Molecular Species in the E. coli Membrane The plasma membrane of E. coli is about 75% pro- tein and 25% phospholipid by weight. How many molecules of membrane lipid are present for each molecule of membrane protein? Assume an average protein M, of 50,000 and an average phospho- lipid M, of 750. What more would you need to know to estimate the fraction of the membrane sur- face that is covered by lipids?