Question

How is photosynthesis an example of an oxidation-reduction reaction?

Answer 1

Oxidation involves the addition of oxygen (or) removal of electrons (or) removal of hydrogen. Reduction involves the addition of electrons (or) removal of oxygen (or) addition of hydrogen.

Complete step by step answer:
Photosynthesis is a process used by plants. In photosynthesis, plants that consume carbon dioxide will absorb light energy and it will help for the preparation of glucose. The reaction of photosynthesis can be written as follows,
$6C{O_2} + 6{H_2}O\xrightarrow{{LIGHT}}{C_6}{H_{12}}{O_6} + 6{O_2}$
Let “x” be the oxidation state of carbon.
The oxidation state of carbon in carbon dioxide can be calculated as,
$x - 2(2) = 0$
Since the oxidation number of oxygen is $- 2$
$\Rightarrow x = + 4$
Thus, the oxidation state of carbon in carbon dioxide is $+ 4$
Similarly, the oxidation state of carbon in glucose can be calculated as,
$6x + 12 - 2(6) = 0$
Since the oxidation number of oxygen is $- 2$ and for hydrogen, it is $+ 1$
$\Rightarrow 6x + 12 - 12 = 0$
$\Rightarrow 6x = 0$
$\Rightarrow x = 0$
Thus, the oxidation state of carbon in glucose is zero.
Therefore, carbon dioxide is reduced to glucose since the oxidation number of carbon changes from $+ 4(carbon dioxide) \to 0(glu\cos e)$. Thus, reduction takes place in photosynthesis.
Let “y” be the oxidation state of oxygen.
The oxidation state of oxygen in water can be calculated as,
$2(1) + y = 0$
Since the oxidation state of hydrogen, it is $+ 1$
$\Rightarrow y = - 2$
Thus, the oxidation state of oxygen in water is $- 2$
We know that a molecule has an oxidation number of zero. Thus, the oxygen molecule has oxidation number zero.
Therefore, water is oxidized to oxygen molecules since the oxidation number of oxygen changes from $- 2(water) \to 0(oxygen. molecule)$ . Thus, oxidation takes place in photosynthesis.
From the above definitions and all, it is clear and confirms that both oxidation and reduction take place in photosynthesis.

Note: The reaction is said to be redox if both oxidations, as well as reduction, take place simultaneously. Oxidizing agents are the compound that has the ability to oxidize other compounds. And reducing agents the compound that has the ability to reduce other compounds.