Final answer:
When a vesicle fuses with the plasma membrane, a phospholipid in the outer layer of the vesicle bilayer ends up on the extracellular face of the plasma membrane, maintaining the bilayer structure and cell integrity.
Step-by-step explanation:
If a phospholipid is located in the outer layer of the bilayer in a vesicle, upon fusion with the plasma membrane, it will end up on the extracellular face of the bilayer. Vesicles typically fuse with the plasma membrane to deliver their content outside the cell, or to incorporate their own membrane components into the cell membrane. Since the phospholipid molecules are arranged with their hydrophilic heads facing the aqueous environments, the head of the phospholipid from the vesicle will face the extracellular fluid, incorporating this way. This orientation maintains the cellular structure and function, ensuring that the hydrophobic tails are kept away from the aqueous environment, which is critical for the integrity and fluidity of the membrane.