Understand Changes in Photosynthesis Graph

[xRadio]

http://img80.imageshack.us/img80/1017/26119684vg0.jpg

This graph shows changes that occur in the amounts of ribulose 1,5 - bisphosphate (RuBP) and phosphoglyceric acid (PGA) in leaf mesophyll cells during a six minute interval of sunlight with the concentration of carbon dioxide being suddenly diminished at the middle of the interval.

Can anyone help me understand what's going on this graph?

 

[jim mcnamara]

Well, to start with PGA concentrations go down. Can you see why? ie., where does PGA come from? It must have something to do with carbon fixation, right? Why? because you just lost your fixable carbon source when the CO2 levels go to almost zero.

RuBP is used for what? Does it seem like RuBP must be used up in the process that creates PGA? Or not? Hint: Calvin's cycle

 

[jim mcnamara]

Here is what you need to read if none of the above makes sense.
http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/dark.htm

 

[_Mayday_]

Carbon Dioxide is a limiting factor, so the rate of photosynthesis will vary.