Because of these changes that extracellular conditions may incur, animals such as humans have in them mechanisms like kidneys by which the interstitial fluid which bathe the cells remain isotonic to the cytoplasm.
A differentially permeable plasma membrane is an important feature of cells as it encloses all the organelles to increase the concentration of reactants (by decreasing the volume) available for chemical processes specific to the organelle. In addition, it protects the cell from the constantly changing external milieu. The unregulated entry of big and ionic molecules is prevented by the hydrophobic interior of the membrane. Impermeable molecules that are nonetheless essential are transported through proteins embedded in the membrane. The most common example is water, which passes through the transmembrane aquaporins.
The water molecule is an 18 g/mol molecule, which is small compared to a 32 g/mol O2 that can pass through the membrane much more freely than water. So how come water molecules still need aquaporins to be able to pass through the amphiphilic plasma membrane? Despite their neutral charge, water molecules are polar molecules which have a transient negative on the O side and a transient positive in between the two H molecules. This polarity makes them attractive to other polar solutes, producing a transient solute-water binding that decreases the thermodynamic activity (or movement). The more impermeable solutes are present the less is its activity. Because energy spontaneously flow from high to low thermodynamic activity, water movement, or osmosis, should go from a low (hypotonic solution) to high concentration (hypertonic solution) of solutes. If no osmosis was observed, the cytoplasm is said to be isotonic to the surrounding solution.
Osmosis happening in cells was replicated in this experiment, with unshelled eggs