the free-energy change (delta G)

A ball on a hill rolls down on its own; to push it back up you have to do work. Chemical reactions have a 'downhill' and an 'uphill' too, and one number tells you which way a reaction will go by itself. That number is the free-energy change, written delta G.

Delta G measures how much usable energy a reaction releases or requires under the conditions in the cell. If delta G is negative, the reaction is downhill (called exergonic): it can proceed on its own and gives off usable energy. If delta G is positive, the reaction is uphill (endergonic): it will not happen on its own and needs an energy input to be pushed forward. If delta G is zero, the reaction is at equilibrium — going equally fast in both directions, doing no net work. Importantly, delta G tells you only the direction and the energy balance, not the speed: a strongly downhill reaction can still be vanishingly slow until an enzyme lowers the barrier in its path.

Delta G is the bookkeeping that makes bioenergetics make sense. Building large molecules and pumping ions uphill have a positive delta G, so the cell pays for them by linking them to a strongly downhill reaction — usually ATP being split — so that the two added together come out negative and proceed. Knowing the sign of delta G lets a biologist predict whether a step needs energy or supplies it, and whether two reactions can be coupled to drive each other. It is the compass for which way the chemistry of life will flow.