Question

Photosynthesis converts solar energy into the chemical energy of a carbohydrate by?

• A. Reducing CO$_2$ and oxidizing H$_2$O
• B. Reducing H$_2$O and oxidizing CO$_2$
• C. Reducing the carbohydrate
• D. Reducing glucose and water

Hint:

• Photosynthesis is a redox reaction powered by light.
• Water (H$_2$O) is the electron donor; it gets oxidized to O$_2$.
• Carbon Dioxide (CO$_2$) is the electron acceptor; it gets reduced to carbohydrate (glucose).
• Remember OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.

Answer 1

The Correct Option is A

The overall equation for photosynthesis is: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light Energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 \text{ (glucose)} + 6\text{O}_2$. This is a redox process:

• Oxidation of Water (H$_2$O): During the light-dependent reactions, water molecules are split, donating electrons. Water is oxidized to oxygen gas (O$_2$). The oxygen atoms in H$_2$O (oxidation state -2) lose electrons and become O$_2$ (oxidation state 0). $2\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\text{H}^+ + 4e^-$
• Reduction of Carbon Dioxide (CO$_2$): During the light-independent reactions (Calvin cycle), carbon dioxide molecules are reduced to form carbohydrates like glucose. CO$_2$ accepts electrons (originally from water, via electron carriers like NADPH). The carbon atom in CO$_2$ (oxidation state +4) gains electrons to become part of glucose (average oxidation state of C is 0).

Therefore, solar energy drives the oxidation of water and the reduction of carbon dioxide. Option (a) correctly states this. Option (b) describes the reverse (akin to respiration). Options (c) and (d) involve products or misidentify the redox processes. \[ \boxed{\text{Reducing CO}_2 \text{ and oxidizing H}_2\text{O}} \]