Chapter 7, Problem 1AC

Suppose an experiment is performed in which plant I is supplied with normal carbon dioxide but with water that contains radioactive oxygen atoms. Plant II is supplied with normal water but with carbon dioxide that contains radioactive oxygen atoms. Each plant is allowed to perform photosynthesis, and the oxygen gas and sugars produced are tested for radioactivity. Which plant would you expect to produce radioactive sugars, and which plant would you expect to produce radioactive oxygen gas? Explain why.

Summary Introduction

To describe:

Among the two types of plants whether plant I (grown with normal carbon dioxide and water containing radioactive oxygen) or plant II (grown with normal water and carbon dioxide containing radioactive oxygen) will produce radioactive sugars and whether plant I or plant II will produce radioactive oxygen gas.

Introduction:

The photosynthesis can be defined as the process through which the plants or the autotrophs produce food using the carbon dioxide, water, and sunlight. The photosynthesis inside the chloroplast takes place in two different stages, light reactions and carbon reactions or dark reactions.

Explanation of Solution

In an experiment Plant I is grown in normal carbon dioxide but with water containing radioactive oxygen atoms and plant, II is grown in radioactive oxygen in carbon dioxide and with normal water.

As the oxygen is released from photolysis of water which takes place during the photosynthesis, plant I will release oxygen-containing radioactivity. Photolysis can be defined as the splitting of the water molecule by the enzymes which are present in photosystem II. Photolysis facilitates the continuous supply of electrons to photosystem II. The overall reaction of photolysis can be shown as below:

${\text{2H}}_{\text{2}}{\text{O}}^{*}+\text{Light photon}\to {\text{4H}}^{\text{+}}+{\text{O}}_{\text{2}}{}^{*}+{\text{4e}}^{-}$

Where,

${\text{H}}_{\text{2}}{\text{O}}^{*}$ is radioactive water

${\text{H}}^{\text{+}}$ is hydrogen ions

${\text{O}}_{\text{2}}{}^{*}$ is radioactive oxygen

${\text{e}}^{-}$ is electron

The sugars are formed in the dark reactions or carbon reactions. During this phase, the plants absorb carbon dioxide which combined with $\text{RuBP}$(ribulose bisphosphate) to form PGA (phosphoglyceric acid). PGA forms G3P (glyceraldehyde-3 phosphate) which in end of cycle combines with another G3P to form glucose. So, carbon dioxide indirectly forms sugars. As the second plant is grown in presence of radioactive carbon dioxide, it will produce radioactive sugars. The overall reactions of Calvin cycle are represented as follows:

${\text{3CO}}_{\text{2}}+\text{9ATP}+\text{6NADPH}\to \text{G3P}+\text{9ADP}+{\text{6NADP}}^{\text{+}}$

$\text{G3P}+\text{G3P}\to \text{Glucose}$

Where,

${\text{CO}}_{\text{2}}$ is carbon dioxide

$\text{ATP}$ is adenosine triphosphate

$\text{NADPH}$ is nicotinamide adenine dinucleotide phosphate

$\text{G3P}$ is glyceraldehydes-3 phosphate

$\text{ADP}$ is adenosine diphosphate

Conclusion

Since, the oxygen is released from the photolysis of water molecule, the plant I (grown with normal carbon dioxide and water containing radioactive oxygen) will produce radioactive oxygen. The sugars are formed from the combining of carbon dioxide with $\text{RuBP}$, so the plant II (grown with normal water and carbon dioxide containing radioactive oxygen) will produce radioactive sugars.