In the oxidative phase of the pentose phosphate pathway, energy is extracted from glucose 6-phosphate to produce NADPH and yield ribose 5-phosphate. The free-energy change for this process is large and exergonic. In the non-oxidative phase of the pentose phosphate pathway, a series of carbon swapping reactions regenerate glucose 6-phosphate from ribose 5-phosphate. These reactions do not consume energy, and the overall free-energy change for the non-oxidative phase is close to zero. In cells that carry out fatty acid synthesis, the two phases of the pathway continuously cycle, from glucose 6-phosphate to ribose 5-phosphate and back to glucose 6-phosphate. If the oxidative phase releases energy (in the form of NADPH) but the non-oxidative phase doesn't consume any energy, how is it possible for them to keep cycling? Explain.