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Difficulty: MediumPlant Nutrition and Photosynthesis

A potted green plant was supplied with water containing oxygen-18 isotope (H218OH_2^{18}O) and placed in a sealed chamber under bright sunlight with unlabeled carbon dioxide (C16O2C^{16}O_2). Which of the following photosynthetic products will contain the 18O^{18}O isotope?

  1. A
    Glucose synthesized during the light-independent stage
  2. Molecular oxygen gas released during photolysisAnswer
  3. C
    Sucrose translocated downward through xylem tissue
  4. D
    Glycerate-3-phosphate formed in the chloroplast stroma

Answer

Molecular oxygen gas released during photolysis
During the light-dependent stage of photosynthesis, light energy absorbed by chlorophyll powers photolysis, the splitting of water molecules (H2OH_2O). Because the water supplied contained isotopic 18O^{18}O, the oxygen gas (O2O_2) evolved as a byproduct is labeled with 18O^{18}O.

Step-by-Step Solution

1
Identify the stage of photosynthesis where water is consumed.
Water (H2OH_2O) is split during photolysis in the light-dependent reaction inside the thylakoid membranes.
Photolysis splits water into hydrogen ions (H+H^+), electrons (e−e^-), and molecular oxygen (O2O_2).
2
Trace the metabolic fate of oxygen atoms from water vs. carbon dioxide.
The oxygen atoms in released O2O_2 gas come exclusively from H2OH_2O, while the oxygen atoms in carbohydrates (such as glucose and glycerate-3-phosphate) come from CO2CO_2.
Isotopic tracing experiments (Ruben and Kamen) proved that all evolved oxygen gas originates from water splitting.

Key Concept

Photolysis of water during the light-dependent stage of photosynthesis
Estimated Time:1m 0s
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Topics

Plant Nutrition and Photosynthesis

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