Cells need to be able to transport very small molecules at quick rates. It is also often necessary to have consistent molecule concentration though out a cell. As a cell expands, volume becomes so great that passive means of transport such as osmosis become inadequate. Even active means, including ion "pumps" in the cell membrane, are not able to take molecules over long distances. Cells can overcome this problem, but only to a certain degree. A neuron, you may know, has a long extension known as an axon. …
