Components of E. coli E. coli cells are rod-shaped, about 2 um long and 0.8 m in diameter. The volume of a cylinder is h, where h is the height of the cylinder. (a) If the average density of E. coli (mostly water) is 1.1 x 10 g/L, what is the mass of a single cell? (b) E. coli has a protective cell envelope 10 nm thick. What percentage of the total volume of the bacterium does the cell envelope occupy? (c) E. coli is capable of growing and multiplying rapidly because it contains some 15,000 spherical ribosomes (diameter 18 nm), which carry out protein synthesis. What percentage of the cell volume do the ribosomes occupy?