Similar to bacteria, eukaryotic cells move chemicals into and out of themselves using antiporters, symporters, and ABC transport systems, but they also employ a process called endocytosis in which areas of the cytoplasmic membrane invaginate and pinch off to form membrane-enclosed vesicles called endosomes (Fig. 1). Endocytosis pathways are subdi-vided into four categories: clathrin-mediated endocytosis, caveolae, macropinocytosis, and phagocytosis.

In clathrin-mediated endocytosis, after the binding of a ligand to a plasma membrane receptor, a signal is sent that causes a cytoplasmic protein called clathrin to coat the mem-brane, followed by formation of a membrane invagination. The invagination pinches off to form a clathrin-coated vesicle in the cytoplasm. Once internalized, the clathrin-coated vesi-cle uncoats (a prerequisite for the vesicle to fuse with other membranes), and individual vesicles fuse to form the early endosome.

Caveolae are small pits in the cell membrane formed by lipid rafts (areas of the membrane rich in cholesterol) that contain the cholesterol-binding protein caveolin. Caveolar endocytosis occurs through flask-shaped invaginations at the plasma membrane and is important in the cellular uptake and intracellular delivery of some bacteria, bacterial toxins (for example, cholera toxin), viruses (for example, polyomavirus), and circulating proteins.

Macropinocytosis is a form of endocytosis that pinches off packets of medium to spawn larger vacuoles about 0.5–5 μm in diameter (also called endosomes). Eukaryotic microbes such as amebas and human white blood cells called neu-trophils and macrophages engulf prey bacteria by a form of endocytosis called phagocytosis, producing even larger vacu-oles (phagosomes). Following their formation, endosomes and phagosomes fuse with another vacuole, called the lyso-some, to produce phagolysosomes (or late-stage endocytic vacuoles). Lysosomes contain various digestive enzymes that will degrade phagolysosome contents to a form and size that can be transported across the vacuolar membrane and into the cytoplasm.

Endocytosis regulates many eukaryotic cell processes, including nutrient uptake, cell adhesion and migration, recep-tor signaling, pathogen entry, antigen presentation, and drug delivery.

Exocytosis is essentially a reversal of the process. Proteins or other compounds are synthesized by the cell and placed into a secretory vesicle that blebs from the Golgi complex. The secretory vesicle with its cargo fuses with the cell membrane and disgorges its contents in a process called exocytosis. Exo-cytosis is used, for example, to release neurotransmitters like acetylcholine and GABA (gamma-aminobutyric acid) from neurons, and digestive enzymes into the stomach or intestine.

FIGURE 1 ■ Endocytosis and exocytosis.

Endocytosis Exocytosis

SecretoryvesicleVesicle

(endosome)

Cellmembrane

eTOPIC 4.2�Eukaryotes Transport Nutrients by EndocytosiseTOPIC 4.2�Eukaryotes Transport Nutrients by Endocytosis

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