as a source of energy or as building blocks for other molecules. Enzymatic digestion breaks down the large polymeric molecules in food into their monomer subunits—proteins into amino acids, polysaccharides into sugars, and fats into fatty acids and glycerol. After digestion, the small organic molecules derived from food enter the cytosol of cells, where their gradual oxidation begins.
Sugars are particularly important fuel molecules, and they are oxidized in small controlled steps to carbon dioxide (CO2) and water (Figure 2–45). In this section, we trace the major steps in the breakdown, or catabolism, of sugars and show how they produce ATP, NADH, and other activated carrier molecules in animal cells. A very similar pathway also operates in plants, fungi, and many bacteria. As we shall see, the oxidation of fatty acids is equally important for cells. Other molecules, such as proteins, can also serve as energy sources when they are funneled through appropriate enzymatic pathways.
Glycolysis is a Central aTp-producing pathway
The major process for oxidizing sugars is the sequence of reactions known as glycolysis—from the Greek glukus, “sweet,” and lusis, “rupture.” Glycolysis produces ATP without the involvement of molecular oxygen (O2 gas). It occurs in the cytosol of most cells, including many anaerobic microorganisms. Glycolysis probably evolved early in the history of life, before photosynthetic organisms introduced oxygen into the atmosphere. During glycolysis, a glucose molecule with six carbon atoms is converted into two molecules of pyruvate, each of which contains three carbon atoms. For each glucose molecule, two molecules of ATP are hydrolyzed to provide energy to drive the early steps, but four molecules of ATP are produced in the later steps. At the end of glycolysis, there is consequently a net gain of two molecules of ATP for each glucose molecule broken down. Two molecules of the activated carrier NADH are also produced.
The glycolytic pathway is outlined in Figure 2–46 and shown in more detail in Panel 2–8 (pp. 104–105) and Movie 2.5. Glycolysis involves a sequence of 10 separate reactions, each producing a different sugar intermediate and each catalyzed by a different enzyme. Like most enzymes, these have names ending in ase—such as isomerase and dehydrogenase—to indicate the type of reaction they catalyze.
Although no molecular oxygen is used in glycolysis, oxidation occurs, in that electrons are removed by NAD+ (producing NADH) from some of the carbons derived from the glucose molecule. The stepwise nature of the process releases the energy of oxidation in small packets, so that much of it can be stored in activated carrier molecules rather than all of it being released as heat (see Figure 2–45). Thus, some of the energy released by oxidation drives the direct synthesis of ATP molecules from ADP and Pi, and some remains with the electrons in the electron carrier NADH.
Figure 2–45 Schematic representation of the controlled stepwise oxidation of sugar in a cell, compared with ordinary burning. (a) if the sugar were oxidized to Co2 and h2o in a single step, it would release an amount of energy much larger than could be captured for useful purposes.
(B) in the cell, enzymes catalyze oxidation via a series of small steps in which free energy is transferred in conveniently sized packets to carrier molecules—most often aTp and naDh. at each step, an enzyme controls the reaction by reducing the activation-energy barrier that has to be surmounted before the specific reaction can occur. The total free energy released is exactly the same in (a) and (B).
large activation energy overcome by the heat from a fre all free energy is released as heat; none is stored