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The internal energy, or E, is the energy contained within a system, such as the energy contained in chemical bonds and the random motion of its particles. A system and its surroundings can transfer energy between them.

The change in internal energy of a system, delta E, is equal to the sum of q, the total heat transferred to the system, and w, the total work done on the system.

In most biochemical reactions, no work is performed; therefore, the change in internal energy is equivalent to another thermodynamic function called enthalpy.

The change in enthalpy of a system or ∆H is accompanied by an equal and opposite change in the enthalpy of the surroundings.

An exothermic process is when the system releases heat to the surroundings and is represented by a negative enthalpy. An endothermic process is when the system gains heat, represented by a positive enthalpy.

Living cells are open systems that exchange matter and energy with their surroundings, through exothermic and endothermic processes, following the first law of thermodynamics.