In chemical reactions where chemicals are being broken-down and energy
is being released this energy is frequently contained in hydrogen ions
and high energy electrons. These can then be used to produce ATP.
Initially the hydrogen ions and electrons are given to NAD
to form NADH2 when these are removed
from the NAD then the energy released can
be used to produce ATP molecules.(this is
usually done using a number of reactions)
This occurs in Respiration when the energy is released from glucose is
converted into ATP using the electron carrier
system