What Is a Free Ribosome?

Mark Wollacott

A free ribosome creates proteins within cells, but unlike normal ribosomes, it floats freely in the cell’s cytosol. It is different because it is not bound within the cell’s nucleus or one of its many organelles. Ribosomes make proteins out of amino acids and are made up of ribonucleic acid (RNA) and proteins.

A free ribosome floats freely within a cell and is not bound to its nucleus.
A free ribosome floats freely within a cell and is not bound to its nucleus.

The proteins created by a free ribosome are released into the cytosol. The cytosol is the liquid contained within a cell, in which organelles and other elements float around. A cell’s non-nucleus contents are also called the cytoplasm. This is different with prokaryotic cells because they have no walled-off nuclei or organelles and, therefore, all reactions take place in the cytosol.

Cytosol makes up 70% of the volume of an animal cell.
Cytosol makes up 70% of the volume of an animal cell.

A cell’s cytosol is high in glutathione. This means free ribosomes are not able to produce proteins that lack disulfide bonds. The proteins a free ribosome makes are called cytosolic proteins. All proteins created by the free ribosome are used in the cytosol and do not pass into the nucleus or any of the organelles.

The free ribosome may work independently, but may also gather into clusters and groups. These clusters are called a polysome, a polyribosome or an ergosome. These clusters usually gather around a single mRNA. An mRNA is a messenger RNA strand of genetic code that acts as a blueprint for protein creation. mRNA is basically a ribosome’s instruction manual containing the exact order amino acids need to be placed in to make the desired protein.

Normal ribosome and free ribosome structures are the same. There are also few differences between the structures of ribosomes found in prokaryotic cells and eukaryotic cells. Both consist of a large unit and a small unit. A ribosome’s size is measured in Svedberg units, shortened to s. A Svedberg unit is based on the sedimentation of a ribosome’s structure under centrifugal force.

A prokaryotic cell has a total value of 70s. It is made up of a large section with a value of 50s and a small one of 30s. Eukaryotic cells have a value of 80s with a large section worth 60s and a small one worth 40s. As they are measured based on sedimentation and not mass, their combined Svedberg units are usually smaller than their values when separated.

The difference between prokaryotic and eukaryotic ribosome size means that scientists have been able to create antibiotics that target bacterial prokaryotic cells, but do not harm eukaryotic cells. In theory, this might harm some ribosomes within the eukaryotic cell, too. This is because the ribosomes contained within a chloroplast or mitochondria are similar to those found in prokaryotic cells. These ribosomes are further evidence that chloroplasts and mitochondria are prokaryotic cells that were absorbed within eukaryotic cells.

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