Scientists have discovered that a version of the Krebs cycle, the heart of the cellular metabolic network, can take place without the cellular proteins known as enzymes. Since the Krebs cycle does not require cellular proteins to occur, researchers now believe that metabolism may predate life. In fact, spontaneous chemical reactions may have served as the foundation for life on Earth.“Metabolism” describes the web of chemical reactions that maintain the living state of cells and organisms. This includes both reactions that synthesize amino acids and lipids that cells need and reactions that break down molecules to generate energy. Cells use lipids and amino acids in membranes and proteins and to create the molecules that are consumed to generate energy.