Endocytosis is best described as which of the following?

Endocytosis is described as a membrane process where a cell engulfs material. This biological process involves the invagination of the cell membrane to form a pocket that encloses extracellular substances. Once the pocket is formed, it deepens and pinches off, bringing the enclosed materials into the cell in a new vesicle. Endocytosis is critical for various cellular functions, including nutrient uptake, immune response, and the regulation of surface receptors.

The other options, while related to different aspects of cell biology and pharmacology, do not accurately define endocytosis. Passive diffusion involves substances moving across the membrane without energy or cellular activity, and thus does not involve any engulfing process. The method of transporting substances with energy relates to active transport, which is distinct from endocytosis, although both processes require energy. Biochemical reactions in drug metabolism refer specifically to the chemical alterations drugs undergo in the body, which is unconnected to the specific process of endocytosis.