Bioenergetics and ATP

Bioenergetics is the study of how living cells change and use energy. It tells us the direction a reaction can go, but not how fast it goes or by what steps.

The first law of thermodynamics: energy can be neither created nor destroyed, only changed from one form into another. The change in a system's energy, ΔE = Q - W (heat taken in minus work done), depends only on its starting and ending states.

The second law: the entropy (disorder) of the universe always increases. How then do living things stay so orderly? They take in ordered energy (food or light) and give out heat and disorder to their surroundings. So their own order is paid for, and the total entropy still rises. Living things do not break the second law.

J. Willard Gibbs (1878) joined the two laws in one equation: ΔG = ΔH - TΔS. ΔG is the useful energy available to do work (free energy). If ΔG is negative, the reaction can happen on its own. The TΔS part is energy that cannot do work.

ATP is the cell's universal currency of energy, like money. It is earned when food is broken down (or, in plants, using light): ADP and phosphate (Pi) are joined to make ATP. It is spent on work: building new molecules, active transport across membranes, muscle contraction, and copying genes accurately. Splitting ATP into ADP and Pi releases 7.3 kcal per mole.