Re: CO2 and O2 diffusion

>From: Craig Bingman <cbingman at netcom_com>

>> (Paul K): (1) When the water is still, is the CO2 diffusing in faster
>>than >>the O2 can diffuse out?  It would seem so, because of the
>>production of >>bubbles.
> (Craig Bingman): Both oxygen and carbon dioxide diffuse in still water.

(Paul): What I meant to say was, is the CO2 diffusing into the plant faster
than the O2 can diffuse out of the plant?

>> (2) when the water is moving over the leaves, shouldn't it allow the CO2 to
>> diffuse in even faster than it was before?  And, if this happens, wouldn't
>> you have an increase in O2 production, and still have production of
>> bubbles?  My hypothesis is that photosynthesis was already CO2-saturated in
>> the stagnent condtion, and so increasing availability of CO2 by decreasing
>> the boundary layer did not increase the rate of photosynthesis. (Paul)
>But increasing the flow rate will also carry away oxygen more readily, so
>it isn't a good test to compare bubble production in stagnant and
>moving water conditions. (Craig)

Let's say you had a situation where the plant was not CO2 saturated under
stagnent conditions,and was producing bubbles, and you then circulated the
water.  The decreased boundary layer would allow greater CO2 uptake in this
case.  If the rate of CO2 uptake by the plant increased as much as the rate
of oxygen loss by diffusion, then you should still get bubbles. That was my
thinking, but now I see a flaw in that argument.  Since I get no bubbles
with water movement and a very good supply of CO2, I should not expect to
see any bubbles with water movement and a lower supply of CO2 and a lower
rate of photosynthesis.  The conclusion is that bubble production should
cease at all CO2 levels when the water is circulated.

Rates of CO2 uptake could be tested by measuring bubble production under
stagnent conditions where you varied the CO2 content. Actually, there are
better ways to measure CO2 uptake than bubble production.  It would be much
more accurate to have a plant in one bottle and no plant in a control
bottle with the same amount of water, expose both to light for an hour, and
then titrate both solutions with a weak base.  The difference between the
control titration (no plant) and the titration of the solution with the
plant would represent the amount of CO2 taken up.

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174
Goofing off instead of working in pleasant, although cooler, Jackson,