9: Metabolism

All living cells require a continuous supply of energy to maintain organization, carry out metabolic reactions, and respond to their environment. Cellular energy (the usable chemical energy that powers cellular processes) is stored, transferred, and used through tightly regulated biochemical pathways. A biochemical pathway is any organized series of enzyme-controlled chemical reactions in a living cell. The term includes pathways involved in energy production, molecule synthesis, cell signaling, DNA repair, and other cellular processes. A metabolic pathway is a type of biochemical pathway specifically involved in metabolism—the breakdown, synthesis, or transformation of molecules. Glycolysis, the citric acid cycle, and fatty acid synthesis are metabolic pathways. Therefore, all metabolic pathways are biochemical pathways, but not all biochemical pathways are considered metabolic pathways. In many introductory biology texts, however, the terms are used interchangeably.

Metabolic pathways may be catabolic or anabolic. Catabolic pathways break larger molecules into smaller molecules and release energy. Cellular respiration, for instance, breaks down glucose and captures some of its energy in ATP. Anabolic pathways use energy to build larger molecules from smaller components. Cells use these pathways to produce proteins, nucleic acids, lipids, and other materials needed for growth, maintenance, and repair.

Rather than releasing energy all at once, cells extract and manage energy through a series of controlled reactions. This gradual release allows cells to capture energy efficiently without producing excessive heat or causing cellular damage. Energy is stored within chemical bonds and transferred through metabolic reactions. Much of this energy is carried by high-energy electrons that move between molecules through oxidation–reduction reactions.

Specialized electron carriers, including NAD⁺ and FAD, accept high-energy electrons and become NADH and FADH₂. NADH is the reduced form of nicotinamide adenine dinucleotide, and FADH₂ is the reduced form of flavin adenine dinucleotide.These carriers temporarily store and transport the electrons to other reactions, where their energy can contribute to ATP production. ATP then directly powers cellular activities such as active transport, movement, and the synthesis of biological molecules.

The substances produced between the beginning and end of a pathway are called metabolic intermediates. An intermediate may continue through the same pathway or enter another pathway, creating an interconnected network of cellular reactions. Some pathways are linear, while others form cycles or branch in multiple directions.

Cells regulate biochemical pathways by controlling enzyme activity. When enough of a final product has accumulated, it may inhibit an enzyme near the beginning of the pathway and slow further production. When energy or a particular product is needed, the pathway may become more active. This regulation balances energy production with energy demand, conserves cellular resources, and maintains metabolic stability.