Cartilage and the two meanings of the word "electro-osmosis" Occasionally, certain phenomena are reasonably well understood by some experts (like me) (thus not being "unsolved" in the usual sense), but most scientists & MDs still don't get the point. The swelling pressure that gives cartilage much of its strength is produced by a form of osmotic pressure that depends on electrical attraction between sugars and ions, instead of a semi-permeable membrane. Long chains of sugars, some of which have sulfate groups covalently bound to them, are covalently connected to fibrous proteins, that are in turn connected to type II collagen fibers. Water diffuses in among these proteins and chains of sulfonated sugars, and the sulfate groups become ionized (with a net negative charge) but are prevented from diffusing freely by their covalent connections to the sugars, which in turn are connected to proteins. The negative charge of these sulfates is balanced by positive ions (cations), including hydrogen ions (please don't call them "protons"!), sodium ions, calcium ions, potassium ions etc. Regardless of which cations are dissolved in the water, their total concentration is considerably higher inside a piece of cartilage, as compared with adjacent body fluids. Ordinary osmotic pressure is caused by random diffusion of water through semi-permeable membranes, when the concentrations of dissolved sugars or salts (or it could be any soluble molecules) is higher on one side of the semi-permeable membrane as compared with the other side. The pressure produced is proportional to the excess number of dissolved molecules on one side of the semi-permeable membrane as compared with the other, and obeys what amounts to the ideal gas law. In other words, a concentration difference of one mole per liter will produce an osmotic pressure of 22.4 atmospheres, which is a LOT! Those interested in SCUBA diving will recall that every 33 feet increase in depth of water exerts one more atmosphere of pressure, so 22 atmospheres of pressure is reached at a depth of over 700 feet. (which is a LOT!) Incidentally, osmosis can occur with solvents other than water, and the osmotic pressure is "a colligative property", like melting point depression, depending on numbers of dissolved molecules, not forces between the molecules, or any other properties. It's an effect of entropy, and proportional to Kelvin temperature. This is not a physics course, however, so don't let these details bother you. Just notice the key ideas. Imagine that somebody had a magic wand with magical properties equivalent to a semi-permeable membrane, by which I mean it could prevent dissolved molecules from diffusing past some border. Then an osmotic force would be exerted on the magic wand, in the direction toward the higher concentration of ions or other dissolved molecules. You may wince at calling ions "molecules"? Inside every piece of cartilage (unless it gets dried out), the effect of a magic wand is produced by electrical attraction forces between positively-charged cations and negatively charged sulfates, attached to those sugars, etc. So an osmotic pressure is produced without a semi-permeable membrane. This is one of the two meanings of the word electro-osmosis. The more sulfates, the more pressure; so pressure often varies continuously between one part of a cartilage and another. Cartilage can grow by expansion, in contrast to bone which can only get bigger by more bone being deposited on its surfaces. That is why all the bones in our body, except the flat ones in the roof of the skull, start out as cartilages. The cartilage is then replaced by bone, little by little, in consistent geometric patterns. Please note that cartilage doesn't change into bone; it gets digested and replaced. Many animals deposit calcium phosphate in the matrix of cartilage, without replacing it. Calcified cartilage is not bone. I wish I knew the mechanism by which cells deposit calcium salts in cartilage; probably it's the same mechanism as cells use for making bone.. If a piece of glass is immersed in water, and an electric voltage gradient is created (such as one uses for electrophoresis). If you look at this piece of glass through a microscope, & there are particles of anything suspended in the water, then you will see a slow but directional flow of water parallel to its surfaces, toward the negative electrode. What are being pulled on by the voltage gradient is not the water molecules, but cations dissolved in the water. Every kind of glass dissociates cations, that may be sodium ions, or potassium ions, or others. While glass is immersed in water, this dissociation creates a small negative charge on the glass itself, and a slight positive charge in the water located within a couple microns distance of the glass. Plastics or other materials that dissociate negative ions will create a thin zone of excess anions next to them, when immersed in water. In either case, if you create a voltage gradient in the water next to such a surface, the voltage will pull on the excess ions, and cause a thin layer of water flow. This electrical pulling of thin layers of water, by excess ions dissolved in the water, is the other meaning of the word electro-osmosis. It's the only meaning of the word that you will find in Wikipedia encyclopedia.