What is endocytosis?

Endocytosis is defined as the process by which cells internalize substances from their external environment by engulfing them within vesicles. This mechanism allows cells to intake a variety of materials, including nutrients, signaling molecules, and even other cells, facilitating numerous cellular functions. During endocytosis, the cell membrane invaginates, or folds inward, to form a pocket around the desired substance. This pocket then pinches off to become a vesicle that transports the substance into the cell.

The other choices refer to different cellular processes. The first option describes exocytosis, which is the process of transporting substances out of the cell. The third option relates to protein synthesis, which involves the creation of proteins from amino acids based on genetic instructions, and is not related to the cell's mechanism of taking in materials. Lastly, passive transport involves the movement of molecules across cell membranes without energy input, typically down their concentration gradient, while endocytosis requires energy to facilitate the formation of vesicles. Thus, the accurate characterization of endocytosis focuses on its role in the movement of substances into the cell through vesicular transport.