Consider the evolution of metabolic pathways. We believe that these pathways evolved from the

Question

a. precursor to the end product, as enzymes became more efficient over time.
b. precursor to the end product, as cells simplified the biochemistry of the external environment.
c. end product to the precursor, as cells gradually evolved according to a predetermined plan.
d. end product to the precursor, as intermediates in the pathway were depleted in the environment.
e. end product to the precursor, as photosynthesis began to supply the energy to power the changes at each stage.

Answer 1

D; The general picture of the evolution of metabolic pathways is that cells used up a necessary compound (the end product) in the ocean and then produced an enzyme that would allow them to change a different compound into the necessary one. When this second compound was used up, they then learned to convert a third into the second, and so on. The process was not directed (choice c), but came out of an ad hoc response to adversity. The process began before the development of photosynthesis (choice e), as photosynthesis itself requires sophisticated metabolic pathways.