Where are ribosomes located in eukaryotic cells?
A. On the rough endoplasmic reticulum only
B. On the smooth endoplasmic reticulum and in the cytoplasm
C. On the rough endoplasmic reticulum and in the cytoplasm
D. In the cytoplasm only
The correct answer and explanation is:
The correct answer is:
C. On the rough endoplasmic reticulum and in the cytoplasm
Explanation:
Ribosomes are the cellular structures responsible for synthesizing proteins by translating messenger RNA (mRNA) into polypeptide chains. In eukaryotic cells, ribosomes are found in two primary locations:
- On the rough endoplasmic reticulum (RER):
The RER appears “rough” under a microscope because its surface is studded with ribosomes. These ribosomes synthesize proteins destined for secretion outside the cell, incorporation into the cell membrane, or use within lysosomes. The ribosomes on the RER produce proteins that enter the lumen of the RER, where they are folded and modified before being transported to their next destination, such as the Golgi apparatus. - In the cytoplasm (free ribosomes):
Ribosomes are also freely floating in the cytoplasm, unattached to any membrane. These free ribosomes typically synthesize proteins that function within the cytoplasm itself, such as enzymes involved in metabolism, or proteins destined for the nucleus, mitochondria, or other organelles.
Why not the other options?
- A. On the rough endoplasmic reticulum only:
This is incorrect because ribosomes also exist freely in the cytoplasm, not just attached to the RER. - B. On the smooth endoplasmic reticulum and in the cytoplasm:
The smooth ER lacks ribosomes entirely. It is involved in lipid synthesis, detoxification, and calcium storage, but it is not a site of protein synthesis. - D. In the cytoplasm only:
This ignores the ribosomes bound to the RER, which are essential for synthesizing membrane-bound and secretory proteins.
Summary
In summary, ribosomes in eukaryotic cells are found both attached to the rough endoplasmic reticulum and freely in the cytoplasm. This dual location enables the cell to produce a wide variety of proteins efficiently, tailored to their specific destinations and functions.