Biology Reference
In-Depth Information
which create more of the cycle intermediates on each iteration (Gánti presents
the malate cycle as an example of such a cycle) (Scheme 4):
Malate
A + X
1
2A + Y
Scheme 4
To appreciate the significance Gánti attaches to such autocatalytic cycles,
we need to bring in the second subsystem of a chemoton, the membrane.
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For Gánti, the membrane not only isolates the autocatalytic system (insuring,
for example, that the concentration of intermediates is sufficient that ordinary
diffusion will bring reactants together) but also allows for the control of admis-
sion and expulsion of materials from the system. (Insofar as it is a selective
semi-permeable barrier, the membrane itself is a sophisticated and complex
mechanism - a nontrivial component for the system to build and maintain.) It is
critical for Gánti's account that the chemoton creates its own membrane, and to
explain how it might do this, Gánti further amends his account of the metabolic
cycle so that it not only generates more intermediates of the cycle but also
components of the membrane, which Gánti represents as
T
(as Gánti has now
moved into the realm of a purely theoretical cycle, he designates it simple as A)
(Scheme 5):
A
1
A + X
2A + T + Y
Scheme 5
Assuming that the membrane-bound system naturally takes the shape of a
sphere, Gánti notes that such a stoichiometric relation would lead to the mem-
brane increasing more rapidly than the volume of metabolites enclosed. The
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The membrane was not part of Gánti's initial account (see Griesemer & Szathmáry, forthcoming) and was
introduced only as he recognized a need, when dealing with reactions occurring in a fluid milieu, to keep
reaction components together in sufficiently high concentrations. Moreover, Gánti's account underplays the
role membranes play in actual living cells - they provide not only a way to create distinct environments,
but also a potent tool for energy storage. In oxidative metabolism, for example, a differential concentration
of protons across a membrane, as a result of the oxidations along the electron-transport system, results in a
proton-motive force that then drives the synthesis of ATP.
