Enzymes
The metabolism is a series of chemical reactions that occur arranged in organisms in order to synthesize macromolecules from simple molecules, with the energy obtained the break chemical bonds of other macromolecules, reserve substances or foods.
As the substances that take part in these reactions are very stable, a lot of energy is necessary for this reaction to take place because if not, the reaction speed would be zero or too slow. This could be solved by increasing the temperature, but it could cause the death of the organism, so living beings use biocatalysts, enzymes, which lower the activation energy necessary for the reaction to occur, thereby increasing its speed.
Animated image: Enzymatic reaction.