Ribosomes are the structures that build proteins, and they are found in essentially every living cell. Their location depends on the type of cell and, in eukaryotic cells, on where the proteins they make will ultimately be used.
In a typical eukaryotic cell—such as a human, animal, plant, or fungal cell—ribosomes are found free in the cytoplasm and attached to the surface of the rough endoplasmic reticulum (rough ER). Ribosomes are also found inside mitochondria, and in plant cells and algae, inside chloroplasts.
The key distinction is that ribosomes are not enclosed by a membrane. They can either float freely in the cytoplasm or associate with particular membranes while carrying out protein synthesis.
Ribosomes in the cytoplasm
Many ribosomes are located freely in the cytosol, the fluid portion of the cell’s cytoplasm. These are often called free ribosomes.
Free ribosomes make proteins that generally remain inside the cell, including many proteins that function in the cytosol itself. They also produce some proteins that are transported into certain organelles, such as the nucleus, mitochondria, or chloroplasts.
Free ribosomes are not permanently attached to one location. A ribosome can associate with the endoplasmic reticulum when it begins producing a protein that needs to enter the secretory pathway.
Ribosomes attached to the rough ER
Another major population of ribosomes is attached to the rough endoplasmic reticulum, a membrane-bound network within eukaryotic cells. The ribosomes give this region its characteristic “rough” appearance when viewed with an electron microscope.
Ribosomes on the rough ER make proteins that are destined for secretion from the cell, insertion into certain cellular membranes, or delivery to compartments such as lysosomes. As a newly forming protein is synthesized, it can be threaded into the ER or inserted into its membrane.
Importantly, these ribosomes are not a special permanent type of ribosome. A ribosome can be free in the cytosol and then become associated with the ER when it begins translating a protein containing the appropriate targeting information.
Ribosomes inside mitochondria and chloroplasts
Eukaryotic cells also contain ribosomes inside certain organelles.
Mitochondria contain their own ribosomes, which are used to make a limited set of proteins encoded by mitochondrial DNA. These ribosomes are distinct from the ribosomes found in the cytosol and reflect the evolutionary history of mitochondria.
Chloroplasts, found in plants and many algae, also contain their own ribosomes. They make some of the proteins needed for chloroplast function from genetic information carried within the chloroplast.
The presence of ribosomes in these organelles is one of several features supporting the idea that mitochondria and chloroplasts evolved from ancient bacteria that became permanent residents inside larger cells.
Where are ribosomes found in prokaryotic cells?
Prokaryotic cells, including bacteria and archaea, do not have a nucleus or membrane-bound organelles such as the ER. Their ribosomes are therefore found in the cytoplasm, where they synthesize proteins.
Bacterial ribosomes are commonly described as 70S ribosomes, whereas the ribosomes found in the cytosol of eukaryotic cells are 80S ribosomes. The “S” refers to a sedimentation unit used to describe how particles behave during centrifugation; it does not simply represent the size of the ribosome.
Some bacterial cells also have ribosomes associated with the inner surface of the cell membrane while they synthesize certain membrane or exported proteins, but they do not have a rough ER.
Are ribosomes in the nucleus?
Ribosomes themselves are not normally found functioning inside the nucleus. However, the nucleus contains the nucleolus, a specialized region where ribosomal RNA is produced and ribosomal subunits are assembled.
The completed ribosomal subunits are then transported out of the nucleus through nuclear pores into the cytoplasm. There, they join to form functional ribosomes capable of translating messenger RNA (mRNA) into protein.
So, the nucleus is important for ribosome production and assembly, but protein synthesis by mature ribosomes takes place primarily outside the nucleus.
A simple way to remember their locations
| Location | What ribosomes are doing there |
|---|---|
| Cytosol | Making proteins used mainly within the cell |
| Rough ER | Making proteins destined for secretion, membranes, or certain organelles in the endomembrane system |
| Mitochondria | Making a subset of mitochondrial proteins |
| Chloroplasts | Making a subset of chloroplast proteins |
| Nucleolus | Producing ribosomal RNA and assembling ribosomal subunits, rather than translating proteins |
The most important answer is therefore: in eukaryotic cells, ribosomes are found free in the cytoplasm and attached to the rough ER, with additional specialized ribosomes inside mitochondria and, in photosynthetic eukaryotes, chloroplasts. In prokaryotic cells, ribosomes are found in the cytoplasm because these cells lack a nucleus and membrane-bound organelles.
