Animated Solution for Chemistry - Organic Chemistry: Synthesis of each molecule of glucose in photosynthesis involves

The Calvin Cycle

Imagine the chloroplast as a microscopic factory. While the light-dependent reactions capture solar energy to produce the high-energy molecules $\mathrm{ATP}$ and $\mathrm{NADPH}$, the actual construction of the sugar molecule happens in the stroma during the dark reaction, famously known as the Calvin Cycle.

This cycle is the heart of carbon fixation, where inorganic carbon dioxide from the atmosphere is stitched together to form organic glucose.

A single molecule of glucose, ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$, is a six-carbon sugar. Because each molecule of carbon dioxide (${\mathrm{CO}}_2$) provides only one carbon atom, the Calvin cycle must process exactly six molecules of ${\mathrm{CO}}_2$ to yield one complete glucose molecule.

This means the cycle must effectively "turn" six times, gathering one carbon atom per turn.

ATP and NADPH

Building complex molecules requires a massive investment of energy. For every single ${\mathrm{CO}}_2$ molecule that enters the Calvin cycle, the plant must spend $3$ molecules of $\mathrm{ATP}$ and $2$ molecules of $\mathrm{NADPH}$.

Since we need to fix $6$ molecules of ${\mathrm{CO}}_2$ to build our glucose, we simply multiply these requirements: - Total $\mathrm{ATP}$ required = $6\times 3=18\mathbf{ATP}$ - Total $\mathrm{NADPH}$ required = $6\times 2=12\mathbf{NADPH}$

When we put it all together, the balanced chemical equation for the synthesis of one glucose molecule in a standard ${\mathrm{C}}_3$ plant looks like this:

Thus, the synthesis of each molecule of glucose involves exactly $18$ molecules of $\mathrm{ATP}$.

Pro Tip: Always read the question carefully! If the examiner asks about a ${\mathrm{C}}_4$ plant, remember that they use an additional pathway to concentrate ${\mathrm{CO}}_2$, which costs an extra $2\mathrm{ATP}$ per ${\mathrm{CO}}_2$. Therefore, a ${\mathrm{C}}_4$ plant requires a total of $30\mathrm{ATP}$ to synthesize one glucose molecule.