Both the citric acid cycle and glycolysis also function as starting points for important biosynthetic reactions by producing vital carbon-containing intermediates, such as oxaloacetate and α-ketoglutarate. Some of these substances produced by catabolism are transferred back from the mitochondria to the cytosol, where they serve in anabolic reactions as precursors for the synthesis of many essential molecules, such as amino acids (Figure 2–59).
electron Transport Drives the synthesis of the majority of the aTp in most Cells
Most chemical energy is released in the last stage in the degradation of a food molecule. In this final process, NADH and FADH2 transfer the electrons that they gained when oxidizing food-derived organic molecules to the electron-transport chain, which is embedded in the inner membrane of the mitochondrion (see Figure 14–10). As the electrons pass along this long chain of specialized electron acceptor and donor molecules, they fall to successively lower energy states. The energy that the electrons release in this process pumps H+ ions (protons) across the membrane—from the innermost mitochondrial compartment (the matrix) to the intermembrane space (and then to the cytosol)—generating a gradient of H+ ions (Figure 2–60). This gradient serves as a major source of energy for cells, being tapped like a battery to drive a variety of energy-requiring reactions. The most prominent of these reactions is the generation of ATP by the phosphorylation of ADP.
Figure 2–57 Simple overview of the citric acid cycle. The reaction of acetyl Coa with oxaloacetate starts the cycle by producing citrate (citric acid). in each turn of the cycle, two molecules of Co2 are produced as waste products, plus three molecules of naDh, one molecule of GTp, and one molecule of faDh2. The number of carbon atoms in each intermediate is shown in a yellow box. for details, see panel 2–9  (pp. 106–107).
O Figure 2–58 The structure of GTP. GTp and GDp are close relatives of aTp and aDp, respectively.
At the end of this series of electron transfers, the electrons are passed to molecules of oxygen gas (O2) that have diffused into the mitochondrion, which simultaneously combine with protons (H+) from the surrounding solution to produce water. The electrons have now reached a low energy level, and all the available energy has been extracted from the oxidized food molecule. This process, termed oxidative phosphorylation (Figure 2–61), also occurs in the plasma membrane of bacteria. As one of the most remarkable achievements of cell evolution, it is a central topic of Chapter 14.
In total, the complete oxidation of a molecule of glucose to H2O and CO2 is used by the cell to produce about 30 molecules of ATP. In contrast, only 2 molecules of ATP are produced per molecule of glucose by glycolysis alone.
amino acids and nucleotides are part of the nitrogen Cycle
So far we have concentrated mainly on carbohydrate metabolism and have not yet considered the metabolism of nitrogen or sulfur. These two elements are important constituents of biological macromolecules. Nitrogen and sulfur atoms pass
Figure 2–59 Glycolysis and the citric acid cycle provide the precursors needed to synthesize many important biological molecules. The amino acids, nucleotides, lipids, sugars, and other molecules—shown here as products—in turn serve as the precursors for the many macromolecules of the cell. each black arrow in this diagram denotes a single enzyme-catalyzed reaction; the red arrows generally represent pathways with many steps that are required to produce the indicated products.
from compound to compound and between organisms and their environment in a series of reversible cycles.