catalyst

Imagine you want to roll a heavy boulder over a hill to the other side. Left alone, the boulder would happily stay in the valley forever, even though the far side is lower and it would be 'happier' down there. The problem is the hill in between. A catalyst is like a helper who digs a tunnel through the hill: the boulder still ends up in the same place, but now the journey is far easier and quicker. The catalyst itself is not used up — it is ready to help the next boulder, and the next.

More precisely, a catalyst is a substance that speeds up a chemical reaction without being consumed or permanently changed by it. It does this by lowering the activation energy — the energy 'hump' that the starting molecules must climb before they can turn into products. The catalyst does not change which reactions are possible or how much energy is released overall; it only changes how fast a reaction that was already going to happen actually happens. A reaction that would take centuries on its own can be sped up millions of times.

In living cells, almost every catalyst is a special protein called an enzyme (a few are made of RNA). Without catalysts, the thousands of reactions that keep you alive — digesting food, copying DNA, releasing energy — would proceed far too slowly to support life at body temperature. The cell cannot simply turn up the heat to speed things up, so it relies on enzymes instead. A common misconception is that a catalyst 'makes a reaction happen that otherwise could not'; in truth it only accelerates a reaction that was already thermodynamically allowed.