The difference in structure and function of different ribosome types is continually being researched and amended, but there are currently two ways in which they can be classified. The first way is to classify them based on the kind of cell they inhabit: archaeal, eukaryotic, or eubacterial. The organelles in these specific types of cells differ in composition, size, and protein to ribonucleic acid (RNA) ratio. The second way in which ribosomes are classified involves whether they are bound to a membrane or remain free-floating within the cell. The terms used to describe this classification are “membrane-bound” and “free,” respectively.
The differences between archaeal, eukaryotic, and eubacterial organelles can be seen if the cells are spun in a centrifuge. Each type has a unique sedimentation pattern and rate as the cell separates in the lab. Measured in Svedberg units, which express the rate at which the sedimentation accumulates, bacterial ribosomes belong to the 70 Svedberg class, whereas both archaeal and eukaryotic ribosomes belong to the 80 Svedberg class. To distinguish further, each type also differs in size and ratio of protein to RNA. For example, the eukaryotic type measures between 25 to 35 nanometers (nm) and has a protein ratio of 1:1, while the other types measure differently.
The structure of membrane-bound and free ribosomes is identical; they differ only in spatial distribution. There are specific actions, however, that one type can perform when it is in one place or the other. Those that are bound to the rough endoplasmic reticulum (rough ER) have a greater ability to produce proteins and enzymes that can be used easily by the cell’s primary plasma membrane. Newly-produced protein chains can be directly inserted into the rough ER by membrane-bound ribosomes, shortening the time and resources needed for transport. This type is also responsible for the majority of proteins that are exported from the cell interior.
The free type make a number of specific necessary proteins, like those needed for the manufacture of hemoglobin, which the membrane-bound type cannot. Free ribosomes are needed when a cell is growing quickly or reproducing, as they can move and relocate easily. They are often found in small clusters within the cytoplasm of the cell and, in this case, they can be called polyribosomes. Free ribosomes are found in abundance in cells that do not export large amounts of protein because close proximity to the rough ER is essential for this process.