Tonicity and Osmolarity

Questions:

  • I was asked how we know if a solution is hypertonic/hypotonic/isotonic, and how we figure it out without putting a cell into solution

My incorrect answer: - I explained that a tonicity is defined based on the relative concentration of solutes in the solutions, which will determine the direction of water flow in solution

The correct answer: - Tonicity is unitless and is defined by the direction of fluid flow in the context of two solutions separated by a semipermeable membrane.

Expanded answer

  • Osmolarity is a measure of concentration of osmotically active particles in solution - depends on number of particles in a volume of solution (Rather than what the type of particle is)
  • Osmolarity differs from molarity as individual ions contribute to the osmolarity

For example: - 1Mol of NaCl in 1 L of water has a molarity of 1mol/L - However in water, \(NaCl(aq) \rightarrow Na^{+}(aq)+Cl^{-}(aq)\) - This means that the osmolality of this solution is 2 Osmol/L (as NaCl dissociates into 2 ions) - 1 mol of glucose in 1L of water has a molarity of 1mol/L - In water, glucose doesn't deionise, meaning its osmolality is still 1 Osmol/L

  • To define tonicity, you need 2 compartments separated by a semipermeable membrane
    • e.g. a cell in solution

Questions (from https://journals.physiology.org/doi/epdf/10.1152/advan.00094.2018): Assumptions for the following questions:

  1. Our idealized cell has an internal concentration of 300mosM. (We will continue this assumption through therest of the text, unless otherwise noted.)
    1. The solutes present in the cell are assumed to be nonpen-etrating and cannot leave the cell.
    1. NaCl is a functionally nonpenetrating solute that behavesas if it cannot cross the cell membrane. (As noted above,in reality, Na+ that leaks into the cell is removed by the Na+-K+-ATPase at a rate that closely matches the rate ofleakage in.)
    1. Urea is a freely penetrating solute that easily crosses thecell membrane. Because of this, urea will distribute throughout body compartments until its concentration isequal in all compartments.
    1. Water freely crosses all cell membranes, dividing theintracellular and extracellular compartments.

Try to answer these questions based on your understandingof osmolarity and tonicity. 1. You place a cell with an internal concentration of 300mosM in a 400 mosM solution. What happens to thevolume and osmolarity of the cell at equilibrium? 2. Which would have a greater effect on the volume of a cellat equilibrium:solution Acomposed of 300 mosM NaCland 200 mosM urea, orsolution Bcomposed of 200mosM NaCl and 300 mosM urea? 3. Given the same solutions as in the previous question (300mosM NaCl/200 mosM urea or 200 mosM NaCl/300mosM urea), which solution would have a greater effecton the osmolarity of the cell at equilibrium?