The conversion of one kind of energy into another, e.g. conversion of the chemical energy of ATP into mechanical energy by muscle contraction; also conversion of a signal as it crosses a barrier, e.g. linkage of hormone binding on the outside of a plasma membrane to the generation of a second messenger inside it; also DNA transfer by a virus that can incorporate into its own genome part of the DNA of a first host and then transfer it to a second host. see conjugation; transformation