Question

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

Answer 1

An organelle is a subcellular structure that, much like an organ in the body, has one or more unique tasks to perform in the cell. The nuclei, which store genetic information, are among the most essential cell organelles; mitochondria, which generate chemical energy; and ribosomes, which assemble proteins.

Complete answer:
Peroxisome- Peroxisome, the membrane-bound organelle found in the eukaryotic cell cytoplasm. Peroxisomes play a crucial role in particular biomolecules being oxidised. They also contribute to the biosynthesis of plasmalogens-known membrane lipids. Peroxisomes perform additional roles in plant cells, including carbon recycling from phosphoglycolate during photorespiration.
Endoplasmic reticulum- In biology, the Endoplasmic reticulum is a continuous membrane system that forms a series of flattened sacs within the eukaryotic cell cytoplasm and serves multiple functions , especially important in protein synthesis, folding, modification and transport. An endoplasmic reticulum comprises all eukaryotic cells. In animal cells, the ER normally makes up more than half of the cell's membrane content.
Mesosome- Mesosomes or chondrioids are folded invaginations of bacteria in the plasma membrane formed by the chemical fixation techniques used for electron microscopy sample preparation. It is a bacterial organelle that appears as an invagination of the plasma membrane and acts either in the replication of DNA and in the division of cells or in exoenzyme excretion.
Ribosome- The rough endoplasmic reticulum, which is an organelle, is bound to ribosomes in eukaryotic cells. It is also found in chloroplast organelles in plant cells and mitochondria. Within the organelle, ribosomes are thus called organelles. Ribosome, a particle that is present in all living cells in large numbers and acts as the site of synthesis of protein. Ribosomes exist in prokaryotic and eukaryotic cells as free particles and in eukaryotic cells as particles bound to the membranes of the endoplasmic reticulum.

Hence, the correct answer is (D).

Note:
In cells, ribosomes are surprisingly abundant. For example , a single actively replicating eukaryotic cell can include as many as 10 million ribosomes. Depending on the type of cell and on factors such as whether the cell is resting or replicating, the size of the ribosomes inside cells varies.