Which cellular structure is responsible for protein synthesis?

Ribosomes are the cellular structures fundamentally responsible for protein synthesis. They serve as the site where amino acids are assembled into proteins based on the instructions carried by messenger RNA (mRNA). Ribosomes can either be free-floating in the cytoplasm or attached to the endoplasmic reticulum, allowing them to synthesize proteins for various functions within the cell, including structural roles and enzymatic activities.

Additionally, while the nucleus houses the DNA and is pivotal in the transcription process (creating mRNA from DNA), it does not directly participate in protein synthesis. Mitochondria primarily function in energy production and metabolism, and though the endoplasmic reticulum assists in the synthesis and processing of proteins (especially the rough endoplasmic reticulum), it is the ribosomes that carry out the actual translation process of assembling amino acids into proteins.