Question

Which one of the following structures is an organelle within an organelle?
A. Ribosome
B. Peroxisome
C. ER
D. Mesosome

Answer 1

The endoplasmic reticulum is a cell organelle that is present in eukaryotic cells. Depending on the presence of certain granular organelles on their membranes, they are categorised into rough ER and smooth ER. These small granular organelles are sites for protein synthesis.

Complete answer:
Ribosomes are organelles that are situated on the membranes of the endoplasmic reticulum, which itself is an organelle. So, ribosomes are organelles that are present within an organelle.
Ribosomes are sites for protein synthesis and are present in both eukaryotic and prokaryotic cells. In prokaryotic cells, they are found in the cytoplasm, and in eukaryotic cells, they are present on the membranes of the ER.
When ribosomes are present on the ER, it is referred to as Rough Endoplasmic Reticulum. When the ER is devoid of ribosomes, it is called Smooth Endoplasmic Reticulum.
Ribosomes are made up of proteins and nucleic acids, i.e. RNA. The proteins are synthesised by the process of translation.

The correct answer is option ( A ) that is Ribosome.

Additional information:
Let us discuss some of the other cell organelles:

Peroxisome:
Peroxisomes are small membrane-bound organelles that are involved in various metabolic reactions. They contain certain oxidative enzymes that facilitate the reactions. Some of the functions of peroxisomes include hydrogen peroxide metabolism, fatty acid oxidation, lipid biosynthesis, and many more.
Mesosome:
Mesosomes are bacterial organelles that are formed by an extension of the plasma membrane into the cell wall. They help in the synthesis of the cell wall, DNA replication, and distribution of secretions and respiration.

Note: Prokaryotes possess 70S ribosomes,
Whereas, 80S ribosomes are found in eukaryotes.
The total number of ribosomes in a cell is based on cellular activity.