Inside of the nucleolus, ribosomal subunits are synthesized using proteins and ribosomal RNA, or rRNA. The nucleolus then transports these subunits to the rest of the cell to be assembled into full ribosomes.
The nucleolus is situated inside of the nucleus in eukaryotic cells. It is round in shape and does not have an outer membrane surrounding it. The rRNA needed to synthesize ribosomal subunits is created inside of the nucleolus itself, while the necessary proteins pass through pores in the nuclear membrane to enter the nucleus. These cellular materials are used to manufacture large and small ribosomal subunits, the two components that make up a complete ribosome. After the subunits are made, they exit the nucleus through the nuclear pores.
Outside of the nucleus, the subunits combine into ribosomes that are responsible for creating the proteins necessary to carry out the cell's various functions. These ribosomes can be found in several areas of a cell, such as the nuclear membrane, cytosol and reticulum. Different cells are responsible for synthesizing different quantities of proteins. Cells that need to produce very large quantities of proteins need greater numbers of ribosomes. The nucleolus is larger in these cells, allowing them to produce more ribosomes. Cells that are rapidly growing tend to require a lot of ribosomes.