For fusion of a vesicle with its target membrane to occur, the membranes have to be brought to within 1.5 nm so that the two bilayers can join (Figure Q13–1). Assuming that the relevant portions of the two membranes at the fusion site are circular regions 1.5 nm in diameter, calculate the number of water molecules that would remain between the membranes. (Water is 55.5 M and the volume of a cylinder is πr²h.) Given that an average phospholipid occupies a membrane surface area of 0.2 nm², how many phospholipids would be present in each of the opposing monolayers at the fusion site? Are there sufficient water molecules to bind to the hydrophilic head groups of this number of phospholipids? (It is estimated that 10^–12 water molecules are normally associated with each phospholipid head group at the exposed surface of a membrane.)

Figure Q13-1

Transcribed Image Text:

vesicle target membrane ] 1.5 nm