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photosynthetic CO2 sources

Bill Williams WEWilliams at osprey.smcm.edu
Fri Mar 10 11:12:35 EST 2000

Doug et al.,

It IS hard to imagine why the water doesn't keep out the CO2, isn't 
it? First, 14C tagging experiments dating all the way back to Melvin 
Calvin definitely identify the source of fixed carbon as the 
atmosphere surrounding the leaves. The "flow" of H2O vapor out of the 
leaf does make it more difficult for the CO2 to get in to the leaf, 
but remember that it's diffusion we're talking about here, and 
there's generally a lot of space between molecules, even in a medium 
as dense as air.  Thus, the outward diffusion of water molecules has 
only a small effect, though for a precise calculation of the CO2 
concentration in a leaf's intercellular spaces one does have to 
correct for this effect. For more on this, see

Jarman PD. 1974. The diffusion of carbon dioxide and water vapour 
through stomata. J. of Exp . Bot.  Vol. 25, No. SS

von Caemmerer S, Farquhar GD. 1981. Some relationships between the 
biochemistry of photosynthesis and the gas exchange of leaves. Planta 

Leuning R. 1983. Transport of gases into leaves. Plant, Cell and 
Environment 6:181-194.

Pearcy RW, Ehleringer J, Mooney HA, Rundel PW, editors. 1989. Plant 
Physiological Ecology: Field methods and instrumentation. London: 
Chapman and Hall. 457 p.

At 14:03 +0000 3/10/00, Douglas Jensen wrote:
>     As I get into the physiological portion of my General Botany course,
>I am coming to a quandry that has bugged me for a few years.  I guess
>it's time to show my physiological phailures publicly and to request
>expert advice.
>     The problem is this.  Water transport is generally powered by
>transpiration.  This means that there is a constant flow of vapor out
>the stomata when they are open.  However, we also teach that the carbon
>dioxide for photosynthesis comes in through the stomata at the same
>time.  I have a difficult time envisioning significant diffusion of CO2
>going against a flow of water vapor.  Is there good support for an
>atmospheric source of CO2?   Could enough carbon be transported through
>the xylem as carbonate?  On the other hand, am I worrying about two
>processes that do not interact significantly?
>Douglas P. Jensen
>Assistant Professor of Biology
>Converse College
>580 East Main Street
>Spartanburg, South Carolina 29302
>fax: (864)596-9201

William E. Williams
Biology Department
Saint Mary's College of Maryland
18952 E. Fisher Rd.
Saint Mary's City, MD  20686-3001

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