How do endocytosis and exocytosis function in the transport of substances across cell membranes?

Endocytosis and exocytosis transport substances across cell membranes by engulfing or releasing them through vesicles.

Endocytosis is the process by which cells take in substances from the external environment by engulfing them in a vesicle. There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis involves the engulfment of large particles, such as bacteria or dead cells. Pinocytosis involves the engulfment of fluid and small particles. Receptor-mediated endocytosis involves the binding of specific molecules to receptor proteins on the cell surface, which triggers the formation of a vesicle.

Exocytosis is the process by which cells release substances into the external environment by fusing a vesicle with the cell membrane. This process is used to secrete hormones, enzymes, and other molecules. Exocytosis is also used to replace or repair components of the cell membrane.

Both endocytosis and exocytosis are important for maintaining the composition of the cell membrane and regulating the internal environment of the cell. These processes are also involved in cell signalling, immune response, and the uptake of nutrients. Dysfunctions in endocytosis and exocytosis have been implicated in various diseases, such as cancer, neurodegenerative disorders, and lysosomal storage diseases.