We can view metabolism at several levels. At the highest level, we have nutrients, such as sugars, fatty acids and amino acids entering cells and carbon dioxide and other waste products (such as urea) exiting. Cells use the incoming materials for energy and substance to synthesize sugars, nucleotides, and other amino acids as building blocks for the carbohydrates, nucleic acids, fatty compounds, and proteins necessary for life. As we zoom in, we can imagine pathways made up of reactions for breakdown and synthesis of each of these compounds. The figure at left shows such a simple schematic and how the pathways are not isolated from each other – molecular products of one are substrates for another. At a deeper level, we can study individual reactions and discover the enormous complexity and commonality of metabolic reactions.
Figure 6.2.1: Anabolic vs Catabolic Processes
In studying metabolism, we recognize that metabolic pathways are manmade concepts with artificial boundaries. Students commonly think of the molecules in the pathways being tied exclusively to those individual pathways, but with the exception of reactions that have physical barriers (such as those occurring within an organelle), metabolic pathways have many common intermediates used in multiple reactions occurring in the same location at the same time and thus cannot be ascribed to any one pathway. The best we can do is understand general directions of pathways in cells.