Question

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

Answer 1

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

Explanation:

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")`