Proteins are synthesized on __ which give the rough endoplasmic reticulum its rough appearance. a. mitochondria b. nucleolus c. ribosomes d. microtubules
The correct answer and explanation is:
Correct Answer: c. ribosomes
Explanation (300 words)
Proteins are synthesized on ribosomes, which are small, dense structures composed of ribosomal RNA (rRNA) and proteins. Ribosomes can be found floating freely in the cytoplasm or attached to the surface of the rough endoplasmic reticulum (RER). The attachment of these ribosomes is what gives the RER its “rough” or studded appearance under a microscope, distinguishing it from the smooth endoplasmic reticulum (SER), which lacks ribosomes.
The rough endoplasmic reticulum is a key organelle in the process of protein synthesis and processing. Proteins made on ribosomes attached to the RER are typically those destined for secretion from the cell, incorporation into the cell membrane, or use in lysosomes. As these proteins are synthesized, they are threaded into the lumen (interior space) of the RER, where they may undergo folding and modification, such as the addition of sugar chains (glycosylation).
Ribosomes themselves are not membrane-bound organelles. They are found in both prokaryotic and eukaryotic cells, though their size and composition differ slightly. In eukaryotic cells, the larger 80S ribosomes are found, while prokaryotic cells have smaller 70S ribosomes.
The other choices in the question are not involved directly in protein synthesis on the RER:
- a. Mitochondria are responsible for energy (ATP) production, not protein synthesis for export.
- b. Nucleolus is the site where ribosomal RNA is synthesized and ribosome subunits are assembled, but it does not synthesize proteins.
- d. Microtubules are components of the cytoskeleton, involved in maintaining cell shape and facilitating intracellular transport, but not protein synthesis.
In summary, ribosomes are the key players in protein synthesis and their presence on the RER gives it its rough appearance.