Re: How do plants in closed terrariums manage to get CO2 for photosynthesis?

Hi, 

I'd like to combine the first two questions together. 

All thephotosynthetic plants and other eukaryotes, reduce CO2 to  
cabohydrate via the same basic mechanism, the photosynthetic carbon 
reduction cycle, originally described as C3 cycle--- Calvin Cycle.  Other 
photosynthetic pathways may use C4 cycle, which is associated with C3 
cycle.  CO2 enters the Calvin cycle caltalyzed by the chloroplast enzyme 
Rubisco, which is very important for photosynthesis.

The low concentration does considerablly constrain the carboxylation of 
plants, because the O2 will compete with CO2 for the reaction substrate of 
Rubisco. But there're two proporties of plant's carboxylation very 
important: first, the negative change in free energy associated with the 
frist step of carboxylation ( if the free energy of a reaction is negative, 
this reaction can process without catalyze, that means easier); secondly, 
the Rubisco is the most aboundant enzyme of plants, and its concentration 
is 500 times of that of CO2. So the affinity of Rubisco to CO2 is sufficiently 
high to ensure reapic caboxylation.  But I am not sure the meaning of 
"closed terrariums" in your question; if it is only consisted of plants and 
completely isolated from outer air, I think the plants can not survive long 
time, because the CO2 released from respiration of plants is much lower 
than that reduced by photosynthesis. 

About the palnts without green leaves, I think they can be devided into 
two kinds: autophyte and heterophyte. The fromer may have or have no 
chloroplast, but they have apoplast similar to choloplast, and they finish 
light reaction of photosynthesis through pigments other than chlotophyll 
a ( maybe also include chlorophyll b), which is aboundant in green plants, 
insteatly they use carotenoid and phycobilin.  These pigments enable 
light reaction, but not efficient as chlorophylls.  That's why there are so 
many green plants on the earth.  For the later, they do not have plasts for 
photosynthesis, and will absorb organic nutrient from outer environment, 
just as animals do. 

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Admin note:

D. Hershey adds the following:

The earth is a closed ecosystem. The amount of carbon on earth and, hence
carbon dioxide, is ultimately finite so it is continously recycled. For
example, bacteria and fungi decompose dead organisms and release carbon
dioxide, animals and plants release carbon dioxide via cellular
respiration, and photosynthetic organisms fix the carbon dioxide via
photosynthesis. All these processes are part of the carbon cycle described
in most biology textbooks.

A closed terrarium is a closed ecosystem just on a much smaller scale. The
supply of carbon dioxide in the terrarium air is very small because of the
limited volume of air.  Therefore, unless the terrarium contains animals
to provide carbon dioxide or microbes break down large amounts of organic
matter in the soil and release carbon dioxide, the plants in a sealed
terrarium cannot gain significant dry weight. They would basically refix
the same carbon dioxide over and over. The carbon dioxide the plants
release during cellular respiration would be refixed via photosynthesis,
then respired, refixed, respired, refixed, etc.

Most terrariums are not completely sealed so carbon dioxide can enter and
plants can gain dry matter via photosynthesis. The inventor of the
terrarium, Nathaniel Bagshaw Ward (1791-1868), supposedly had a "sealed
terrarium" that survived four years but it is questionable if it was
completely sealed because of the substantial plant growth he observed.

Another attempt at a closed terrarium on a larger scale was the big
Arizona greenhouse called Biosphere Two . It was
originally supposed to be a sealed terrarium for two years containing
several people, many animals and all sorts of plants. However, it got out
of balance, and they had to add more oxygen because the amount of
photosynthesis was less than required to supply adequate oxygen to all the
animals.

References:

Hershey, D.R. 1996. Doctor Ward's accidental terrarium. American Biology
Teacher. 58:276-281.