A protein that ultimately functions in the plasma membrane of a cell is most likely to have been synthesized __________.

• A. in the ribosomes of the mitochondria
• B. on free cytoplasmic ribosomes
• C. in the smooth endoplasmic reticulum
• D. in the plasma membrane
• E. in the rough endoplasmic reticulum

Answer: (E)

Verified step by step guidance

1. Understand the role of the rough endoplasmic reticulum (RER) in protein synthesis. The RER is studded with ribosomes, which are the sites of protein synthesis, particularly for proteins that are to be secreted, incorporated into the plasma membrane, or sent to an organelle.
2. Recognize that proteins destined for the plasma membrane are typically synthesized in the RER. This is because the RER is involved in the initial stages of protein processing and modification, which are crucial for membrane proteins.
3. Consider the pathway of protein synthesis and transport. Proteins synthesized in the RER are often transported to the Golgi apparatus for further modification and sorting before being sent to their final destination, such as the plasma membrane.
4. Differentiate between the roles of free ribosomes and ribosomes on the RER. Free ribosomes in the cytoplasm generally synthesize proteins that function within the cytosol, whereas ribosomes on the RER synthesize proteins for membranes or export.
5. Conclude that the synthesis of a protein that functions in the plasma membrane is most likely initiated in the rough endoplasmic reticulum, as this organelle is specialized for producing membrane-bound and secretory proteins.