How many ribosomes are actively synthesizing proteins at any instant in an E. coli cell growing with a 45-min doubling time? The birth size of E. coli is 1-μm diameter and 2-μm length. The water content is 75%. About 60% of the dry material is protein, and the rate of amino acid addition per ribosome is 20 amino acids per second. The average molecular weight of free amino acids in E. coli is 126.