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direct them to their final destination. Most of these proteins are first modified by the addition of branched sugar groups to make "glycoproteins." Most proteins in the plasma membrane, for instance, are glycoproteins. The full range of functions of these sugar groups is unknown, but they may help the protein to fold correctly after synthesis, act in cell-cell recognition and adhesion, and promote appropriate interactions with other proteins.

Proteins are further modified and sorted in the Golgi apparatus, a set of flattened membrane disks that is continuous with the ER. Here proteins and lipids are packaged in vesicles that bud off and travel along the cytoskeleton to their final destination. Fusion of the vesicle membrane with the target membrane delivers the contents to the target organelle. Proteins and other materials that the cell exports travel to the plasma membrane via vesicles. Fusion of the vesicle with the plasma membrane delivers the contents to the exterior of the cell.

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