Final answer:
Secretory pathway proteins have targeting signals, sequences of amino acids, that direct them to specific organelles. These signals are often cleaved off upon reaching the destination. Proteins are packaged into vesicles to either fuse with organelles or to be secreted outside the cell.
Step-by-step explanation:
Many secretory pathway proteins contain additional targeting signals to ensure they are directed to the correct organelles after synthesis. These targeting signals are essentially sequences of amino acids that act as 'train tickets,' guiding the proteins to their designated locations within the cell. For instance, specific signal sequences at the amino or carboxyl end of a protein can direct the protein to organelles such as mitochondria or chloroplasts in plant cells. Once a protein reaches its destination, these signal sequences are typically cleaved off.
Proteins that are destined for secretion or inclusion in cell membranes go through a series of modifications in the endomembrane system. After folding and modification, including glycosylation in the rough endoplasmic reticulum (RER), the proteins are then packaged into secretory vesicles. These vesicles either fuse with other organelles for internal use, such as lysosomes containing hydrolytic enzymes, or with the plasma membrane to release their contents outside the cell by exocytosis.