Which phrase best describes the structure and role of rough endoplasmic reticulum (rough ER) in a cell?

Introduction / Context:
The endoplasmic reticulum (ER) is a key organelle in eukaryotic cells that exists in two forms: rough ER and smooth ER. Understanding the structure and function of rough ER is important for topics such as protein synthesis, secretion, and membrane production. This question asks you to identify the description that best matches rough ER.

Given Data / Assumptions:

Rough ER is different from smooth ER in both structure and function.
Ribosomes are involved in protein synthesis.
The question is about rough ER, not other organelles or solutions.

Concept / Approach:
Rough ER is characterized by the presence of ribosomes attached to its cytosolic surface, giving it a "rough" appearance under the microscope. These ribosomes synthesize proteins that are destined for secretion, insertion into membranes, or use in lysosomes. The lumen of rough ER also participates in folding and modification of newly synthesized polypeptides. Therefore, the best description must mention ribosomes and protein synthesis.

Step-by-Step Solution:
Step 1: Recall that rough ER has ribosomes attached to its surface. Step 2: Remember that ribosomes on rough ER synthesize proteins that enter the ER lumen or membrane. Step 3: Compare each option and see which one mentions ribosomes and protein synthesis clearly. Step 4: Choose the option that states rough ER is studded with ribosomes and is involved in protein synthesis.

Verification / Alternative check:
As an additional check, think about how rough ER is often located near the nucleus and continuous with the nuclear envelope. This positioning allows mRNA to be quickly translated by ribosomes on the ER surface. Many secretory cells such as those in the pancreas have extensive rough ER, further confirming its role in protein production.

Why Other Options Are Wrong:
Contains fewer solutes than the cell: This phrase is vague and does not describe rough ER specifically; it could apply to many solutions.
Stored in the central vacuole: The vacuole is a separate organelle; rough ER is not stored inside it.
Protected by vesicles: Vesicles bud from the ER and Golgi to transport materials but do not simply "protect" rough ER in this way.

Common Pitfalls:
Students sometimes mix up rough ER with smooth ER, which lacks ribosomes and is involved in lipid synthesis and detoxification. Focusing on the presence or absence of ribosomes is a reliable way to distinguish these two types of ER.

Final Answer:
Rough ER is best described as being studded with ribosomes and involved in protein synthesis.