Advanced Placement (AP) Biology Practice Exam

Which structure is primarily responsible for the synthesis of proteins?

1. Ribosome

2. Lysosome

3. Golgi apparatus

4. Mitochondria

The correct answer is: Ribosome

The ribosome is the key structure responsible for protein synthesis in cells. It functions as the site where messenger RNA (mRNA) is translated into polypeptide chains, ultimately forming proteins. Ribosomes can be found freely floating in the cytoplasm or attached to the endoplasmic reticulum, and they play a crucial role in assembling amino acids in the sequence specified by the mRNA transcript. During the process of translation, ribosomes read the sequence of the mRNA and, with the help of transfer RNA (tRNA) molecules, incorporate the appropriate amino acids to build a protein. This process is essential for nearly every cellular function, as proteins serve as enzymes, structural components, signaling molecules, and much more. Other cellular structures, while vital for overall cellular health and function, do not directly synthesize proteins. The lysosome is primarily involved in digestion and waste processing, the Golgi apparatus modifies, sorts, and packages proteins synthesized in the rough endoplasmic reticulum for transport or secretion, and mitochondria are primarily responsible for energy production through ATP synthesis.