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Every cell must eat, communicate with the world around it, and quickly respond to changes in its environment. To help accomplish these tasks, cells continually adjust the composition of their plasma membrane and internal compartments in rapid response to need. They use an elaborate internal membrane system to add and remove cell-surface proteins, such as receptors, ion channels, and transporters (Figure 13–1). Through the process of exocytosis, the secretory pathway delivers newly synthesized proteins, carbohydrates, and lipids either to the plasma membrane or the extracellular space. By the converse process of endocytosis, cells remove plasma membrane components and deliver them to internal compartments called endosomes, from where they can be recycled to the same or different regions of the plasma membrane or be delivered to lysosomes for degradation. Cells also use endocytosis to capture important nutrients, such as vitamins, cholesterol, and iron; these are taken up together with the macromolecules to which they bind and are then moved on to endosomes and lysosomes, from where they can be transported into the cytosol for use in various biosynthetic processes.