Geraard Spergen wrote:


Differing chemical reaction rates cannot explain why plants have more
C13 to begin with. You have to explain why C13 is more likely to
become part of a plant than to become part of something else... or
perhaps you could propose that flora C12 is more likely to absorb an
itinerant neutron than fauna C12.

Chemistry is mostly about electrons, photons are absorbed in the
nucleus. I reasoned (perhaps incorrectly) that photosynthesis
involves photons being absorbed by neutrons and that C13 had a higher
cross section for photon absorption than did C12 and that this might
account for plants having a higher proportion of C13 than
non-photosynthesizing organisms. It may be a dumbass theory, but it
can't hold a candle to creationism.


Photosynthesis is about chlorophyll absorbing multiple red photons and
using that sum energy to cleave water and co2 to make sugar, sort of.
Photons in the visible region are mainly absorbed by the electrons in the
bonding orbitals of the molecules (or the outer electrons for atoms (inner
electronic transitions are generally in the x-ray/vuv region for atoms)).
Nuclear spin transitions, I vaguely recall, can be in the visible region,
but their transitions strengths are weak and nearly always visible spectra
of atoms and molecules are determined by the electronic structure. Nuclear
energy transitions are up in the gamma ray region. Anyway, I am too lazy
to look this next bit up but I think it is more or less correct, the thing
about chlorophyll that is interesting is that it absorbs photons and
transfers that energy into vibrational modes. That vibrational energy
builds up in the porphin ring and is then used to initiate breaking apart
of CO2 and H2O. Or something like that anyway.

Biologically produced carbon compounds have *less* of the heavier isotopes,
not more. Isotopic fractionation occurs because in general heavier things
don't move as fast (both in a vibrational sense and in a translational
sense) and it is the kinetic energy of motion, to a large degree, that
makes molecules react. There is sort of a double whammy for plants in that
the CO2 fractionates going through the stoma since the 13CO2 is not as
efficiently transported into the plant to begin with, this is mainly a
translational effect. Once in the plant, the heavier isotope CO2 doesn't
react as readily (which I think is the vibrational effect).

--
Bill Asher