Metabolism

Metabolism (from the Greek *metabolē*, meaning "change") is the sum of all chemical reactions that keep living organisms alive. Its three core jobs are: turning food energy into a form cells can use; converting food into the building blocks of macromolecules like proteins, lipids, nucleic acids, and certain carbohydrates; and getting rid of metabolic waste. These enzyme-driven reactions let organisms grow, reproduce, hold their shape, and react to their surroundings. The term can also cover every chemical reaction in a living thing, including digestion and moving substances within and between cells. More narrowly, the reactions happening inside cells are called intermediary (or intermediate) metabolism.

Metabolic reactions fall into two categories. Catabolism breaks compounds down—for instance, cellular respiration turns glucose into pyruvate—and usually releases energy. Anabolism builds compounds up (biosynthesis), such as making proteins, carbohydrates, lipids, and nucleic acids, and generally consumes energy. These reactions are organized into metabolic pathways, where one chemical is changed into another through a series of steps, each sped up by a specific enzyme. Enzymes are essential because they let organisms drive energy-requiring reactions that wouldn't happen on their own, by linking them to spontaneous, energy-releasing reactions. As catalysts, enzymes speed up reactions and also allow the cell to regulate how fast a metabolic reaction occurs, adjusting to changes in the cell's environment or signals from other cells.

An organism's metabolic system decides which substances are nutritious and which are poisonous. For example, some prokaryotes use hydrogen sulfide (H₂S) as a nutrient, even though this gas is toxic to some animals. Yet because H₂S is a gasotransmitter, certain mammals—including humans—produce it naturally in tiny amounts, where it plays vital signaling and regulatory roles. The basal metabolic rate measures the total energy consumed by all these chemical reactions.

A striking feature of metabolism is how similar the basic pathways are across vastly different species. The carboxylic acids that serve as intermediates in the citric acid cycle, for instance, appear in all known organisms, from the bacterium *E. coli* to elephants. This similarity likely comes from these pathways appearing early in evolution and being kept because they work so well. In diseases like type II diabetes, metabolic syndrome, and cancer, normal metabolism gets disrupted. Cancer cells also have a distinct metabolism compared to normal cells, and these differences can be targeted for therapy.

Most structures in animals, plants, and microbes are built from four basic molecule classes: amino acids, carbohydrates, nucleic acids, and lipids (often called fats). Because these molecules are vital for life, metabolic reactions either focus on making them during cell and tissue construction or on breaking them down for energy through digestion. These biochemicals can be linked into polymers like DNA and proteins, the essential macromolecules of life.

Proteins are chains of amino acids joined by peptide bonds. Many proteins are enzymes that catalyze metabolic reactions. Others have structural or mechanical roles, like the cytoskeleton that maintains cell shape. Proteins also matter in cell signaling, immune responses, cell adhesion, active transport across membranes, and the cell cycle. Amino acids contribute to cellular energy metabolism by feeding carbon into the citric acid cycle, especially when glucose is scarce or cells face metabolic stress.

Lipids are the most diverse biochemical group. Their main structural use is in biological membranes, like the cell membrane, and they also store chemical energy. Lipids have a long, non-polar hydrocarbon chain with a small, oxygen-containing polar region. They are usually hydrophobic or amphipathic and dissolve in organic solvents like ethanol, benzene, or chloroform. Fats are a large group containing fatty acids and glycerol; a glycerol molecule linked to three fatty acids by ester bonds is a triacylglyceride. Variations exist, including backbones like sphingosine in sphingomyelin and hydrophilic groups like phosphate in phospholipids. Steroids, such as sterol, are another major lipid class.

Carbohydrates are aldehydes or ketones with many hydroxyl groups, existing as straight chains or rings. They are the most abundant biological molecules, storing and transporting energy (starch, glycogen) and providing structure (cellulose in plants, chitin in animals). The basic units are monosaccharides like galactose, fructose, and most importantly glucose. Monosaccharides can link together into polysaccharides in nearly limitless ways.