Speeds up the reaction but does not change k

The answer is catalyst.

A catalyst is characterized by acceleration the rate of a chemical reaction but not modifiy the equilibrium, which is what k represents (the constant of equilibrium of the reaction).

The catalyst manages that the reaction goes through a different pathway, with a lower activation energy, so the transition compound is reached faster and the whole reaction goes faster, but the final species and their equilibriium concentrations do not change.

Answer( precipitate );

Answer/when the cell uses energy;