Definition of exocytosis in cell transport: Which statement correctly describes exocytosis of proteins?

Introduction / Context:
Cells move large cargo by vesicular transport. Exocytosis is the outward, secretory pathway delivering proteins, lipids, and signaling molecules to the plasma membrane or extracellular milieu.

Given Data / Assumptions:

• We consider secreted proteins, neurotransmitters, and membrane proteins being delivered to the surface.
• Question asks for the correct definition.

Concept / Approach:
In exocytosis, secretory vesicles derived from the Golgi apparatus fuse with the plasma membrane. The vesicle lumen becomes continuous with the extracellular space, releasing soluble cargo and inserting membrane proteins/lipids into the plasma membrane.

Step-by-Step Solution:
Differentiate exocytosis (outward) from endocytosis (inward). Select the option explicitly stating movement out to extracellular space. Eliminate options describing uptake or internal spaces. Confirm terminology matches standard cell biology usage.

Verification / Alternative check:
SNARE-mediated membrane fusion and calcium-triggered exocytosis (e.g., synaptic vesicles) exemplify this process.

Why Other Options Are Wrong:
(b, d) misuse of terms; (c) describes endocytosis; (e) is unrelated to the general definition.

Common Pitfalls:
Confusing directionality; forgetting that vesicle lumen becomes extracellular after fusion.

Final Answer:
Secretion of proteins out of the cell across the plasma membrane into the extracellular space.