Final answer:
Ribosomes are composed of rRNA and proteins, with rRNAs constituting about 60% of the ribosome by mass. Prokaryotic ribosomes are 70S, while eukaryotic cytoplasmic ribosomes are 80S and mitochondria and chloroplasts contain 70S ribosomes similar to prokaryotic ones.
Step-by-step explanation:
The question concerns the structure of ribosomes, which are essential cellular components for protein synthesis. A ribosome is made up of both rRNA and proteins, and functions to bind mRNA and tRNA to synthesize proteins. Ribosomes are composed of two subunits, and the proportions of their rRNA and protein content vary slightly, with rRNAs making up about 60% of the ribosome by mass. In prokaryotes such as E. coli, the ribosomes are 70S, consisting of a 50S large subunit and a 30S small subunit. In contrast, eukaryotic ribosomes are 80S, with a 60S large subunit and a 40S small subunit, and these ribosomes are found both in the cytoplasm and on the rough endoplasmic reticulum. Additionally, eukaryotic cells have 70S ribosomes within mitochondria and chloroplasts, resembling those of prokaryotes.