The internal energy of a biochemical system includes any kind of energy that might be changed by any chemical or biochemical reaction. Examples include the kinetic energy of motion and the energy of vibration and rotation of every atom, molecule and ion in the system. Other examples include all of the energy stored in the chemical bonds between atoms and the energy of noncovalent interactions between molecules and ions. The internal energy of a system is a function of its state. That is, the internal energy depends only on the initial and final states of the system, not on the path taken to get from the initial state to the final state. The thermodynamic state of a system is defined by prescribing the amounts of all substances and any two of the following three system variables:
1) Temperature (T)
2) Pressure (P)
3) Volume (V)
An open system can exchange energy with its
surroundings and may therefore change its internal energy. This
change is called 
E. Internal energy exchanges
can only involve heat (q) or work (w). This is the first law of
thermodynamics: E = q-w, where a positive value
of q indicates heat absorbed by the system from its surroundings
and a positive value of w indicates work is done by the system
on its surroundings. Conversely, a negative value of q means that
heat flows from the the system to its surroundings and a negative
value of w means that the surroundings do work on the system.
When V is changed against a constant P, w =
PV or, using the ideal gas law,
w=