Final answer:
Proteins are first synthesized on ribosomes and then modified in the endoplasmic reticulum. The Golgi apparatus tags them for their destination, and vesicles are involved in their transport and secretion.
Step-by-step explanation:
After their synthesis, proteins are modified and sorted by the endoplasmic reticulum and the Golgi apparatus located in the cell's endomembrane system. An example of such a modification is the addition of phosphate groups or other small molecules. Proteins are synthesized on ribosomes, and then they enter the endoplasmic reticulum where they are modified.
These modified proteins are then tagged in the Golgi apparatus to ensure they reach their correct destinations. Vesicles then package and transport these proteins either to other parts of the cell or to the plasma membrane for secretion.