Page 165 - Algae

148 Algae: Anatomy, Biochemistry, and Biotechnology

It is worthwhile to emphasize that any photon that is absorbed by any chlorophyll molecule is
energetically equivalent to a red photon because the extra energy of an absorbed photon of shorter
wavelength (,680 nm) is lost during the quick fall to the red energy level that represents the lowest
excited level.

Proton Transport: Mechanism of Photosynthetic Phosphorylation
The interplay of PSI and PSII leads to the transfer of electrons from H 2 O to NADPH, and the
concomitant generation of proton gradient across the thylakoid membrane for ATP synthesis.
The thylakoid space becomes markedly acidic with pH approaching 4. The light-induced trans-
membrane proton gradient is about 3.5 pH units. The generation of these protons follows
two routes:

. For the splitting of two water molecules and the release of one oxygen molecule four proton
are released in the thylakoid space.
. The transport of four electrons through the cytochrome b 6 f complex leads to the trans-
location of eight protons from the stroma to the thylakoid space.

Therefore about 12 protons for each O 2 molecule released are translocated. The proton-motive
force Dp, that is, the force created by the accumulation of hydrogen ions on one side of the thyla-
koid membrane, consists of a pH gradient contribution and a membrane-potential contribution. In
chloroplasts, nearly all of Dp arises from the pH gradient, whereas in the mitochondria the contri-
bution from the membrane potential is larger. This difference is due to the thylakoid membrane per-
2þ
meability to Cl 2 and Mg . The light-induced transfer of H þ into the thylakoid space is
2 2þ
accompanied by the transfer of either Cl in the same direction or Mg in the opposite direction
þ
(1 per 2H ). Consequently, electrical neutrality is maintained and no membrane potential is gen-
erated. A pH of 3.5 units across the thylakoid membrane corresponds to Dp of 0.22 V or a DG
(change in Gibbs free energy) of 24.8 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 DG associated with a reaction indicates that it can happen spon-
taneously. About three protons flow through the F 0 F 1 -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 appro-
priately positioned for the subsequent light-independent reactions, in which CO 2 is converted into
carbohydrates. The overall reaction can be expressed as:

8 photons and 4e
þ
4NADP þ 2H 2 O þ 4ADP þ 4P i ! 4NADPH þ 4ATP þ O 2 (3:3)
This equation implies that each H 2 O is split in the thylakoids under the influence of the light to give
1
off O 2 molecule, and that the two electrons so freed are then transferred to two molecules of
2
þ
þ
NADP , 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 (P i ) so that the
energy is stored in high energy compounds. NADPH and ATP are the assimilatory power required
to reduce CO 2 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
þ
NADP and do not come back.