6. a substance that enters earth's cycles and does harm to living things is
a. oxygen.
b. hydrogen.
c. freshwater.
d. a pollutant.

Animal bodies (and even plant bodies for that matter) are complex enough that life couldn't exist without enzymes. enzymes are usually large, complex biological catalysts that are expensive to make. enzymes speed up the rate of chemical reactions without being used up during the reaction (they can be reused many times). they are highly specific in the reactions they regulate. knowing what enzymes are and how they function, why does this explain why complex life forms could not exist if there were no enzymes.

Environmental differences within ecosystems are generally caused by

Which macromolecule catalyzes chemical reactions this be considered enzyme chemical reactions thus he considering enzymes