Biology Reference
In-Depth Information
Fig. 11.37 The
chemiosmotic hypothesis
proposed by P. Mitchell in
1961 (adopted from Mitchell
1961)
(L)
(R)
O + H
+
2 H
2 e
-
2 H
+
OH
-
2 (ADP + P)
-
2 H
2
O
2 OH
-
H
+
2 ATP
NADH and succinate during respiration. His basic idea is that the
chemical
energy of,
say, NADH is first converted into the
osmotic
energy (hence the adjective “chemios-
motic”) of the proton gradient across the mitochondrial inner membrane (inside high
pH and outside low pH) and associated membrane potential (inside negative and
outside positive) which subsequently drives the synthesis of ATP (Scheme (
11.55
)):
1
2
ð
Chemical Energy of NADH
Þ >
ð
Proton Gradient
Þ>
ð
ChemicalEnergy of ATP
Þ
(11.55)
where
Process 1
indicates the translocation of protons across the mitochondrial
inner membrane driven by respiration (see the upper box in Fig.
11.37
below), and
Process 2
indicates the proton gradient-driven phosphorylation of ADP to ATP (see
the lower box in Fig.
11.37
). The key postulates of the Mitchell hypothesis are as
follows:
1. The membrane-embedded respiratory enzymes (symbolized by the upper box in
Fig.
11.37
)
somehow
separate the electron (indicated by e
the encircled nega-
tive charge) and the proton (H
+
) from the hydrogen atom (H) and move the
former across the membrane (from the left side, L, to the right side, R), leading
to the generation of a transmembrane proton gradient and a membrane potential
(not shown) and attendant acidification of the L compartment and alkalinization
of the R compartment.
2. The osmotic energy stored in the proton gradient (also called the electrochemical
gradient of protons or the “proton-motive force”, PMF) then drives the abstrac-
tion of the hydroxyl ion (OH
) from the L compartment and the proton from the
R compartment to effectuate the synthesis of ATP from ADP and P
i
at the
reaction center embedded inside the M phase (see the lower box in
