Final answer:
A common pool of ribosomes is used to synthesize proteins for both cytosolic localization and those destined for the ER, with bound ribosomes attaching to the rough ER for protein synthesis and modification.
Step-by-step explanation:
The true statement among the options provided is: A common pool of ribosomes is used to synthesize both the proteins that stay in the cytosol and those that are destined for the ER.
The ribosomes that synthesize proteins destined for the endoplasmic reticulum (ER), cell membrane, or secretion are referred to as being bound ribosomes since they attach to the membrane of the rough endoplasmic reticulum (RER) during translation.
The RER has ribosomes on its surface, which gives it a rough appearance, and this is where proteins that are destined for export or cell membrane insertion are synthesized.
The synthesized proteins enter the lumen of the RER, where they can undergo modifications, such as folding and glycosylation.
On the other hand, the smooth endoplasmic reticulum (SER) lacks ribosomes on its surface and is involved in lipid synthesis.
It is not correct that all ribosomes start protein synthesis attached to the ER, nor that most proteins are synthesized inside the smooth ER. Indeed, ribosomes in the cytoplasm also synthesize proteins that remain within the cytosol.