There are two types of metabolic pathways: catabolic, involving the breakdown of biochemicals into simpler compounds, and anabolic, involving the synthesis of biochemicals from simpler molecules. Each living cell has thousands of distinct metabolic reactions. Each reaction is catalyzed by an enzyme and is linked to other reactions through a pathway. How can you keep them all straight? It is nearly impossible to memorize them. The purpose of this chapter is to provide an organizational framework to metabolism that allows you to view it as something other than a collection of disjointed pathways.
Photosynthetic organisms fix CO 2 to form organic molecules, such as glucose. The carbon in CO 2 is in the +4 oxidation state, while the carbon in glucose (C 6H 12O 6) has an oxidation state of zero. Thus, carbon fixation must involve the reduction of carbon. Along with this process, something else must be oxidized—the oxygen of water is converted to molecular oxygen. In chemical terms, the oxygen in water is oxidized from the ‐2 state to zero—the oxidation state of elemental O 2. If the glucose is used by muscle cells during exercise, it can be broken down aerobically (with the participation of molecular oxygen) to CO 2 and water (effectively the reverse of photosynthesis), or anaerobically (without molecular oxygen being involved) to lactic acid, both of which are represented in Figure 1 .
Lactic acid's empirical formula is C 3H 6O 3, so the carbons have a net oxidation number of zero, the same as in glucose (C 6H 12O 6). However, the carbons in lactic acid do not have the same oxidation number. The carboxyl carbon of lactic acid is more oxidized (+3) than the methyl carbon at the other end, which has three of its four bonds to hydrogen and therefore has an oxidation number of ‐3. A key feature of metabolic pathways is that the oxidation of one component is balanced by the reduction of another. The net result is that no electrons are lost or gained in the process.