Ribosomes are small granular, non-membranous structures that are involved in the synthesis (production) of proteins.
Ribosomes are found in two locations. Some of them are attached to the rough endoplasmic reticulum. Other ribosomes are free and are found in the cytoplasm.
Free ribosomes produce proteins that stay within the cell or are imported into the mitochondria and other organelles.
As mentioned earlier, in the section about the rough endoplasmic reticulum, the ribosomes attached to the rough endoplasmic reticulum produce proteins that eventually either become part of the plasma membrane of the cell or are secreted by the cell.
Ribosomes are made of two chemicals: proteins and a type of RNA called ribosomal RNA (rRNA).
An interesting quandary for evolutionists is the question of how did ribosomes evolve. If ribosomes are used to make proteins but part of ribosomes are proteins, how did ribosomes evolve? You need ribosomes to make proteins but you need proteins to make ribosomes.
Ribosomes consist of two parts: a large subunit and small subunit. (See the dark blue and the lighter blue in the image below.) When proteins are made at the site of the ribosomes, the two subunits “clamp” to a molecule called messenger RNA. Messenger RNA contains a sequence of nucleotides that are complementary to a sequence of nucleotides in DNA. (The messenger RNA molecules is the gray line running diagonally in the image below.) So, in essence, what happens at the site of the ribosomes is the ribosome “reads” the sequence of nucleotides in a messenger RNA molecule and uses that sequence to produce a protein with a particular sequence of amino acids. (The amino acids are the green circles in the image below.)
The cell can produce larger number of protein molecules by having multiple ribosomes bind to and read each messenger RNA. This structure consisting of many ribosomes attached to a messenger RNA molecule is called a polyribosome or polysome. This is illustrated below.