Golgi apparatus membrane turnover

The control could be established if at any one time there was at the membrane more vesicles than those that were fusing. However in order to sustain the increase in secretion it would be necessary to transmit the signal back to the synthetic system and packaging process at the Golgi apparatus so that a new steady state system was obtained. This would produce the requisite number of vesicles containing the polysaccharides necessary to maintain the new rate of fusion and secretion. The turnover of the Golgi apparatus can be very fast in plant cells and times of 5-40 minutes have been calculated (36). In vitro experiments which involved isolation of membrane fractions from maize-root cells also showed that Ca was necessary for membrane fusion (37). Analysis of the membranes indicated that the Ca dependence involved membrane proteins and one of these was a Ca and Mg -dependent ATPase (38). 

There is no specific mechanism incorporated into the model to determine the size of cellulose chains or the means of their termination and so to predict the inbuilt degree of polydispersity. Again, such addenda are not difficult to contrive. It could well be, for example, that the ratio between the rate of membrane flow from the Golgi apparatus to the vicinity of the plasmalemma and the turnover number of the GDPGlc-cellulose glucosyltransferase determines chain size and that there is no specific code for terminating the chains. 

---