I understand that a prokaryotic cell has ribosomes smaller than 70s.

But what i don't understand is what 70s accutually means. Please could someone explain what 70s means to me?

thank you

S is a unit of density. Check this site for a clear explanation.

http://student.ccbcmd.edu/courses/bio141/lecguide/unit1/prostruct/proribo.html

Certainly! The term "70s" refers to the size of ribosomes in prokaryotic cells. Ribosomes are responsible for protein synthesis in all cells. These molecular machines are made up of two subunits, each with a different size. In prokaryotes, such as bacteria, the smaller subunit is around 30s (but more precisely 30S) and the larger subunit is around 50s (but more precisely 50S). When these two subunits combine, they form a complete ribosome with a size of around 70s (but more precisely 70S).

To explain further how this size is determined, it is based on a technique called sedimentation rate. Sedimentation rate refers to how fast a molecule or structure settles or moves in a solution under the force of gravity. Ribosomes are subjected to centrifugation, a technique that involves spinning the sample at a very high speed. During centrifugation, larger structures settle more quickly than smaller ones. By comparing the sedimentation rate of ribosomes to that of known molecular markers, scientists can assign them a specific size designation, such as 70S for prokaryotes.

It's important to note that eukaryotic cells, which make up plants, animals, fungi, and other multicellular organisms, have larger ribosomes. Their ribosomes consist of a smaller subunit, roughly 40s (40S), and a larger subunit, approximately 60s (60S). When combined, these subunits form eukaryotic ribosomes with a size of about 80s (80S).

In summary, the "70s" (or 80s for eukaryotes) refers to the sedimentation rate and size of ribosomes in prokaryotic (or eukaryotic) cells, specifically describing the combination of the smaller and larger subunits that make up the complete ribosome.