Which sequence describes exocytosis?

• A. Vesicle containing substance travels to plasma membrane, membrane of vesicle fuses with plasma membrane, materials secreted
• B. Plasma membrane engulfs material to form vesicle
• C. Vesicle moves to nucleus to release contents
• D. Materials diffuse through pores

Answer: (A)

Exocytosis is the process by which secretory vesicles fuse with the plasma membrane to release contents outside the cell. The sequence goes: a vesicle containing the substance moves toward the plasma membrane, the vesicle membrane fuses with the plasma membrane, and a pore opens to release the contents into the extracellular space. This mechanism is how cells secrete neurotransmitters, hormones, and digestive enzymes. The other described options describe endocytosis (the cell membrane engulfs material to form a vesicle), an impossible vesicle path to the nucleus, or diffusion through pores, which doesn’t account for vesicle-mediated secretion.