Good Afternoon,

No one in my biology department at university has been able to answer this question and I hope someone here can remedy that situation. What is the origin of ribulose bisphosphate? This is the 5-carbon sugar that is utilized as a substrate by the RuBisCo enzyme (ribulose bisphosphate carboxylase/oxygenase) to bind with CO2 to make an unstable 6-carbon sugar, half of which is used by plants to make glucose in the Calvin cycle (after 2 cycles) but the other 3-C sugar is used to . . . make more ribulose bisphasphate! (after 5 cycles). If my vexation isn't already apparent, if the Calvin cycle needs ribulose bisphosphate for photosynthesis, and it is a product of photosynthesis, where the heck did it come from in the first place? The most I've been able to find out is that it existed in the free bacteria that would eventually evolve into chloroplasts (the organells that perform photosynthesis). So I know how it got into plants in general, but my original question remains unanswered. Is it one of those original organic compounds that is to have come about spontaneously on the early earth? Is it an assemblage of them? Is it an exaptation of some kind? Is it simply unknown?

Thanking you in advance for your information,

Nevyn O'Kane

Ribulose 5-phosphate (RuP) which is needed to produce Ribulose 1,5-biphosphate (RuBP) by the phosphoribulokinase with an input of one ATP is also the end-product of the pentose phosphate pathway that generates NADPH.

The overall reaction of the pentose phosphate pathway can be summarised as:
Glucose 6-phosphate + 2 NADP+ + H2O ? ribulose 5-phosphate + 2 NADPH + 2 H+ + CO2

To answer your question, RuP is generated within the cell.

Last edited by Manabu Sakamoto (15th Mar 2008 15:55:33)