Proteins must enter the endoplasmic reticulum to be

Proteins are fed into the ER during translation if they have an amino sequence called a signal peptide. In general, proteins bound for organelles in the endomembrane system (such as the ER, Golgi apparatus, and lysosome) or for the exterior of the cell must enter the ER at this stage.

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a scenario where a cell may be needed to perform a form of endocytosis is when transporting large molecules. a scenario where a cell may be needed to perform a form of exocytosis is when releasing the large molecule from the cell.