Which process involves the formation of vesicles as the plasma membrane pinches inward?

The process involving the formation of vesicles as the plasma membrane pinches inward is known as endocytosis. In this process, the plasma membrane actively invaginates, creating a pocket that pinches off to form a vesicle that brings substances into the cell. This is crucial for the uptake of larger molecules or particles that cannot pass through the lipid bilayer directly.

Endocytosis plays a vital role in various cellular functions, including nutrient uptake, receptor-mediated signaling, and immune responses. It encompasses several mechanisms, such as phagocytosis and pinocytosis, where cells internalize different types of substances. The inward pinching of the plasma membrane that characterizes endocytosis allows for a controlled and selective uptake of materials from the extracellular environment.

The other processes mentioned, such as exocytosis, involve vesicles fusing with the plasma membrane to release content outside the cell, while diffusion refers to the passive movement of molecules down their concentration gradient without vesicle formation. Phagocytosis, although a type of endocytosis focusing specifically on the uptake of large particles or cells, is just one of the several forms under the broader category of endocytosis.