Question

What is the value of the activation energy of the uncatalyzed reaction?

Answer 1

there is no specific amounts of value of activation energy to start a reaction - it varies.

Answer 2

in the context of bioenergy:
If the reaction hasn't started yet, we begin with zero. the value to start (activation energy) needed depends on the reactants. Think of it as the amount of energy needed to either start the breakdown of molecules (catabolism) or build molecules together (anabolism). This is why enzymes are important in these processes, as one of the many functions is to lower the amount of activation energy required for metabolic pathway.