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Re: CO2 use at nightime?
>Everybody knows that plants don't use CO2 at night right?
>Not so fast! Thinking about Roger's calculations on CO2 use got me to
>thinking. Plants don't use the CO2 directly, it has to go through
>chemical precursor stages, right? That would imply that plants
>continue to absorb and store CO2 in various chemical forms in
>preparation for the days photosynthesis. What do you think?
CO2 is used directly by plants; there is no chemical precursor or
storage product for CO2. In the chloroplast, CO2 enters into the
Calvin Cycle and is added to a five-carbon compound, ribulose
1,5-bisphosphate. The resulting six-carbon compound is split, giving
two molecules of a three-carbon compound, 3-phosphoglycerate. This
3-carbon compound is reduced and phosphorylated to form the
carbohydrate, triose phosphate. Some of the triose phosphate
molecules are used to form sucrose and starch, while the rest is used
to regenerate ribulose 1,5-bisphosphate needed for the continuation of
the Calvin cycle.
The enzyme responsible for carboxylating ribulose 1,5-bisphosphate is
named, appropriately enough, ribulose bisphosphate carboxylase, or
rubisco, for short. It is a water-soluble protein found in the
aqueous phase of the chloroplast outside of the photosynthetic
As each molecule of CO2 used requires 2 molecules of NADPH and 3
molecules of ATP, the Calvin Cycle can continue as long as there is
available NADPH and ATP (these compounds are re-generated from ADP and
NADP via photosynthesis).
Rubisco is found in photosynthetic and chemosynthetic organisms. I
even found it in bacteria living endosymbiotically within the gill
tissues of a marine lucinid clam. Check out:
Fisher, M. and S. Hand. 1984. Chemoautotrophic symbionts in the
bivalve Lucina floridana from seagrass beds. Biological Bulletin 167:
especially section 5.7, for much more detail on photosynthesis.