Multiple Choice
When chloroplast pigments absorb light, __________.
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9. Photosynthesis
Introduction to Photosynthesis
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In a plant, which of the following reactions produces molecular oxygen ($\mathrm{O_2}$)?
A
The fixation of carbon dioxide in the Calvin cycle
B
The splitting of water during the light-dependent reactions
C
The synthesis of glucose from triose phosphates
D
The reduction of NADP$^+$ to NADPH
Verified step by step guidance1
Understand the context: The question is asking about the process in plants that produces molecular oxygen ($\mathrm{O_2}$). This is related to photosynthesis, which consists of two main stages: the light-dependent reactions and the Calvin cycle (light-independent reactions).
Recall the role of water in the light-dependent reactions: During these reactions, light energy is absorbed by chlorophyll in the thylakoid membranes of chloroplasts. This energy is used to split water molecules (a process called photolysis). The chemical equation for this is: $2\mathrm{H_2O} \rightarrow 4\mathrm{H^+} + 4\mathrm{e^-} + \mathrm{O_2}$.
Identify the source of molecular oxygen ($\mathrm{O_2}$): The splitting of water (photolysis) releases electrons, protons ($\mathrm{H^+}$), and molecular oxygen ($\mathrm{O_2}$). This is the only step in photosynthesis where $\mathrm{O_2}$ is produced.
Eliminate incorrect options: The Calvin cycle involves the fixation of carbon dioxide ($\mathrm{CO_2}$) into organic molecules, but it does not produce $\mathrm{O_2}$. Similarly, the synthesis of glucose from triose phosphates and the reduction of NADP$^+$ to NADPH occur in photosynthesis but do not involve the production of $\mathrm{O_2}$.
Conclude: The splitting of water during the light-dependent reactions is the correct answer because it directly produces molecular oxygen ($\mathrm{O_2}$) as a byproduct of photolysis.
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