What happens to the 3-carbon compounds that do not leave the Calvin cycle to be made into organic compounds?

Question

Oliver Antoine · Accepted Answer

The glyceraldehyde-3-phosphate (#"G3P"#) that is not used to make organic compounds is turned into ribulose-1,5-bisphosphate (#"RuBP"#) to be recycled.

In the first two stages of the Calvin cycle — carbon fixation and reduction — 3 molecules of #"RuBP"# and 3 molecules of #"CO"_2# are converted into 6 molecules of #"G3P"#.

(From www.khanacademy.org)

#"3RuBP" + color(white)(l)"3CO"_2 → "6G3P"#
#"3×5C"color(white)(ll) + "3×1C" → "6×3C"#

However, only one of the #"G3P"# molecules goes to make other compounds.

The other five are recycled to regenerate RuBP.

#"5G3P" →color(white)(l) "3RuBP"#
#"5×3C" →color(white)(l) "3×5C"#

The net result is that 3 molecules of #"CO"_2# are converted to 1 molecule of #"GDP"#:

#color(red)(cancel(color(black)("3RuBP"))) + "3CO"_2 → stackrelcolor(blue)(1)color(red)(cancel(color(black)(6)))"G3P"#
#color(red)(cancel(color(black)("5G3P")))color(white)(mmmmm) → color(red)(cancel(color(black)("3RuBP")))#
#stackrel(————————————)("3CO"_2color(white)(mmmmmll) → "1G3P")#

Eli Ayoub · Answer

undefined The regeneration phase of the Calvin cycle recycles the 3-carbon compounds that do not exit the cycle, guaranteeing the cycle's ongoing operation.