The ER, Golgi apparatus, endosomes, and lysosomes work in tandem to modify, sort, and package proteins and lipids. An integrated membrane trafficking network facilitates the back and forth shuttling of molecules within different organelles in the same cell or across the cell membrane.
The transport of soluble and membrane proteins is mediated by transport vesicles that collect cargo from one cellular compartment and deliver it to another by fusing with the target organelle membrane. The Rab family of proteins acts as molecular markers on the target organelle and guides the vesicle. Once the cognate vesicle has arrived at the destination, SNAREs mediate the fusion of the two membranes to deliver the cargo. The membrane-bound vesicle protects the cargo from external changes in the cytosol during transit.
If the vesicle fuses with the plasma membrane, the contents are delivered out of the cell in a process called exocytosis. Conversely, the cell uptakes molecules from the extracellular space in endocytosis. Depending on the nature and type of cargo, such as invading pathogen, soluble molecules, or receptor-specific molecules, the endocytic process could be phagocytosis, pinocytosis, and receptor-mediated endocytosis, respectively.
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