Equilibrium and dynamic osmotic behaviour of aqueous solutions with varied concentration at constant and variable volume

Abstract

Osmosis is essential for the living organisms. In biological systems the process usually occurs in confined volumes and may express specific features. The osmotic pressure in aqueous solutions was studied here experimentally as a function of solute concentration (0.05-0.5 M) in two different regimes: of constant and variable solution volume. Sucrose, a biologically active substance, was chosen as a reference solute for the complex tests. A custom made osmotic cell was used. A novel operative experimental approach, employing limited variation of the solution volume, was developed and applied for the purpose. The established equilibrium values of the osmotic pressure are in agreement with the theoretical expectations and do not exhibit any evident differences for both regimes. In contrast, the obtained kinetic dependences reveal striking divergence in the rates of the process at constant and varied solution volume for the respective solute concentrations. The rise of pressure is much faster at constant solution volume, while the solvent influx is many times greater in the regime of variable volume. The results obtained suggest a feasible mechanism for the way in which the living cells rapidly achieve osmotic equilibrium upon changes in the environment.

Figure 1

Schematic of the two experimental osmotic regimes: (a) open cell (variable volume); (b) closed cell (constant volume).

Figure 2

Solvent influx n _L [mol] as a function of osmotic pressure P _osm [bar] in the regime of variable solution volume.

Figure 3

Comparison of equilibrium osmotic pressure values, P _osm as a function of solute concentration under regimes of constant and variable solution volume. Experimental data are from the readings of the electronic pressure sensor, respectively: (□) constant solution volume; (◊) variable solution volume; (▲) variable solution volume (values corrected for dilution). The dotted line indicates the theoretical dependence (see (1)).

Figure 4

Osmotic pressure P _osm versus time t dependence for three different initial sucrose concentrations at the two regimes: (1) 0.5 M (constant volume); (2) 0.2 M (constant volume); (3) 0.1 M (constant volume); (4) 0.5 M (variable volume); (5) 0.2 M (variable volume); (6) 0.1 M (variable volume).

Figure 5

Solvent influx n _L as a function of elapsed time t dependences for the three studied solute concentrations: (a) constant volume regime: (1) 0.5 M; (2) 0.2 M; (3) 0.1 M (n _L is expressed in millimoles); (b) variable volume regime: (4) 0.5 M; (5) 0.2 M; (6) 0.1 M (n _L is expressed in moles).

Figure 6

Solvent rates of transfer dependences dn _L/dt as a function of elapsed time t for the three solute concentrations: (a) constant volume regime: (1) 0.5 M; (2) 0.2 M; (3) 0.1 M (n _L is expressed in millimoles); (b) variable volume regime: (4) 0.5 M; (5) 0.2 M; (6) 0.1 M (n _L is expressed in moles).