Which organelle contains most of the cell's ribosomes and synthesizes proteins?

• A. Rough Endoplasmic Reticulum
• B. Smooth Endoplasmic Reticulum
• C. Nucleus
• D. Mitochondria

Answer: (A)

Ribosomes are the machines that build proteins by translating mRNA. In most eukaryotic cells, most ribosomes are attached to the rough endoplasmic reticulum, which gives it its characteristic "rough" appearance. The rough ER is the main site where proteins destined for secretion, insertion into membranes, or delivery to lysosomes are synthesized and processed as they are threaded into the ER lumen or embedded in its membrane. The smooth ER lacks ribosomes, so it doesn’t contribute to ribosome abundance and handles other tasks like lipid synthesis and detoxification. The nucleus contains the DNA and the nucleolus where ribosomal subunits begin to form, but the actual ribosomes performing translation are in the cytosol or on the rough ER, not housed primarily in the nucleus. Mitochondria have their own smaller set of ribosomes for a limited set of proteins, but they don’t contain the majority of the cell’s ribosomes. So, the organelle that contains most of the cell’s ribosomes and conducts the bulk of protein synthesis is the Rough Endoplasmic Reticulum.