8.  Colligative properties: osmotic pressure

description of the phenomena, laws, limitations, determination of the molecular weight, biological importance

Description of the phenomena

Osmotic pressure:

·       the pressure required to stop the net flow of solvent molecules through the semipermeable membrane

·       the solvent goes from the least concentrated side to the more concentrated side through the semipermeable membrane (left to right on the figures), this process is called osmosis

·       the goal of osmosis is to equalize the solute concentrations on both sides of the membrane

·       driving force: the difference in solute concentration across the membrane creates osmotic pressure

·       example: in biological systems, the cell membrane acts as a semipermeable membrane, maintaining osmotic balance between the cell interior and its surroundings.

Semipermeable membrane:

·       a membrane which only allows certain molecules (mainly solvent molecules) to pass through while blocking others (mainly solute molecules)

Formulas: Van’t Hoff’s Law for dilute solutions
            ∏=∏₀*C (or can be Π=iCRT)
                       
            ∏=osmotic pressure
            ∏₀=RT/C₀ → osmotic pressure is temperature dependent
            C₀= 1 mol/l
            C=molarity (n_solute/V_solution (in l)) →proportional with the number of dissolved               molecules
            (van’t Hoff factor (i) might need to be considered here as well depending on the                    solute)
                       
            ∏=g*Δh
                       
            ∏=osmotic pressure
            g= 9,81 m/s²
            Δh= height difference between the two sides of the membrane (as seen on the                        figure 2)

Consequences of temperature dependence:

·       if t=20°C → ∏=2,4 MPa

·       2,4 MPa is 24 times the atmospheric pressure
            Consequence 1:

§  the corresponding hydrostatic pressure is 240 so even for a 10^-4 mol/l concentration solution Δh is 2,4 cm

§  this means that large molecular weight solutes (peptides, polymers) can also be measured

Consequence 2:

·       The large pressure difference acts on the semipermeable membrane so it should have high mechanical stability

·        the two concentrations at the two sides of the membrane shouldn’t differ too much (isotonic, hypotonic and hypertonic solutions)

Limitations:

·       only works for sufficiently dilute solutions

·       only works if solute-solute reaction can be neglected

·       the membrane has to be perfectly semipermeable

·       precise control of temperature (osmotic pressure varies with temperature)

Determination of molecular weight

·       osmotic pressure can be measured with an osmometer

·       T can be measured with a thermometer

·       R is a constant

·       from these 3 we can calculate the molarity

·       The volume of the solution can also be measured

·       from the volume of the solution and the molarity we can determine the moles of the solute

·       we should know the mass of the solute used for creating the solution

·       from the moles of the solute and the mass of the solute we can calculate M

·       M=

Biological importance

·       cell function and stability

o   osmotic pressure is crucial for maintaining the shape, volume and function of cells

o   osmotic pressure regulates water movement in and out of cells, preventing the cell from swelling or shrinking

·       transport of nutrients and waste

o   osmotic pressure drives the movement of water and nutrients of water and dissolved substances across cell membranes

o   this movement is essential for nutrient uptake and waste removal

·       blood plasma

o   osmotic pressure due to albumin is vital for maintaining blood pressure and volume

o   it is also important for fluid balance between blood and tissues

·       kidney function

o   osmotic pressure plays a key role in the kidney’s ability to filter blood, reabsorb water and concentrate urine

o   these steps are essential for waste excretion and fluid balance

·       plant turgor pressure

o   in plants osmotic pressure within cells (turgor pressure) is necessary for maintaining rigidity and structural integrity which both contribute to growth and movement

Reverse osmosis
           

·       we use double the osmotic pressure to set the balance towards the less concentrated side (left) of the membrane

·       this method is used in desalination, and this is how non-alcoholic beer is created