Phagocytosis is the process by which living cells called phagocytes envelop or engulf other cells or particles, whereas pinocytosis is a process in which liquid droplets are ingested by living cells. Both are forms of endocytosis, which is the process in which cells surround external substances and take them within the cell.
Phagocytosis can be performed by certain single-celled organisms like the amoeba or by one of the human body's cells, such as a white blood cell. An organism like the amoeba uses phagocytosis as a form of feeding. In contrast, the white blood cells use phagocytosis as a defense mechanism against bacteria, viruses, dust particles and other foreign bodies. During the process of phagocytosis, the particle and the cell, or phagocyte, must first adhere to each other. This is accomplished in various ways, depending on the chemical makeup of the particle's surface. Once they have adhered to each other, the phagocyte surrounds the particle and ingests it. The speed of this process is dependent on the size of the particle.
Pinocytosis involves the same general process as phagocytosis. The main difference is that a droplet of liquid instead of a foreign particle is engulfed by the cell. Pinocytosis can be used to transport a droplet of liquid through a cell without disruption of the cell's cytoplasm.