All cells of living organisms depend on a protective barrier, called a membrane, that will ensure that the contents in and out of the cell are appropriately separated and balanced. In animal cells, especially, the cell´s plasma membrane works as a semi-permeable barrier that divides two environments that are equally submerged in water: the extracellular, and the intracellular environments. But it does not only divide water from both compartments; it also maintains a regulatory role in solute concentration, given that the laws of physics do not change, and water will always follow after solutes, and will go to where there is less concentration of water molecules.
Osmosis, is one of the many ways in which cells are able to maintain the proper balance of both water, and solutes, in the extracellular and intracellular spaces, and plasma membranes (and plant cell walls) play a major role here. Essentially, these membranes are made of a center of lipids (completely hydrophobic), and two layers of phospholipid heads, that interact with water. These molecules are not rigidly and tightly packed together, but are somewhat fluid and permeable. It also contains a series of proteins that form sorts of transport chains and gates, that will ensure that water, solutes and molecules can appropriately cross in and out of the cell.
Water usually moves by osmosis in both plant cells and animal cells, thanks to the membrane´s permeability (in plants the cell wall) to it. Water, will thus move from one side to the other given two factors: concentration of water, and concentration of solutes; with water always following towards places with higher concentration of solutes, and lower concentration of water molecules. This balance of water and solutes on both spaces is vital to ensure that water does not leak in great amounts on either end, either bursting the cell, or dehydrating it to the point of death. And this is what the characteristic of semipermeability does; it allows water to diffuse through osmosis, but under certain conditions, which are kept under regulation by the membrane through other mechanisms like facilitated transport, and active transport, of solutes and molecules.
However, osmosis does not apply to solutes; these depend on the different proteins, channels, and carriers, that are present on membranes to be able to pass. Although, it is important to mention that cells keep their balance by having certain levels of leakage, especially for K+, in order to maintain stability, the norm is that in order for these to pass in and out of cells, they must count with cell membranes.
It is finally, because of membranes that the normal laws of physics regarding water concentration and solute concentrations, do not apply exactly as they should. These barriers ensure that a cell may live by keeping the strict balance to ensure survival.
