Energy is the ability to do work, and generating and utilizing energy are the functions of metabolism, the sum total of the chemical reactions in cells. When a molecule undergoes a chemical reaction, chemical bonds are broken and re-formed to generate one or more products. If the products of a reaction are more stable than the reactants, the reaction releases energy that can be stored in ATP or electrochemical gradients. Reactions that release energy are called exergonic; if they require energy to proceed, they are endergonic. Inside cells, metabolic reactions are powered by a steady stream of matter and energy that flows through the system: nutrients are ingested and broken down, and their products are reused or discarded. This constant throughput of matter and energy keeps the cell far from equilibrium.
Metabolic maps depict the complexity of metabolism in cells, showing how the product of one reaction becomes a reactant in the next. Enzymes carry out the reactions of metabolism in linear sequences called metabolic pathways. Enzymes are selective, each recognizing and interacting with a specific molecule or molecules to facilitate a particular reaction. Many enzymes are also regulated, with the activity of the enzyme being adjusted to the cell’s changing needs. Feedback inhibition is one of the mechanisms that make enzymes responsive to those changing needs.
Metabolic regulation extends to tissue and organs, often under the control of hormones. Much of what we know about metabolism comes from studies of metabolic disorders, conditions where enzymes and hormones don’t function as they should. A genetic mutation that disrupts the activity of a single enzyme can block or severely impair an entire metabolic pathway, preventing the production of needed molecules or causing the buildup of toxic ones. As biologists learn about metabolic pathways, they are discovering how to manipulate metabolism. Metabolic engineering is already being used to develop new drugs to treat specific diseases. It also offers the possibility of renewable energy, such as biofuels that may reduce our reliance on fossil fuels.
