Consider a protein that is made in the rough endoplasmic reticulum. You observe that when the synthesis of the protein is completed, the protein is located in the ER membrane. Where else in the cell might this protein be found?

• A. In the aqueous interior of a lysosome, functioning as a digestive enzyme
• B. In the cytoplasm, functioning as an enzyme in carbohydrate synthesis
• C. In the internal space of the Golgi apparatus, being modified before the protein is excreted
• D. In a mitochondrion, functioning in ATP synthesis
• E. Embedded in the plasma membrane, functioning in the transport of molecules into the cell

Answer: (E)

Verified step by step guidance

1. Understand the initial location of the protein: The protein is synthesized in the rough endoplasmic reticulum (ER) and is located in the ER membrane. This suggests that the protein is likely to be a membrane protein.
2. Consider the pathway of membrane proteins: Proteins synthesized in the rough ER can be transported to the Golgi apparatus for further modification. From the Golgi, they can be directed to various destinations, including the plasma membrane.
3. Evaluate the possible destinations: Membrane proteins can be embedded in the plasma membrane, where they often function in transport, signaling, or cell recognition.
4. Eliminate unlikely options: The protein is unlikely to be found in the aqueous interior of a lysosome or in the cytoplasm, as these locations are not typical for membrane proteins. Similarly, it is unlikely to be in a mitochondrion, as proteins for mitochondria are usually synthesized in the cytoplasm and imported into the mitochondria.
5. Conclude the most likely location: Given the protein's initial location in the ER membrane and the typical pathway for membrane proteins, it is most likely to be embedded in the plasma membrane, functioning in the transport of molecules into the cell.