In what type of cell structure are ribosomes found?

Ribosomes are primarily found on the rough endoplasmic reticulum (RER). The RER is characterized by its studded appearance due to the presence of ribosomes on its cytoplasmic surface, which are involved in synthesizing proteins. These ribosomes translate messenger RNA (mRNA) into polypeptide chains, which then enter the endoplasmic reticulum for folding and processing.

The smooth endoplasmic reticulum, in contrast, lacks ribosomes and is involved in lipid synthesis and detoxification processes. The nucleus is the control center of the cell, housing the cell's genetic material, but it does not contain ribosomes itself. The plasma membrane serves as the boundary of the cell but is not involved in ribosome function or synthesis. Therefore, identifying ribosomes within the context of the rough endoplasmic reticulum is crucial for understanding their role in protein synthesis within the cell.