| Comments |
(Gennis 1989 p.273 top paragraph:) "Speed is essential. An ion channel in its open configuration typically can allow the passage of 10^6-10^8 ions/sec across the bilayer. Such a large flux is the experimental criterion distinguishing channels from transporters (see Table 8.3). The large current passed by such ion channels means that opening a relatively small number of channels can cause a large, rapid change in the electrical properties of the membrane. It will be useful to consider some realistic numbers at this time and to see how to estimate the response time of the membrane." (Gennis 1989 p.275 top paragraph:) "Table 8.3 shows the turnover numbers of several ion channels and transporters. Note that the ion channels and pores are characterized by very high turnover values. In contrast, the lactose permease from E. coli has a maximal turnover of only about 30 sec^-1. If the ion channel in the previous example were to operate at this rate, it would require a 10 million-fold increase in the channel density to achieve the same specific membrane conductance. This would be physically impossible." More accurate ref needed |