So far we have been looking at the machinery for degradation of glucose to pyruvate and ultimately to C0₂ and water. Whenever there is an excess of pyruvate, and energy is not needed immediately, pyruvate can be reconverted to glucose for storage as glycogen in the liver. (Recall from Chapter 21 that glycogen is a branched-chain starchlike molecule.) This reverse process is gluconeogenesis, which simply means “new glucose generation" (right). It is almost equivalent to glycolysis in reverse since, except for three controlling steps, it uses the same intermediate compounds, the same reactions in reverse, and even the same enzymes.

It is logical from the standpoint of economy that glucose buildup should use some of the same intermediates and enzymes as glucose degradation. What is surprising is that it also appears that a part of this gluconeogenesis scheme has been picked up bodily and adapted for use in the dark reactions of photosynthesis, even though the starting point for glucose manufacture in photosynthesis is C0₂ instead of pyruvate. We will see evidence later in this chapter that respiration may have evolved from photosynthesis. It also appears that photosynthesis may have taken over some of the chemistry of glycolysis and gluconeogenesis. These borrowings illustrate the idea that nothing is ever really new in evolution. Just as hands and feet came from fins, and lungs from gills, so respiration borrowed from photosynthesis, and photosynthesis from glucose metabolism.