Question

the chemical reaction for photosynthesis is 6co₂ + 12h₂o + light energy c₆h₁₂o₆ + 6o₂ + 6h₂o
ff the input water is labeled with a radioactive isotope of oxygen, ¹⁸o, then the oxygen gas released as the reaction proceeds is also labeled with ¹⁸o. which of the
following is the most likely explanation?
during the light reactions of photosynthesis, water is split, the hydrogen atoms combine with the co₂, and oxygen gas is released.
during the light reactions of photosynthesis, water is split, removing electrons and protons, and oxygen gas is released.
during the calvin cycle, water is split, regenerating nadph from nadp+, and oxygen gas is released.
during the calvin cycle, water is split, the hydrogen atoms are added to intermediates of sugar synthesis, and oxygen gas is released.

Explanation:

During the light reactions of photosynthesis, water (\(H_2O\)) is split. The reaction for this splitting (photolysis) is \(2H_2O
ightarrow4H^{+}+4e^{-}+O_2\). The oxygen gas (\(O_2\)) released comes from the oxygen in water. In the Calvin cycle, water is not split in the way described in the first, second, and fourth options. The first option wrongly mentions hydrogen atoms being added to sugar intermediates during Calvin - cycle water - split (water is not split in the Calvin cycle for this purpose). The second option has an incorrect description of water - split function in the Calvin cycle (NADPH regeneration in Calvin cycle is not related to water splitting). The fourth option incorrectly states that hydrogen combines with \(CO_2\) during a water - split event (hydrogen from water splitting in light reactions is used for \(NADPH\) formation, not directly combining with \(CO_2\) in the described way). Only the third option correctly relates the water - split (which releases electrons, protons, and oxygen gas) to the light reactions.

Answer:

During the light reactions of photosynthesis, water is split, removing electrons and protons, and oxygen gas is released.