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Ingestion by the Cell—Endocytosis
If a cell is to live and grow and reproduce, it must obtain nutrients and other substances from the sur-rounding ﬂuids. Most substances pass through the cell membrane by diffusion and active transport.
Diffusion involves simple movement through the membrane caused by the random motion of the mol-ecules of the substance; substances move either through cell membrane pores or, in the case of lipid-soluble substances, through the lipid matrix of the membrane.
Active transport involves the actual carrying of a substance through the membrane by a physical pro-tein structure that penetrates all the way through the membrane. These active transport mechanisms are so important to cell function that they are presented in detail in Chapter 4
Very large particles enter the cell by a specialized function of the cell membrane called endocytosis. The principal forms of endocytosis are pinocytosis and phagocytosis. Pinocytosis means ingestion of minute particles that form vesicles of extracellular ﬂuid and particulate constituents inside the cell cytoplasm. Phagocytosis means ingestion of large particles, such as bacteria, whole cells, or portions of degenerating tissue.