law, the pressure is proportional to the temperature [math]T[/math] (degrees Kelvin [math][K][/math]) of the fluid, [math]p=(N/V) R \, T[/math], where [math]V=[/math] volume, [math]N=[/math] number of moles and [math]R \approx[/math] 8.3 [J/K.mol] is the ideal gas constant.
Water molecules are in continuous random motion and pass through water-permeable pores in the membrane in both directions. If no solute is blocked by the membrane, these molecular exchanges balance on average and there is no net osmotic flow. If one side contains solute particles that cannot enter the membrane pores or the hydrophobic membrane interior, the interactions between the solute, water and membrane make the exchange asymmetric. The blocked solute particles are then osmotically active: they exert a lower counterpressure on the water molecules passing the membrane.[6][7] The osmotic pressure, [math]\Pi[/math], which is equal to the pressure difference [math]\Delta p[/math] required to stop the net water flux across the membrane, is therefore proportional to the molar concentration [math]c_{osm}[/math] of osmotically active particles and to the absolute temperature [math]T[/math], [math]\quad \Pi \propto c_{osm} T[/math].
For an ideal dilute solution, the osmotic pressure is given by the van ’t Hoff relation [math]\Pi=c_{osm} \, R\, T[/math] where [math]c_{osm}[/math] is the concentration of osmotically active particles. If the solute concentration [math]c[/math] is expressed as molar concentration of dissolved chemical species, the formula is often written [math]\Pi=i \, c \, R \, T [/math], where [math]i[/math] is the van ’t Hoff factor. For concentrated electrolyte solutions, corrections for non-ideal behaviour are needed.
If [math]c[/math] is expressed in mol/l, [math]R=[/math] 8.314 kPa l mol⁻¹ K⁻¹ gives [math]\Pi[/math] in kPa.
For example, for an idealized NaCl solution with 35 g NaCl/l, [math]\; c_{osm}[/math] is roughly 1.2 osmoles/l and [math]\Pi \approx [/math] 3,000 kPa =30 bar. Real seawater is not an ideal NaCl solution, but this estimate gives the right order of magnitude.
The thermodynamic condition for osmotic equilibrium is equality of the chemical potential of the solvent on the two sides of the membrane. Adding impermeant solute lowers the chemical potential of the solvent in the solution. If the solution compartment is allowed to rise in pressure, the pressure increase raises the solvent chemical potential until equilibrium is restored. The pressure difference required to stop net solvent transport is the osmotic pressure [math]\Pi[/math] given by the van ’t Hoff relation.
External links
Wikipedia articles Osmosis and Osmotic pressure.
References
- ↑ Pinet P.R. 1998. Invitation to Oceanography. Jones and Bartlett Publishers, p. 508
- ↑ Nelson, P.G. 2017. Osmosis and thermodynamics explained by solute blocking. Eur. Biophys. J. 46: 59–64
- ↑ Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K. and Walter, P. 2022. Molecular Biology of the Cell, 7th ed. W.W. Norton & Company, New York.
- ↑ Verkman, A.S. 2013. Aquaporins. Current Biology 23: R52–R55.
- ↑ Rivera-Ingraham, G.A. and Lignot, J-H. 2017. Osmoregulation, bioenergetics and oxidative stress in coastal marine invertebrates: raising the questions for future research. Journal of Experimental Biology 220: 1749-1760
- ↑ Kiil, F. 1982. Mechanism of osmosis. Kidney International 21: 303–308
- ↑ Manning, G.S. and Kay, A.R. 2023. The physical basis of osmosis. J. Gen. Physiol. 155(10), e202313332
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