Biology 11th Edition by Gerald Audesirk,Teresa Audesirk,Bruce Byers

Exercise 12

Because there was no oxygen in the earliest atmosphere, the first cells must have derived energy by ______________ metabolism of organic molecules. Oxygen was introduced into the atmosphere when some microbes developed the ability to ______________ and released oxygen as a by-product. Oxygen was ______________ to many of the earliest cells, but some cells evolved the ability to use oxygen in ______________ respiration, which provided far more ______________.