Sugars and fats are the major energy sources for most nonphotosynthetic organisms, including humans. However, most of the useful energy that can be extracted from the oxidation of both types of foodstuffs remains stored in the acetyl CoA molecules that are produced by the two types of reactions just described. The citric acid cycle of reactions, in which the acetyl group (–COCH3) in acetyl CoA is oxidized to CO2 and H2O, is therefore central to the energy metabolism of aerobic organisms. In eukaryotes, these reactions all take place in mitochondria. We should therefore not be surprised to discover that the mitochondrion is the place where most of the ATP is produced in animal cells. In contrast, aerobic bacteria carry out all of their reactions, including the citric acid cycle, in a single compartment, the cytosol.
The Citric acid Cycle Generates naDh by oxidizing acetyl Groups to Co2
In the nineteenth century, biologists noticed that in the absence of air cells produce lactic acid (for example, in muscle) or ethanol (for example, in yeast), while in its presence they consume O2 and produce CO2 and H2O. Efforts to define the pathways of aerobic metabolism eventually focused on the oxidation of pyruvate and led in 1937 to the discovery of the citric acid cycle, also known as the
Figure 2–54 How stored fats are mobilized for energy production in animals. low glucose levels in the blood trigger the hydrolysis of the triacylglycerol molecules in fat droplets to free fatty acids and glycerol. These fatty acids enter the bloodstream, where they bind to the abundant blood protein, serum albumin. special fatty acid transporters in the plasma membrane of cells that oxidize fatty acids, such as muscle cells, then pass these fatty acids into the cytosol, from which they are moved into mitochondria for energy production.
Figure 2–55 Pathways for the production of acetyl Coa from sugars and fats. The mitochondrion in eukaryotic cells is where acetyl Coa is produced from both types of major food molecules. it is therefore the place where most of the cell’s oxidation reactions occur and where most of its aTp is made. amino acids (not shown) can also enter the mitochondria, to be converted there into acetyl Coa or another intermediate of the citric acid cycle. The structure and function of mitochondria are discussed in detail in Chapter 14.
tricarboxylic acid cycle or the Krebs cycle. The citric acid cycle accounts for about two-thirds of the total oxidation of carbon compounds in most cells, and its major end products are CO2 and high-energy electrons in the form of NADH. The CO2 is released as a waste product, while the high-energy electrons from NADH are passed to a membrane-bound electron-transport chain (discussed in Chapter 14), eventually combining with O2 to produce H2O. The citric acid cycle itself does not use gaseous O2 (it uses oxygen atoms from H2O). But the cycle does require O2 in subsequent reactions to keep it going. This is because there is no other efficient way for the NADH to get rid of its electrons and thus regenerate the NAD+ that is needed.