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

Jul 16, 2016

The glyceraldehyde-3-phosphate ($\text{G3P}$) that is not used to make organic compounds is turned into ribulose-1,5-bisphosphate ($\text{RuBP}$) to be recycled.

#### Explanation:

In the first two stages of the Calvin cycle — carbon fixation and reduction — 3 molecules of $\text{RuBP}$ and 3 molecules of ${\text{CO}}_{2}$ are converted into 6 molecules of $\text{G3P}$.

$\text{3RuBP" + color(white)(l)"3CO"_2 → "6G3P}$
$\text{3×5C"color(white)(ll) + "3×1C" → "6×3C}$

However, only one of the $\text{G3P}$ molecules goes to make other compounds.

The other five are recycled to regenerate RuBP.

$\text{5G3P" →color(white)(l) "3RuBP}$
$\text{5×3C" →color(white)(l) "3×5C}$

The net result is that 3 molecules of ${\text{CO}}_{2}$ are converted to 1 molecule of $\text{GDP}$:

color(red)(cancel(color(black)("3RuBP"))) + "3CO"_2 → stackrelcolor(blue)(1)color(red)(cancel(color(black)(6)))"G3P"
$\textcolor{red}{\cancel{\textcolor{b l a c k}{\text{5G3P")))color(white)(mmmmm) → color(red)(cancel(color(black)("3RuBP}}}}$
stackrel(————————————)("3CO"_2color(white)(mmmmmll) → "1G3P")