Glycolysis and the pathways of cellular respiration do not operate in isolation. Rather, there is an interchange of molecules into and out of these pathways, to and from the metabolic pathways for the synthesis and breakdown of amino acids, nucleotides, fatty acids, and other building blocks of life (see Figure 8.14). Carbon skeletons (i.e., the carbon backbones of organic molecules) can enter the catabolic pathways and be broken down to release their energy, or they can enter anabolic pathways to be used in the formation of the macromolecules that are the major constituents of the cell. These relationships are summarized in Figure 9.13. In this section we will explore how pathways are interrelated by the sharing of intermediate molecules, and we will see how pathways are regulated by the inhibitors of key enzymes.

Question

DNA can be an energy source. When DNA is hydrolyzed, the nucleotides can be metabolized into intermediates in the citric acid cycle. The intermediates are then oxidized and the energy released to reduced coenzymes, which release their energy when oxidized in the mitochondria to form ATP. However, because it is the genetic material and must be preserved, DNA is protected from hydrolysis after it is made by being sequestered in the cell nucleus.