What is osmotic pressure?

Osmotic pressure is both a dynamic property and an colligative property of sols; its effect depends only on the number or concentration of particles, and is independent of the size, shape, and properties of those particles. Since lyophobic sols are thermodynamically unstable systems that coagulate again over time, the osmotic pressure method is not suitable for measuring them; whereas the number-average molar mass of macromolecular solutions can be determined using this method. As shown in the figure, on either side of the semipermeable membrane aa’ there is a pure solvent on one side and a solution on the other. Since only the solvent can pass through the semipermeable membrane, and the chemical potentials of the solvents on the two sides are not equal (μA > μA’), the solvent tends to permeate into the solution. To prevent solvent penetration, the p2 pressure is increased; when the macroscopic solvent penetration rates on two sides are equal, the pressure difference p2-p1 at this point is known as osmotic pressure. If the solution is ideal, the van’t Hoff equation can be obtained; in this equation, the lowercase mB represents the mass molar concentration of the solute, while the uppercase MB represents the molar mass of the solute. Once the osmotic pressure is measured, the molar mass can be calculated. Macromolecular solutions are non-ideal, and the osmotic pressure formula is different; in this formula, a2 is the second virial coefficient. Plotting π/mB against mB yields a straight line, and by extrapolating this line to mB → 0, the number-average molar mass can be calculated from the intercept.

It is a water pressure caused by the concentration difference inside and outside the cells!

　　The higher the concentration of the solution, the greater the osmotic pressure. Osmotic pressure: Water molecules pass through the semipermeable membrane into the sucrose solution, while the solute (which is sucrose) does not pass through the semipermeable membrane. There are more water molecules in a unit volume of pure water than in a unit volume of sucrose solution; therefore, the number of water molecules that pass from the beaker through the semipermeable membrane into the funnel per unit time is greater than the number of water molecules that enter the beaker from the funnel. Water molecules always move from areas with more water to areas with less water, that is, from solutions with lower concentration to solutions with higher concentration. Thus, a hydrostatic pressure is generated. If a pressure is applied above the solution, with a magnitude just sufficient to prevent net penetration of water molecules, the value of this pressure represents the osmotic pressure of that solution at that concentration; it is denoted by the symbol π, and its unit is atm. The value of π can be determined using an osmometer, or it can be expressed by the hydrostatic pressure exerted by the solution above the water surface at osmotic equilibrium (h×s × specific gravity = volume × specific gravity). It can also be calculated using the formula proposed by Vant Hoff: π = icRT or π = n/VRT, where c is the molar concentration of the solution ； i = osmotic coefficient; π is the osmotic pressure of the solution (kPa), V is the volume of the solution (dm3), n is the amount of substance of the solute, and R is the molar gas constant. 　　②Osmotic pressure: When a semipermeable membrane is used, with water as the solvent on one side and a solution on the other, water penetrates through the semipermeable membrane toward the solution side. The pressure applied on the solution side to prevent the movement of water is called osmotic pressure. The movement of water stops because this pressure equals the chemical potential energy of the water passing through the membrane. The osmotic pressure П of a solution with molar concentration Cs is approximately П＝CsRT (R: gas constant, T: absolute temperature). If molar concentration based on volume is used, measured values that are larger than the theoretical values can be obtained. In plant cells with well-developed vacuoles, there exists a relationship of PV = constant value with respect to the cell volume V ； However, in cells with abundant cytoplasm such as animals, the above formula also holds true when the cell volume, which includes the osmotically inactive portion (the non-aqueous phase), is subtracted from V. In plant cells, the cytoplasm and cell sap maintain osmotic pressure balance. The osmotic pressure of the cytoplasm determines the water content in it, thereby affecting physical and chemical properties such as the viscosity of the cytoplasm. Furthermore, osmotic pressure can also generate turgor pressure to regulate cell growth and expansion. Cells have the ability to regulate osmotic pressure, a process known as osmoregulation. Just like animal body fluids, the osmotic pressure of the internal environment in infiltrated tissues also has a significant physiological impact. Osmotic pressure can be expressed in terms of air pressure, freezing point depression (Δ), or osmolar concentration.