What does exocytosis refer to?

Exocytosis refers specifically to the process by which a cell expels or discharges substances. This occurs when vesicles containing molecules fuse with the plasma membrane, releasing their contents outside the cell. This mechanism is essential for a variety of cellular functions, including the secretion of hormones, neurotransmitters, and digestive enzymes.

In contrast, the other options describe different processes: the uptake of substances into a cell is called endocytosis; movement of substances across a cell membrane can occur via various mechanisms such as diffusion or active transport; while absorption of nutrients typically pertains to the uptake of substances necessary for cellular function but doesn't specifically denote the process of exocytosis. Thus, the definition corresponding to exocytosis aligns with the discharge of substances from a cell.