What is the definition of endocytosis?

Endocytosis is defined as the process by which a cell takes in materials that are too large to pass through the cell membrane directly. This mechanism allows cells to internalize a variety of substances, such as large molecules, particles, or even other cells, by engulfing them. The cell membrane invaginates, forming a pocket that pinches off to form a vesicle containing the ingested material. This is crucial for cellular functions, including nutrient uptake and receptor-mediated signaling.

In contrast, other processes mentioned in the incorrect options do not describe endocytosis. The expulsion of materials from the cell refers to exocytosis, which is a different process where vesicles fuse with the plasma membrane to release their contents. Cell division relates to mitosis or meiosis, where the cell divides, and cellular respiration describes the metabolic pathways that convert glucose and oxygen into energy, involving the production of ATP. These definitions illustrate the specific nature of endocytosis in contrast to the other cellular processes described in the question.