Description of sources for metabolic energy, which mainly lead to the synthesis of ATP, the most important energy molecule within cells. The abiotic formation of pyrrole enabled the biotic formation of porphyrins, which include chlorophylls in plants and hemes in animals. The importance of thermodynamics and chemical equilibria is described. Energy barriers between different compounds provide a limit to how easily or quickly they can be changed into the other one. Explanation of why some amino acids are essential. Description of the likely origin of life and of LUCA, the last universal common ancestor.