It always occurs in favor of the gradient, in order to equalize the concentrations on both sides of the membrane. It does not involve energy expenditure.
Water moves freely through the membrane, always from the lowest solute to the highest concentration. The pressure with which water is forced through the membrane is known as osmotic pressure.
Osmosis is not influenced by the nature of the solute, but by the number of particles. When two solutions contain the same amount of particles per unit volume, even if they are not the same type, they exert the same osmotic pressure and are isotonic. If separated by a membrane, water will flow in both directions proportionally.
When comparing solutions of different concentrations, the one that has the most solute and therefore the highest osmotic pressure is called hypertonic, and the lowest solute concentration and lowest osmotic pressure is hypotonic. Separated by a membrane, there is greater flow of water from the hypotonic to the hypertonic solution until the two solutions become isotonic.
Osmosis can cause changes in cell volume. A human red blood cell isotonic to a 0.9% sodium chloride solution (“physiological solution”). If placed in a medium with higher concentration, it loses water and withers. If it is in a more diluted (hypotonic) medium, it absorbs water by osmosis and swells and may rupture (hemolysis).
If a paramecium is placed in a hypotonic medium, it absorbs water by osmosis. Excess water is eliminated by increasing the pulse rate of the pulsatile (or contractile) vacuole.
Marine protozoa do not have a pulsatile vacuole as the external environment is hypertonic.
The osmotic pressure of a solution can be measured in a osmometer. The evaluated solution is placed in a closed glass tube with a semipermeable membrane, introduced into a container containing distilled water, as shown in the figure.
By osmosis, water enters the solution by raising the liquid level in the glass tube. Since distilled water is present in the container, the concentration of particles in the solution will always be greater than outside the glass tube. However, when the weight of the liquid column within the glass tube is equal to the osmotic force, the flow of water ceases. It is concluded, then, that the osmotic pressure of the solution is equal to the hydrostatic pressure exerted by the liquid column.