Reactions that break complex molecules into simpler ones and release energy are called:

1. A cell's metabolism refers to the chemical reactions that occur within it. 2. Those reactions run in two opposite directions. 3. Breaking down complex molecules into simpler ones with a release of energy is catabolism. 4. Building complex molecules, which requires energy, is anabolism. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 6 'Metabolism', sections 6.1-6.5_