Function of the rough endoplasmic reticulum

Steps:

• Identify membrane X as rough ER due to attached ribosomes visible in electron micrographs.
• Recognize ribosomes on the membrane perform protein synthesis.
• Note the ER's tubular network facilitates protein folding and transport in vesicles.
• Confirm combined roles match option B.

Why B is correct:

• Rough ER definition: ribosomes synthesize proteins, and the membrane transports them to Golgi for secretion or use.

Why the others are wrong:

• A: Lipid synthesis occurs on smooth ER, lacking ribosomes.
• C: Rough ER also transports proteins, not synthesis only.
• D: Ribosomes enable synthesis, so transport is not the sole function.

Final answer: B