Question

why do living things use enzymes instead of heat as a source of activation energy?

Answer 1

Living things use enzymes instead of a heat source of activation
energy because they speed up chemical reactions as well as the
metabolism in those living things.

Answer 2

Adding sufficient heat would harm or kill a living thing, also enzymes are far more efficient. A single enzyme can often carry out thousands of reactions per second. Living organisms use enzymes for activation energy because they speed up chemical reactions.