JLAB Biology SOL Complete Practice Test 2025

The ribosome is primarily responsible for protein synthesis within the cell. It serves as the site where messenger RNA (mRNA) is translated into polypeptide chains, which then fold into functional proteins. Ribosomes can be found freely floating in the cytoplasm or attached to the endoplasmic reticulum, forming rough ER. They consist of ribosomal RNA (rRNA) and proteins, working together to link amino acids in the sequence dictated by the mRNA, ultimately leading to the formation of proteins that perform various essential functions within the organism.

In contrast, while the nucleus plays a critical role in storing genetic information and is involved in the transcription of DNA to mRNA, it does not directly synthesize proteins. The cell membrane acts as a barrier that regulates the entry and exit of substances but is unrelated to protein synthesis. The cytoplasm does contain ribosomes and is the medium in which protein synthesis occurs, but it is not the specific site responsible for synthesizing proteins themselves. Thus, the immediate and decisive role in protein synthesis is carried out by the ribosome.