Transport of large molecules across the bilayer

The story is different for large solutes. Large molecules, particularly the hydrophilic ones, find it too diflBcult to pass through the lipid bilayer. To circumvent this difficulty, nature invokes a novel mechanism that keeps the solvation of these hydrophilic moieties nearly intact but transports them to the inside of the cell, within a sack formed by spontaneous fluctuation, to be metaphorical. In reality, the molecule to be transported is enclosed by a part of the membrane which then forms a vesicle that can move through a part of the bilayer and then opens up inside the cell. A reverse process is used to transport large molecules from the inside of the ceU to the exterior of the cell. 

The essence of such a transport is that the hydrophUic large molecules remain solvated in water. The surface energy to create a vesicle by fluctuation (which involves a local dent in the layer and pinching off from the rest) is not too high. Both are possible due to the abundance of water at the surface of the bilayer. The vesicle formation is almost like the formation of a reverse micelle and the delivery system is also quite simple. 

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