In the cells of eukaryotic organisms, the subunits that will become ribosomes are produced within the subnuclear structure called the nucleolus, and the subunits then combine in the cell's cytoplasm to become functioning ribosomes. Because the ribosomes are the protein manufacturers of a cell, the size of the nucleolus can take up as much as 25 percent of the volume of the nucleus in cells that require large amounts of protein. Most eukaryotic cells contain only one nucleolus, but certain species can contain several.
Ribosomal subunits within the nucleolus exist as a granular material that will not fully mature into working ribosomes until it is exported outside of the nucleus and into the cell's cytoplasm. The export process requires the large molecules that comprise the preribosomal units to interact with various export receptors. These interactions enable the subunits to pass through the water-repelling central channel of the nucleus' pore complex and enter into the surrounding cytoplasm.
Mature and functioning ribosomes are comprised of more than 50 proteins and contain their own ribosomal RNA, which can also be noted as rRNA. In a process referred to as translation, the ribosomes build protein structures for the cell by reading messenger RNAs, or mRNAs, and binding transfer RNAs, or tRNAs, to amino acids. Ribosomes are found floating throughout the cell's cytosol, which is the fluid portion of the cytoplasm, and many are also found attached to the membrane-like layer of the nuclear envelope called the endoplasmic reticulum.
In prokaryotic cells, which do not possess a nucleus, ribosomes are produced within the cytoplasm. Because of the lack of a nucleus, all ribosomes in prokaryotic cells are free-floating.