What does ’S’ refer in a 70S and 80S ribosome?

 

 

'S' refers to Svedbergs unit for sedimentation coefficient. Sedimentation coefficient depicts that how fasta cell organelle sediments during the ultracentrifugation. 
in cells heavier the structure , higher is the sedimentation coefficient.
The value os S is equal to 10-13 seconds (1 s = 1*1 =-13 seconds).