Vesicle surface/volume ratio

Upon increasing the amphiphile concentration an evolution toward more asymmetric shapes (rodlike or disklike) and decreasing surface/volume ratio is observed. Eventually cylindrical (capped) micelles, bilayers (extended open sheet with rounded edges), and closed vesicles are formed. 

The area of colloids, surfactants, and fluid interfaces is large in scope. It encompasses all fluid-fluid and fluid-solid systems in which interfacial properties play a dominant role in determining the behavior of the overall system. Such systems are often characterized by large surface-to-volume ratios (e.g., thin films, sols, and foams) and by the formation of macroscopic assembhes of molecules (e.g., colloids, micelles, vesicles, and Langmuir-Blodgett films). The peculiar properties of the interfaces in such media give rise to these otherwise unlikely (and often inherently unstable) structures. 

If you have followed closely what has been said before, you might now be wondering why dopamine should be too polar to cross the membranes that represent the blood brain barrier, but should readily cross those of the presynaptic vesicles. The answer to this apparent paradox (aside from the fact that the blood brain barrier actually consists of four membranes in series) lies in the vastly different surface-to-volume ratios of the two compartments. Think of a pinhole in a thimble vs. one in a swimming pool. 

Many of the cells listed in Table 7.1 and 7.2 are involved in active membrane flow and other mass-cooperative transport phenomena. Since cubic membranes offer a high surface to-volume ratio, they may also be actively involved in these processes, perhaps as membrane storage bodies, or as transport guides. It is of interest to note that aggregates of "s3maptic vesicles" often resemble cubic membranes (see Chapter 5 and [136]). This can be taken as an indication of a possible on-off mechanism of membrane continuity, which might accovmt for a regulative capacity of the release of transmitter substance. 

Vesicles can be classified according to their volume (V) to surface (S) ratio, namely their dimensionless reduced volume v ed = 6y/ V/S < 1. 

These considerations are important also in view of the processes of division and/or fusion of vesicles. In particular, when a vesicle divides up, and the total surface area remains constant, the total volume must decrease. This means that water must be eliminated in the process, so as to keep the volume to surface ratio constant. Conversely, when two vesicles fuse with each other, with a constant surface area (no fresh surfactant being added), the total volume must increase to keep the volume/surface constant and water must come in. This important, characteristic feature of vesicles is represented in Figure 9.27. 

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