metabolism
/ muh-TAB-uh-lizm /
A living cell is like a tiny city that never closes. Food and raw materials come in, finished goods go out, garbage gets hauled away, and the lights stay on around the clock. None of this happens by magic: it takes a constant flow of chemical activity. The whole sum of that activity, every chemical reaction a cell runs to stay alive, is called metabolism.
More precisely, metabolism is the complete set of chemical reactions that occur inside a cell or an organism. These reactions are organized into long connected sequences called pathways, where the product of one reaction becomes the starting material for the next, like an assembly line. Almost every step is helped along by a protein called an enzyme, which speeds the reaction up enormously without being used up itself. Metabolism has two broad directions: building larger molecules from smaller ones, and breaking larger molecules down to release usable energy.
Metabolism matters because it is the difference between a living thing and a pile of chemicals. A rock does not metabolize; you do, every second, even while asleep. The reactions are honestly complex, with thousands of them running at once and tightly regulated so the cell makes what it needs when it needs it. The rest of this field unpacks how cells handle energy specifically, because energy is what powers the entire enterprise.
After you eat a sandwich, your cells break the starch and protein into small pieces, burn some for energy, and use others to build new muscle and repair tissue — all of that, together, is metabolism in action.
Metabolism is the whole network of build-up and break-down reactions, not a single process.
In everyday speech "metabolism" often means how fast you burn calories, but in biology it means the cell's entire chemical network — building included, not just burning.