Let's tale a closer look at how ATP is produced from a molecule of glucose, our fuel Only the carbon skeleton is shown to keep things simple The first step is called glycolysis and it takes place outside the mitochondria To begin the process, some energy has to be invested Next, the molecule is split in half Now the molecule, NAD+ an electron carrier picks up electrons and a hydrogen atom from the carbon molecule becoming NADH Keep track of the electron carriers They play an important role by transporting electrons to reactions in the mitochondria In the final steps of glycolysis some ATP is produced but not much For every glucose molecule only two net ATPs are produced outside the mitochondrion However, glycolysis has produced pyruvic acid which still has a lot of energy available
