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Difficulty: MediumPlant Biology and Photosynthesis

During the process of photosynthesis, green plants capture sunlight to convert inorganic compounds into chemical energy and release oxygen gas. What substance is split by light energy during the light-dependent reactions to produce this released oxygen gas?

  1. A
    Carbon dioxide
  2. WaterAnswer
  3. C
    Glucose
  4. D
    Chlorophyll

Answer

Water (H2OH_2O)
Water (H2OH_2O) is absorbed by plant roots and transported to the chloroplasts. In the thylakoid membranes, light energy triggers photolysis, splitting water into hydrogen ions, electrons, and diatomic oxygen gas (O2O_2), which is released into the atmosphere as a byproduct.

Step-by-Step Solution

1
Identify the primary chemical equation and stages of photosynthesis
The overall reaction is 6CO2+6H2O+light→C6H12O6+6O26CO_2 + 6H_2O + \text{light} \rightarrow C_6H_{12}O_6 + 6O_2, divided into light-dependent reactions and light-independent reactions (Calvin cycle).
Understanding where reactants enter and products exit clarifies the source of oxygen gas.
2
Analyze the light-dependent reaction mechanism
Photons absorbed by chlorophyll drive the photolysis of water (2H2O→4H++4e−+O22H_2O \rightarrow 4H^+ + 4e^- + O_2).
Splitting water supplies replacement electrons to the photosystem electron transport chain and releases molecular oxygen (O2O_2) into the atmosphere.
3
Distinguish the role of water from that of carbon dioxide
Water provides the oxygen atoms released as atmospheric oxygen gas, whereas carbon dioxide provides the carbon and oxygen atoms that end up incorporated into glucose.
Confusing the fates of water and carbon dioxide is a common point of error.

Key Concept

Photolysis of water during the light-dependent reactions of photosynthesis
Estimated Time:45s
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Topics

Plant Biology and Photosynthesis

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