A Remarkable Conflict Between Biochemical Dogma And Radical Concept : Traditional Concept : By the 1950s it had been clearly established that oxidative phosphorylation involved the stepwise transfer of electrons through a series of carriers to molecular oxygen. But how the energy derived from these electron transfer reactions was converted to ATP remained a mystery. The general assumption was that ADP was converted to ATP by direct transfer of high-energy phosphate groups from some other intermediate. Thus it was postulated that high-energy intermediates were produced as a result of electron transfer reactions and that these intermediates drove ATP synthesis by phosphate group transfer. Concept Proposed By Peter Mitchell : The fundamental proposal of the chemiosmotic hypothesis was that the “intermediate” that coupled electron transport to ATP synthesis was a proton electrochemical gradient across the membrane. Mitchell postulated that such a gradient was produced by electron transport and that the flow of protons back across the membrane in the energetically favorable direction was then coupled to ATP synthesis

1. Chemiosmotic Theory
A Remarkable Conflict Between Biochemical
Dogma And Radical Concept
Presented By
Sukhendu Roy Chowdhury

2. Traditional Concept
By the 1950s it had been clearly
established that oxidative
phosphorylation involved the stepwise
transfer of electrons through a series of
carriers to molecular oxygen. But how
the energy derived from these
electron transfer reactions was
converted to ATP remained a mystery.
The general assumption was that ADP
was converted to ATP by direct
transfer of high-energy phosphate
groups from some other intermediate.
Thus it was postulated that high-energy
intermediates were produced as a
result of electron transfer reactions and
that these intermediates drove ATP
synthesis by phosphate group transfer.
Concept Proposed By
Peter Mitchell
The fundamental proposal of the
chemiosmotic hypothesis was that the
“intermediate” that coupled electron
transport to ATP synthesis was a proton
electrochemical gradient across the
membrane. Mitchell postulated that
such a gradient was produced by
electron transport and that the flow of
protons back across the membrane in
the energetically favorable direction
was then coupled to ATP synthesis
Mitchell’s hypothesis was greeted with skepticism and remained the subject of acrimonious
debate for more than a decade.

3. In Plants, Chloroplast Is the Site
of Photophosphorylation
Fig. Schematic picture of the overall
organization of the membranes in the
chloroplast.
Oxidative Phosphorylation
Occurs In Mitochondria
Fig. Three-dimensional representation of a
mitochondrion

4. Chemiosmotic Hypothesis To Chemiosmotic Theory
Chemiosmotic mechanism of photosynthetic ATP formation was provided by
an elegant experiment carried out by Andre Jagendorf and coworkers . They
suspended chloroplast thylakoids in a pH 4 buffer, and the buffer diffused
across the membrane, causing the interior, as well as the exterior, of the
thylakoid to equilibrate at this acidic pH. They then rapidly transferred the
thylakoids to a pH 8 buffer, thereby creating a pH difference of 4 units across
the thylakoid membrane, with the inside acidic relative to the outside. They
found that large amounts of ATP were formed from ADP and Pi by this process,
with no light input or electron transport. This result supports the predictions
of the chemiosmotic hypothesis.

5. Mechanisms Of Electron Transport In Chloroplast
The transfer of electrons and protons in the thylakoid
membrane is carried out vectorially by four protein
complexes. Water is oxidized and protons are released in
the lumen by PSII. PSI reduces NADP+ to NADPH in the
stroma, via the action of ferredoxin (Fd) and the
flavoprotein ferredoxin–NADP reductase (FNR). Protons
are also transported into the lumen by the action of the
cytochrome b6f complex and contribute to the
electrochemical proton gradient. These protons must
then diffuse to the ATP synthase enzyme, where their
diffusion down the electrochemical potential gradient is
used to synthesize ATP in the stroma. Reduced
plastoquinone (PQH2) and plastocyanin transfer electrons
to cytochrome b6 f and to PSI, respectively.

6. Basic Feature Of The Chemiosmotic Coupling Conception Is A
Membrane Located ‘ATPase’ System
The ATP is synthesized by a large (400 kDa) enzyme complex known by ATP synthase or
ATPase (after the reverse reaction of ATP hydrolysis), and CFo–CF1 . This enzyme consists of
two parts : a hydrophobic membrane-bound portion called CFo and a portion that sticks out
into the stroma called CF1.
Fig. Structure of ATP synthase.
Proton Motive Force
A difference of solute concentration between two compartments separated
by a biological membrane generates a tendency to equilibrium, which, in the
case of protons, is called proton motive force (PMF). This is measured in
terms of the potential energy resulting from the difference in concentration
between the two compartments. With a charged solute, the potential has
two components: the chemical (concentration) and the electrical potentials.
thus, it is named electrochemical potential.

7. Similarities Of Photosynthetic And
Respiratory Electron Flow In Bacteria,
Chloroplasts, And Mitochondria
Fig. In all three, electron flow is coupled to proton translocation,
creating a transmembrane proton motive force (∆p). The energy in the
proton motive force is then used for the synthesis of ATP by ATP
synthase. (A) A reaction center in purple photosynthetic bacteria
carries out cyclic electron flow, generating a proton potential by the
action of the cytochrome bc1 complex. (B) Chloroplasts carry out
noncyclic electron flow, oxidizing water and reducing NADP+. Protons
are produced by the oxidation of water and by the oxidation of PQH2
(Q) by the cytochrome b6 f complex. (C) Mitochondria oxidize NADH to
NAD+ and reduce oxygen to water. Protons are pumped by the enzyme
NADH dehydrogenase, the cytochrome bc1 complex, and cytochrome
oxidase. The ATP synthases in the three systems are very similar in
structure.

8. In 1978 Peter Mitchell was awarded the Nobel Prize in Chemistry for his contribution to the
understanding of biological energy transfer through the formulation of the chemiosmotic
theory.
Peter Mitchell took a philosophical view of his revolutionary proposal:
“In the exact sciences, cause and effect are no more than events linked in sequence. Biochemists
now generally accept the idea that metabolism is the cause of membrane transport. The
underlying thesis of the hypothesis put forth here is that if the processes that we call metabolism
and transport represent events in a sequence, not only can metabolism be the cause of transport,
but also transport can be the cause of metabolism”

9. References
• The Cell A Molecular Approach EIGHTH EDITION , Geoffrey M. Cooper,BOSTON UNIVERSITY.
• Plant Physiology,Fifth Edition, Lincoln Taiz Professor Emeritus University of California, Santa Cruz Eduardo
Zeiger Professor Emeritus University of California, Los Angeles .