Therefore about 12 protons for each O2 molecule released are translocated. The proton-motive
force Δp, that is, the force created by the accumulation of hydrogen ions on one side of the thylakoid
membrane, consists of a pH gradient contribution and a membrane-potential contribution. In
chloroplasts, nearly all of Δp arises from the pH gradient, whereas in the mitochondria the contribution
from the membrane potential is larger. This difference is due to the thylakoid membrane permeability
to Cl- and Mg2+. The light-induced transfer of H+ into the thylakoid space is
accompanied by the transfer of either Cl- in the same direction or Mg2+ in the opposite direction
(1 per 2H+). Consequently, electrical neutrality is maintained and no membrane potential is generated.
A pH of 3.5 units across the thylakoid membrane corresponds to Δp of 0.22 V or a ΔG
(change in Gibbs free energy) of -4.8 kcal/mol. The change in Gibbs free energy associated
with a chemical reaction is a useful indicator of whether the reaction will proceed spontaneously.
This energy is called free energy because it is the energy that will be released or freed up to do work.
As the change in free energy is equal to the maximum useful work which can be accomplished by
the reaction, then a negative ΔG associated with a reaction indicates that it can happen spontaneously.
About three protons flow through the F0
-ATPase complex per ATP synthesized,
which corresponds to a free energy input of 14.4 kcal/mol of ATP, but in which only 7.3 kcal
are stored in the ATP molecule, a yield of about 50%. No ATP is synthesized if the pH
gradient is less than two units because the gradient force is then too small. The newly synthesized
ATP is released into the stromal space. Likewise, NADPH formed by PSI is released into the
stromal space. Thus ATP and NADPH, the products of light reactions of photosynthesis, are appropriately
positioned for the subsequent light-independent reactions, in which CO2
is converted into
carbohydrates. The overall reaction can be expressed as:
This equation implies that each H2
O is split in the thylakoids under the influence of the light to give
molecule, and that the two electrons so freed are then transferred to two molecules of
, along with H+
s, to produce the strong reducing agent NADPH. Two molecules of ATP
can be simultaneously formed from two ADP and two inorganic phosphates (Pi) so that the
energy is stored in high energy compounds. NADPH and ATP are the assimilatory power required
to reduce CO2
to carbohydrates in the light-independent phase. The generation of ATP following
this route is termed non-cyclic phosphorylation because electrons are just transported from water to
and do not come back.
An alternative pathway for ATP production is cyclic phosphorylation, in which electrons from
PSI cycle in a closed system through the phosphorylation sites and ATP is the only product formed.
Electron arising from P700
are transferred to ferredoxin and then to the cytochrome b6f
Protons are pumped by this complex as electrons return to the oxidized form of reaction center
through plastocyanine. This cyclic phosphorylation takes place when NADP is unavailable
to accept electrons from reduced ferredoxin because of a very high ratio of NADPH to NADP+
The electrochemical potential of the proton gradient drives the synthesis of ATP through an
ATP-synthase situated, as we have seen, anisotropically in the thylakoid membrane.