Ribosomes are molecular machines composed of ribosomal RNA and proteins that translate messenger RNA (mRNA) into protein sequences. They consist of a large and small subunit, both built in the nucleolus and assembled in the cytoplasm. Free ribosomes synthesize proteins destined for the cytoplasm; membrane-bound ribosomes (on the rough ER) synthesize proteins for secretion, the membrane, or organelles. The ribosome reads the mRNA codon by codon, catalyzing peptide bond formation with each incoming aminoacyl-tRNA.
Follow a single polypeptide from gene to functional protein: transcription in nucleus → mRNA export → ribosome assembly → elongation → release. Trace where free vs. bound ribosomes send their products.
Every protein in a cell — enzyme, structural fiber, signaling molecule — is made by a ribosome. You have already learned that organelles divide the cell into functional compartments. Ribosomes are the manufacturing plants within those compartments, and understanding their structure and placement tells you a great deal about where proteins end up and what they do.
A ribosome consists of two subunits, each built from ribosomal RNA (rRNA) and proteins. The two subunits are assembled separately in the nucleolus (a region inside the nucleus), exported through nuclear pores, and only come together on an mRNA strand when translation begins. The ribosome's central job is to read the mRNA sequence, three nucleotides (one codon) at a time, and catalyze the formation of a peptide bond between successive amino acids brought by transfer RNAs (tRNAs). It is essentially a moving factory: it advances along the mRNA, extends the polypeptide chain, and releases it when it reaches a stop codon.
The distinction between *free* and *membrane-bound* ribosomes is not a structural difference — the ribosomes themselves are identical — it is a locational difference that determines protein destination. Free ribosomes float in the cytoplasm and produce proteins that will stay in the cytoplasm, enter the nucleus, or go to mitochondria and chloroplasts. Membrane-bound ribosomes are docked to the rough endoplasmic reticulum and produce proteins destined for the secretory pathway: proteins to be exported from the cell, embedded in membranes, or delivered to lysosomes and other organelles. The docking happens during translation itself: the ribosome begins synthesis in the cytoplasm, and if the emerging protein contains a signal sequence, the ribosome is recruited to the ER membrane so the protein is threaded directly into the ER lumen as it is made.
The structural difference between prokaryotic (70S) and eukaryotic (80S) ribosomes matters far beyond academic taxonomy. Antibiotics like streptomycin, tetracycline, and erythromycin exploit specific features of the bacterial 70S ribosome that are absent or different in the 80S version. By binding to bacterial ribosomes and not eukaryotic ones, these drugs halt bacterial protein synthesis while leaving the patient's cells (and mitochondria, which have their own 70S-like ribosomes) largely unaffected. This selectivity is the mechanistic basis for antibiotic therapy — a direct application of the structural biology you are learning here.