Chapter 8, Problem 1QC

Summary Introduction

To determine:

How to produce carbohydrates containing heavy oxygen ( ${}^{18}\text{O}$) isotope, whether by watering the plants with ${\text{H}}_{\text{2}}{}^{\text{18}}\text{O}$ or inject ${\text{C}}^{\text{18}}{\text{O}}_2$ into the air.

Introduction:

The ATP and NADPH produced in the light reactions drive the second stage of photosynthesis, the Calvin cycle. In Calvin cycle, the ${\text{CO}}_{\text{2}}$ is incorporated into organic compounds, a process called carbon fixation. A total of three carbon dioxide molecules must enter the Calvin cycle to produce carbohydrates.

Explanation of Solution

Calvin cycle occurs in stroma and it uses $\text{ATP}$ and $\text{NADPH}$ to convert ${\text{CO}}_{\text{2}}$ to carbohydrates. Each turn of Calvin cycle fixes one carbon. The actual sugar product of the Calvin cycles is not glucose, but the three-carbon sugar, glyceraldehydes-3-phosphate. For the net synthesis of one glyceraldehyde-3-phospahte molecule, the cycle must take place three times, fixing three molecules of ${\text{CO}}_{\text{2}}$. To make one glucose molecule would require six cycles and fixation of six ${\text{CO}}_{\text{2}}$ molecules.

The water is used in cycles only for the reduction reactions as when water molecules splits it provides electrons. To produce carbohydrates containing the heavy oxygen ( ${}^{18}\text{O}$) isotope, the plants should injected with ${\text{C}}^{\text{18}}{\text{O}}_2$ into air.

Conclusion

${\text{C}}^{\text{18}}{\text{O}}_2$ should be injected into air to produce the carbohydrates containing heavy oxygen

( ${}^{18}\text{O}$).