A researcher made an interesting observation about a protein made by the rough endoplasmic reticulum and eventually found in a cell's plasma membrane. The protein in the plasma membrane was actually slightly different from the protein made in the ER. The protein was probably altered in the __________.

• A. smooth endoplasmic reticulum
• B. plasma membrane
• C. transport vesicles
• D. rough endoplasmic reticulum
• E. Golgi apparatus

Answer: (E)

Verified step by step guidance

1. Understand the role of the rough endoplasmic reticulum (ER): The rough ER is involved in the synthesis of proteins that are usually destined for the cell membrane, secretion, or lysosomes. Proteins synthesized here are often transported to the Golgi apparatus for further processing.
2. Recognize the function of the Golgi apparatus: The Golgi apparatus is responsible for modifying, sorting, and packaging proteins for secretion or delivery to other organelles. It is a key site for post-translational modifications, such as glycosylation, phosphorylation, and proteolytic processing.
3. Consider the journey of a protein: After synthesis in the rough ER, proteins are transported to the Golgi apparatus in vesicles. Here, they undergo modifications that can alter their structure and function.
4. Identify the site of alteration: Given that the protein in the plasma membrane is slightly different from the one made in the ER, it is likely that these modifications occurred in the Golgi apparatus, where proteins are processed and matured.
5. Conclude the reasoning: Since the Golgi apparatus is the primary site for protein modification after synthesis in the rough ER, it is the most probable location where the protein was altered before being incorporated into the plasma membrane.