Which process is phagocytosis a form of?

Phagocytosis is a specific type of endocytosis, which is the process by which cells internalize substances from their external environment. During phagocytosis, a cell engulfs large particles, such as pathogens or dead cells, by extending its plasma membrane around them. This membrane extension forms a phagosome, which later fuses with lysosomes to digest the engulfed material.

Endocytosis encompasses various mechanisms of cellular uptake, including phagocytosis (the uptake of large particles), pinocytosis (the uptake of liquids and small solutes), and receptor-mediated endocytosis (which involves specific recognition of molecules). Since phagocytosis falls under the broader category of endocytosis, it is accurately categorized as such.

The other processes listed—exocytosis, passive transport, and facilitated diffusion—do not involve the internalization of substances by engulfing them. Exocytosis is the process of transporting materials out of a cell, while passive transport and facilitated diffusion describe the movement of substances across a cell membrane without the need for energy, primarily in the direction of the concentration gradient. Hence, endocytosis is the correct classification for phagocytosis.