Final answer:
The true statement about protein synthesis for the ER is that proteins are initially translated by cytosolic ribosomes and then targeted to the ER after a signal sequence emerges during translation. The SRP directs the ribosome to the ER where translation resumes and the protein enters the ER lumen for further modification and sorting.
Step-by-step explanation:
The correct statement regarding the synthesis of proteins destined for the endoplasmic reticulum (ER) is B) Proteins destined for the ER are translated by cytosolic ribosomes and are targeted to the ER when a signal sequence emerges during translation. This process involves the synthesis of proteins on cytosolic ribosomes; as soon as a signal sequence emerges from these ribosomes, a signal recognition particle (SRP) binds to it. The translation is momentarily halted and the SRP directs the ribosome to the ER membrane. Once the complex binds to a receptor on the ER, the SRP is removed, and translation resumes, with the growing polypeptide being threaded into the lumen of the ER.
These proteins include those that are secreted from the cell, inserted into the plasma membrane, or sent to other organelles. They are synthesized on bound ribosomes and generally do not touch the cytosol once they start being synthesized. Once in the ER lumen, these proteins may undergo modifications such as folding or addition of sugars, critical for their functionality and further transport to their cellular destinations such as the Golgi apparatus, from where they are sorted and dispatched.