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Chapter 21: Problem 14

The overall equation for photosynthetic \(\mathrm{CO}_{2}\) fixation is \\[6 \mathrm{CO}_{2}+6 \mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{O}_{2}\\] \(A l l\) the \(\mathrm{O}\) atoms evolved as \(\mathrm{O}_{2}\) come from water; none comes from carbon dioxide. But \(12 \mathrm{O}\) atoms are evolved as \(6 \mathrm{O}_{2}\), and only \(6 \mathrm{O}\) atoms appear as \(6 \mathrm{H}_{2} \mathrm{O}\) in the equation. Also, \(6 \mathrm{CO}_{2}\) have \(12 \mathrm{O}\) atoms, yet there are only \(6 \mathrm{O}\) atoms in \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} .\) How can you account for these discrepancies? (Hint: Consider the partial reactions of photosynthesis: ATP synthesis, NADP' reduction, photolysis of water, and the overall reaction for hexose synthesis in the Calvin-Benson cycle.)

Short Answer

Expert verified
The discrepancies are explained by the detailed reactions taking place during photosynthesis. The oxygen in \(O_2\) is derived from \(H_2O\) in the process of photolysis where water is split. The carbon dioxide contributes oxygen to the formation of glucose in the Calvin-Benson cycle, accounting for the oxygen atoms in glucose. Therefore, all oxygen atoms are accounted for in these processes.

Step by step solution

01

Understanding the Given Equation

The overall equation for photosynthesis shows an exchange of atoms between the reactants and the products. Carbon dioxide (\(CO_2\)) and water (\(H_2O\)) are transformed into glucose (\(C_6H_{12}O_6\)) and oxygen (\(O_2\)). According to the question, all the oxygen atoms in the oxygen gas produced come from water, and not from carbon dioxide.
02

Inspecting the Partial Reactions of Photosynthesis

In the process of photosynthesis, there are several partial reactions such as ATP synthesis, NADP+ reduction, photolysis of water, and hexose synthesis in the Calvin-Benson cycle. Photolysis of water results in the production of \(O_2\), split from the water molecule. During photolysis, each water molecule also donates 4 electrons to the photosystem which is used in the reduction of NADP+ to NADPH. In the Calvin-Benson cycle, each CO2 molecule contributes one carbon atom to the production of glucose.
03

Accounting for the Discrepancies

In photolysis, only the Oxygen atom from water is released as \(O_2\), and during the Calvin cycle, the Oxygen atom from \(CO_2\) is incorporated into the glucose molecule, which explains why the Oxygen in \(O_2\) comes only from \(H_2O\) and not \(CO_2\) in photosynthesis. The extra Oxygen atoms from CO2 do not disappear but are used in the creation of glucose in the Calvin-Benson cycle.

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