When the fusion pore is fully formed, lipids surrounding the water channel orient themselves radially with their tails towards the centre of the traversing channel (see red lipids in molecular dynamics snapshots in figure ). Therefore, the amount of C4 tail beads in transient contact with water molecules (either from intra or extra-organelle spaces) is increased due to the natural toroidal geometry of the fusion pore (although other geometries have been observed ). When the pore collapses and the bilayers recovery to their planar and parallel configurations, lipid tails return inside the bilayer core and lipids expose their phosphate groups toward the water interfaces. During this complex lipid reorganization the amount of C4:water (C4:PW) contacts are a measure of how many water molecules are trapped, on average, inside the bilayers.