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Difficulty: MediumPlant Nutrition and Photosynthesis

During a plant physiology experiment, a potted plant is supplied with carbon dioxide containing radioactively labeled oxygen-18 (C18O2\text{C}^{18}\text{O}_2) while exposed to bright sunlight. In which of the following photosynthetic products will the radioactive 18O^{18}\text{O} isotope primarily be detected?

  1. Glucose synthesized during the light-independent reactions in the stromaAnswer
  2. B
    Oxygen gas released into the atmosphere during the photolysis of water
  3. C
    Mineral salts translocated downward through xylem vessels from the leaves
  4. D
    Molecular oxygen evolved as a direct byproduct of cyclic photophosphorylation

Answer

The radioactively labeled oxygen (18O^{18}\text{O}) from carbon dioxide will primarily appear in glucose synthesized during the light-independent reactions (Calvin cycle).
Carbon dioxide enters the light-independent stage (Calvin cycle) where it combines with ribulose 1,5-bisphosphate (RuBP). The oxygen atoms present in CO2\text{CO}_2 form part of the backbone of 3-phosphoglycerate and are subsequently reduced into organic carbohydrates like glucose. Therefore, isotopic 18O^{18}\text{O} supplied as C18O2\text{C}^{18}\text{O}_2 ends up in glucose.

Step-by-Step Solution

1
Identify the origin of released oxygen gas (O2\text{O}_2) in photosynthesis.
Photolysis of water (H2O→2H++2e−+12O2\text{H}_2\text{O} \rightarrow 2\text{H}^+ + 2\text{e}^- + \frac{1}{2}\text{O}_2) during the light-dependent reaction produces atmospheric oxygen.
Water molecules are split in photosystem II inside the thylakoid lumen to supply electrons.
2
Trace the biochemical pathway of carbon dioxide (CO2\text{CO}_2) in photosynthesis.
Carbon dioxide is fixed by RuBisCO during the light-independent stage (Calvin cycle) and reduced to 3-phosphoglycerate and glyceraldehyde-3-phosphate (G3P).
The oxygen atoms attached to carbon in CO2\text{CO}_2 remain bound through the carbon fixation sequence.
3
Determine the final destination of 18O^{18}\text{O} from C18O2\text{C}^{18}\text{O}_2.
The labeled oxygen is incorporated directly into hexose sugars (glucose) and water formed during stromal dark reactions.
Since CO2\text{CO}_2 is used to build organic molecules, its isotopic oxygen tag ends up in carbohydrate molecules.

Key Concept

Origin of oxygen atoms in photosynthetic products (Photolysis vs. Calvin Cycle Carbon Fixation)
Estimated Time:1m 15s
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Topics

Plant Nutrition and Photosynthesis

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