anabolism and catabolism

Imagine a workshop that does two opposite jobs. In one corner, workers take small parts and assemble them into furniture; in the other corner, workers take old furniture apart for the spare parts and the bit of stored energy locked inside. A cell runs both kinds of work at once, and the two together make up its metabolism.

Anabolism is the building side: it joins small molecules into larger ones, such as linking amino acids into proteins or sugars into starch. Building costs energy, so anabolism spends energy. Catabolism is the breaking-down side: it splits large molecules into smaller ones, such as digesting starch back into sugar or burning sugar for fuel. Breaking apart releases energy, so catabolism captures energy. The energy released by catabolism is what pays for the building work of anabolism, usually carried between them by a shuttle molecule called ATP.

Keeping these two in balance is central to life. A growing child or a healing wound leans toward anabolism; a sprinter or a starving body leans toward catabolism. A common mix-up is to think one is good and the other bad — both are essential, and a healthy cell constantly does both, switching emphasis depending on what it needs.