I'm confused about the role of hydrostatic pressure compared to osmotic pressure.

Q1:If I have a U-tube with a membrane permeable only to water molecules and equal volumes of water on either side but only 1 side (side B) has NaCl, the osmotic pressure would cause water to move from side A to side B,correct ?

Q2.But then hydrostatic pressure would cause water to move back to side A. So, the water would move from side A to side B until the effect caused by the hydrostatic pressure = effect caused by osmotic pressure ?

Q3. The last statement wouldn't be correct If I said that "water moves until the hydrostatic pressure=osmotic pressure" would it ?

and lastly, My friend said the water would move until the hydrostatic pressure on both sides was equal Q4. If water is moving from side A to side B then the we have more water molecules at side B, how would side A ever reach the hydrostatic pressure at side B ? Do I have a misunderstanding in the concept of hydrostatic pressure ? In this context, I understand that it is the pressure exerted by the water molecules on the selectively permeable membrane.

The more I google hydrostatic pressure the more lost I get because all sources seem to explain in terms of equations and physics and I'm only taking this for an introductory course in physiology.

  • 1
    $\begingroup$ Do the answers at biology.stackexchange.com/questions/89609/osmotic-pressure help you? $\endgroup$
    – Bryan Krause
    Apr 5 '20 at 21:14
  • $\begingroup$ I understand everything thats written in that page, I think of osmotic pressure as a vacuum too but I might have a misunderstanding in hydrostatic pressure and what equals what at equilibrium $\endgroup$
    – Alara
    Apr 6 '20 at 3:20

Osmosis is defined as the flow of water/solvent molecules through a semipermeable membrane from a region of low to high solute concentration, until equilibrium is established.

To counter osmotic flow, some pressure must be applied to the solution in order to prevent pure solvent from going through the semipermeable membrane separating the two liquids; this is known as the osmotic pressure.

The osmotic pressure is the pressure required to counter, not sustain, osmosis.

The osmotic pressure can be approximated by using the following formula: $\Pi = i M R T$ .

U- Tube showing osmotic pressure. On the left side of the U-tube is an aqueous solution, and on the right side is pure water. The pure water is trying to dilute the solution by travelling through the semipermeable membrane. Eventually the added weight of the extra water on the left causes enough pressure to stop osmosis.

Osmotic pressure is the pressure that needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane. Osmotic pressure can also be explained as the pressure necessary to nullify osmosis. One way to stop osmosis is to increase the hydrostatic pressure on the solution side of the membrane; this ultimately squeezes the solvent molecules closer together, increasing their “escaping tendency.” The escaping tendency of the solution can be raised until it eventually equals that of the molecules in the pure solvent; at this point, osmosis will cease. The osmotic pressure is the pressure required to achieve osmotic equilibrium.

Osmotic pressure. Osmotic pressure is the pressure required to stop osmosis.

The osmotic pressure (II) of an ideal solution can be approximated by the Morse equation:

$\Pi = i M R T$

Here, i is the van ‘t Hoff factor, M is the molarity of the solution, R is the gas constant, and T is the absolute temperature in Kelvin. We can see from this equation that the amount of solute present in the solution will directly affect the osmotic pressure of the system.


What is the osmotic pressure of a 1.35 M solution of NaCl at 25 $^\circ$C?

First, fill in all of the necessary information, and then solve:

i = 2 (NaCl breaks into two particles)

M = 1.35 $\frac{moles}{L}$

R = 0.0821 $\frac{L\times atm}{K \times mol}$

T = 25 $^\circ$C + 273 = 298 K

$\Pi = 2 \times 1.35 \times 0.0821 \times 298$

$\Pi = 66.1\ atm$



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