This chapter begins our study of metabolism, which may at first, second and third glance appear to be a bewildering array of many enzymatic reactions. It is certainly not possible to memorize the details of all the metabolic pathways in a cell. Our emphasis throughout the course will be on understanding the function of metabolic pathways, and how the structure and regulation of specific enzymes forms the basis of metabolic control. Often recognizing the function of a pathway makes all the rest of the information fall into place, and the answers to the questions below become logical, rather than just a task of memorization. This will make the subject easier to remember and more relevant.
All organisms operate by the same metabolic principles and share many metabolic pathways .
Organisms that can produce organic carbon from CO 2 are called autotrophs , while organisms that must have a source of organic carbon are heterotrophs . Organisms are also classified by whether or not they need oxygen to live. The function of O 2 is to act as an electron acceptor in energy generating oxidation-reduction reactions . Organisms that don't use O 2 for this purpose substitute other electron accepting molecules. Figure 18.3 illustrates how the metabolism of these different types of cells drives the carbon and oxygen cycles in the biosphere and shows that sunlight is the major energy input into the biosphere.