Cells use energy-requiring bulk transport mechanisms to transfer large particles or large numbers of small particles into or out of the cell. The cells envelop the particles in spherical membranes called vesicles or vacuoles. Vesicles that transport material into the cell are built from the cell membrane. These vesicles encapsulate external molecules and transport them into the cell in a process called endocytosis.

Pinocytosis ("cellular drinking") is one of three main types of endocytosis. In pinocytosis, the cell repeatedly takes in fluid from the surrounding environment using tiny vesicles. Pinocytosis occurs in many cell types. In the small intestine, bristle-like protrusions called microvilli use pinocytosis to absorb nutrients from food. Egg cells use pinocytosis to obtain nutrients before fertilization.

In pinocytosis and other forms of endocytosis, vesicles form when sections of the cell membrane sink inward, creating tear-shaped pockets that surround the material being taken into the cell. In pinocytosis, the imported material consists of fluid and other molecules. As the membrane reconnects, the vesicles pinch off, separating from the membrane. In the process, the vesicles enter the cell, taking the enclosed substances with them.

Specific characteristics distinguish pinocytosis from the other forms of endocytosis—phagocytosis and receptor-mediated endocytosis. While phagocytosis ("cell-eating") takes in large particles, pinocytosis transports fluid and smaller particles. The vesicles produced during pinocytosis are substantially smaller compared to those in phagocytosis. Additionally, unlike receptor-mediated endocytosis, pinocytosis is non-selective, although specific molecules induce it. In other words, pinocytosis can be thought of as an equal-opportunity importer that takes in both particles and any surrounding extracellular fluid.