Ribosome
Ribosomes read messenger RNA three bases at a time and link amino acids into proteins. They have no membrane and occur in every living cell, from bacteria to humans.
- 1NucleusOrganelle · double membrane
- 2MitochondrionOrganelle · double membrane
- 3RibosomeMolecular machine · no membrane
- 4Endoplasmic reticulumOrganelle · single membrane network
- 5Golgi apparatusOrganelle · single membrane stacks
- 6LysosomeOrganelle · single membrane
- 7Cell membraneBoundary · phospholipid bilayer
- 8CentriolesMicrotubule pair · organizes the spindle
Animal cell: what you are looking at
A flexible, roughly rounded cell 10–30 µm across, bounded only by its membrane. It has a pair of centrioles near the nucleus and many lysosomes, but no wall, no chloroplasts, and no large central vacuole.
Centrioles. A pair of short microtubule cylinders at right angles; they organize the spindle during cell division.
The ribosome is outlined in coral; it occurs in animal cells, plant cells, fungi, protists, bacteria and archaea. Hover or tab through the key to light up the other structures, and click any of them to open its page.
Compare all eight organelles ↗Know its structure
| Found in | Animal cells, Plant cells, Fungi, Protists, Bacteria and archaea |
| Membranes | None: built from ribosomal RNA and dozens of proteins |
| Typical size | About 20–30 nm across, far too small to see with a light microscope |
| Number per cell | Roughly 20,000 in a growing E. coli cell; several million in a mammalian cell |
| Structure | Two subunits: 80S (60S + 40S) in eukaryotes, 70S (50S + 30S) in prokaryotes, with three tRNA sites (A, P, E) |
| Discovered | Spotted by George Palade in 1955 with the electron microscope; the name “ribosome” dates from 1958 |
What it does
- Translates mRNA into protein. Matches each codon with a tRNA and forms a peptide bond, at about 15–20 amino acids per second in bacteria and 5–6 in eukaryotes
- Sorts proteins by where it sits. Free ribosomes make proteins for the cytosol; ribosomes docked on the rough ER make secreted and membrane proteins
- Checks the message. Stalls on damaged or truncated mRNA so rescue factors can recycle the ribosome and destroy the faulty message
Ribosome: common questions
Wondering about the why? Start here.
What does a ribosome do?
A ribosome reads messenger RNA three bases at a time and joins the matching amino acids into a protein. Transfer RNAs carrying amino acids enter its A site, the growing chain is passed to them in the P site, and the empty tRNA leaves from the E site, at about 15 to 20 amino acids per second in bacteria and 5 to 6 in eukaryotes. The 146-amino-acid β-chain of hemoglobin takes a human ribosome roughly 30 seconds.
Translate a sequence yourselfIs a ribosome an organelle?
It depends on the definition. A ribosome has no membrane, so it is not a membrane-bound organelle; it is a molecular machine of ribosomal RNA and proteins about 20 to 30 nm across, and it occurs in every living cell, including bacteria. Eukaryotic ribosomes are 80S (a 60S and a 40S subunit) while prokaryotic ones are 70S (50S and 30S), a difference antibiotics such as tetracycline exploit to hit bacteria and spare us.
Read: prokaryotes vs. eukaryotesWhat is the difference between free and bound ribosomes?
Free ribosomes float in the cytosol and make proteins that stay there, such as glycolysis enzymes. Bound ribosomes sit on the rough endoplasmic reticulum and feed their growing chains into the ER lumen, producing secreted proteins, membrane proteins, and lysosomal enzymes. They are the same ribosomes: a signal sequence at the start of the new protein decides whether the ribosome docks on the ER.
Follow proteins into the ER