Metabolism is all the chemical reaction that take place in a living organism. Metabolic reactions use or release energy and can be divided into anabolic reactions and catabolic reactions.
Anabolic reactions require an input of energy to synthesize complex molecules from simpler ones. Synthesizing sugar from CO2 is one example. Other examples are the synthesis of large proteins from amino acid building blocks, and the synthesis of new DNA strands from nucleic acid building blocks. These biosynthetic processes are critical to the life of the cell, take place constantly, and demand energy provided by ATP and other high-energy molecules like NADH (nicotinamide adenine dinucleotide) and NADPH (Figure 1).
Catabolic reactions release energy by degradation (or breakdown) of complex molecules into simpler ones. The breakdown of sugars illustrates how a single molecule of glucose can store enough energy to make a great deal of ATP, 36 to 38 molecules. ATP is an important molecule for cells to have in sufficient supply at all times. Molecular energy stored in the bonds of complex molecules, such as carbohydrates, is released in catabolic pathways and harvested in such a way that it can be used to produce ATP. Other energy-storing molecules, such as fats, are also broken down through similar catabolic reactions to release energy and make ATP (Figure 1).
It is important to know that the chemical reactions of metabolic pathways don’t take place spontaneously. Each reaction step is facilitated, or catalyzed, by a protein called an enzyme. Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.
Anabolic pathways assemble large molecules form smaller ones. Catabolic pathways break large molecules into small pieces.