The amount of energy needed for a reaction to occur is known as the

The minimum amount of energy required for a reaction to occur is known as the activation energy. It is the minimum amount of energy needed to activate atoms or molecules to a condition in which they can undergo chemical transformation.

The source of the activation energy needed to push reactions forward is typically heat energy from the surroundings.  Heat energy speeds up the motion of molecules , increasing the frequency and force with which they collide. It also moves atoms and  bonds  within the molecules slightly helping them reach their transition state.

a. decomposes use carbon from dead organisms as fuel for cellular respiration, returning carbon dioxide back into the atmosphere.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.