What process describes a vesicle fusing with the plasma membrane to release its contents into the extracellular fluid?

Exocytosis is the process where a vesicle, which is a small membrane-bound sac within the cell, fuses with the plasma membrane. This fusion allows the vesicle to release its contents into the extracellular fluid. This mechanism is essential for various cellular functions, including the secretion of hormones, neurotransmitters, and other important substances that the cell needs to transport outwards.

During exocytosis, the vesicle membrane combines with the plasma membrane, resulting in the opening of the vesicle to the outside of the cell. This release can be tightly regulated, ensuring that substances are delivered precisely and at the right time, which is critical for maintaining cellular communication and homeostasis.

The other processes mentioned do not involve the fusion of vesicles with the plasma membrane for the purpose of releasing contents. Endocytosis involves the uptake of materials into the cell, facilitated diffusion refers to the movement of molecules across membranes via transport proteins without using energy, and active transport requires energy to move substances against their concentration gradient.