Why does the oxygen produced in the photosynthesis come from water and not carbon dioxide?

Question

In the photosynthesis equation:$$\\ce{6CO2 + 6H2O ->[sunlight] C6H12O6 + 6O2}$$The only place where we have 6 molecules of $\\ce{O2}$ is in $\\ce{6CO2}$. Then it reacts with $\\ce{6H2O}$ to form $\\ce{C6H12O6}$ and $\\ce{6O2}$ that apparently comes from $\\ce{CO2}$. So why do we say that the $\\ce{O2}$ produced by plants comes from $\\ce{H2O}$ and not $\\ce{CO2}$? I don't know if I'm the one who's understanding something wrong or is it the photosynthesis formula which is wrong?

Answer 1

To understand this, you have to understand the biochemistry of photosynthesis. To keep it simple, photosynthesis has two pathways: a light dependent and a light-independent. The light dependent pathway occurs in the thylakoid of the chloroplasts in plants. Through a series of protein complexes, water is reduced to form ATP and NADPH, two products that will make the carbohydrates in the light independent pathway. When water is reduced, O₂ is formed. It is this step, involving a protein complex in the light dependent pathway where we get the phrase, "oxygen comes from water".

The CO₂, in contrast, is exclusively used in the light indepedent pathway, and eventually becomes carbohydrates.