Final answer:
The true statement is that proteins destined for the ER are translated by cytosolic ribosomes which, upon recognizing a signal sequence, become bound to the ER where they resume translation and insert the growing protein into the ER lumen.
Step-by-step explanation:
The statement that is TRUE is: b) proteins destined for the ER are translated by cytosolic ribosomes. This process begins when ribosomes in the cytosol initiate the translation of mRNA into a polypeptide chain. When a protein is destined for the endoplasmic reticulum (ER), the translation process includes a signal sequence. As these amino acids emerge, they are recognized by a signal recognition particle (SRP), which pauses translation and directs the ribosome to the ER. Once the ribosome is bound to the ER, the SRP is released, and translation continues with the growing polypeptide chain being inserted into the ER lumen for further processing.
These ribosomes, while initially free in the cytosol, become bound ribosomes once they are associated with the ER. They will synthesize proteins that are to be incorporated into the cell membrane, secreted from the cell, or deployed to various organelles within the cell. This is why the ribosomes on the rough ER are responsible for the production of these types of proteins. Any polypeptide chains that are made on the rough ER transport their growing peptide chains into the ER lumen directly during the translation process.