Vesicles undergoing reactions changing their surface material are sensitive to pressure and surface tension differences accumulating from changes in surface to volume ratios. Ultimately, these can induce phase changes such as vesicle fission and so it is important to approach these effects in modelling a vesicular protocell life-cycle. If the vesicle is permeable to water, then water transport can offset these stresses. The permeability of vesicles depends on the choice of amphiphiles and conditions: natural phospholipids form membranes that are relatively impermeable to membranes under natural neutral pH conditions, and biological cells have special mechanisms to transport water. Fatty acids based vesicles (such as oleic acid) are much more permeable to water.
In order to be able to model a range of permeabilities independently of other amphiphile properties we introduce water transport as a reversible association of water molecules with amphiphiles with release of water from these complexes occuring to either side of he membrane. The rates of this equilibration determine the rate of water transport.
Without water transport, vesicle growth can result in a tubular shape instability.
The vesicles are prevented from retaining spherical symmetry, by a mismatch between their volume and membrane surface area. Such effects have been explored by many authors, with a variety of modelling approaches (e.g. Lipowsky, Swetina).
Being able to handle such effects within our efficient mprDPD framework allows us to address the vesicle life-cycle in its entirety (see later sections).