Baker, I was hoping someone else might do this. Sorry to be the one to tell you, but it sounds like your chemistry teacher was more interested in stopping students drinking distilled water in the labs, than providing a proper understanding of what really happens.
Osmosis is effectively “selective diffusion” occurring across a semi-permeable membrane. In reality, the membrane is more likely to be differentially permeable or selectively permeable (but we leave that for another day). Diffusion occurs where there is an unequal distribution of the concentration of a fluid substance (gas, liquid, solution). The concentration will even out due to random movement of the particles. Put simply, the probability of a particle moving from a region of higher concentration to a region of lower concentration is greater than vice versa as there are more particles in the region of high concentration and less in the region of lower concentration. This will continue until the concentrations are equal.
All living things are made up of cells. Each cell is surrounded by a cell membrane. This membrane is not a just a sheet with tiny holes in it. It is a complex assemblage of lipids and proteins, containing mechanisms that can facilitate the active transport of materials across the membrane, as required. Water molecules are a particularly small molecule, having a molecular weight of only 18, and the cell membrane is freely permeable to its passage either in or out of the cell. This is why all living cells are only capable of living in a fluid environment and multi-cellular living things existing outside this environment must have some form of effectively waterproof covering and a source of water to replace that which is inevitably lost by cells interacting with the external environment.
Cells exposed to pure water will take on water as the cytoplasm of cells is lower in water concentration due to the many solutes present. The increased volume of water in the cell causes expansion of the cell. This increased internal pressure of water is physically measureable as hydrostatic pressure. When the osmotic pressure of water moving into the cell is equal to the hydrostatic pressure inside the cell, then equilibrium is reached and no more water will be taken in overall. At the same time solutes would move out of the cell if the cell membrane allowed them to. However, cell membranes are not freely permeable to most of their solutes and will they will actively seek to retain those few that they are partially permeable to. Excess consumption of water simply leads to an increased volume of urine. The nephrons, the ‘filtering’ units of the kidneys, do not have to work as hard at active secretion of unwanted waste due to the dilute nature of the urine but active reabsorption of desired solutes would require more effort. The net effect of producing more dilute urine is probably zero up to a fair volume. It certainly puts much less strain on the kidneys than insufficient water or a significant excess intake of various solutes.
Now drinking something like sea water is dangerous. It results in dehydration for the above reasons. Severe dehydration results in electrolyte imbalance and the impaired functioning of cells, including the cell membrane – effectively a double whammy. Just as an important aside, if you were lost and getting dehydrating, even drinking your own urine, a rather weak salt solution, will likely do more harm than good. It is advised against by survival manuals, which is what annoyed the hell out me about the Bear Grylis episode (one of several) where he recommended it. Interestingly, they actually had a disclaimer accompany that episode.
I hope that sets the record straight for you Baker. It is worth doing a little reading on, but try not to get bogged down in the complexities and variations of cell homeostasis.
