Carl Pike C_Pike at ACAD.FANDM.EDU
Thu Jan 8 11:25:00 EST 1998

There are two ideas I've heard about, and they're not mutually exclusive.
I don't profess to be an expert in the subject, so I'd welcome any
corrections by others.

1. The oxygenation reaction by rubisco is an inevitable consequence of the
reaction mechanism at the active site.  In the primordial atmosphere, the
oxygen concentration was very low, so the oxygenation didn't occur to any
significant extent.  Thus there would have been no selection pressure for a
carboxylation reaction mechanism that avoided oxygenation.  Now, with
rubisco as the heart of the photosynthetic system, cells are "stuck" with
it.  The complicated cycle of reactions that follows the oxygenase "makes
the best of a bad thing", by recovering much of the carbon that would
otherwise be lost (and indeed may be lost in some algae that just excrete
the 2-carbon products).

2. Under conditions of low carbon dioxide (such as closed stomata) but high
light, the inability to run carboxylation by rubisco would lead to a pile
up of ATP and the inability to utilize the trapped energy in the light
reactions.  This could lead to photoinhibitory damage.  Thus the
oxygenation reaction and the resulting cycle provide a "safety valve" to
allow that energy to be dissipated.

Carl S. Pike                             (717) 291-3958
Department of Biology                    FAX (717) 399-4548
Franklin and Marshall College            Internet  C_PIKE at ACAD.FANDM.EDU
P.O. Box 3003
Lancaster, PA  17604-3003  USA

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