Final answer:
Proteins synthesized on ribosomes associated with the rough endoplasmic reticulum (RER) enter this organelle during their synthesis, where they can be further modified and prepared for their specific roles within the cell.
Step-by-step explanation:
Proteins enter which organelle as they are being synthesized? Proteins synthesized on the ribosomes attached to the rough endoplasmic reticulum (RER) can either enter its interior lumen or become part of the RER membrane. This is a fundamental step in protein synthesis and modification, which are crucial processes in cellular function. Once inside the RER, the protein contents may bud into transport vesicles that eventually fuse with Golgi vesicles. Here, proteins can be further modified and packaged, ready for their specific roles inside or outside the cell, including being secreted by exocytosis, sent to lysosomes, or integrated into the plasma membrane.