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Difficulty: MediumPlant Nutrition and Photosynthesis

During photosynthesis, the photolysis of water takes place within the thylakoid lumen of chloroplasts. Which of the following correctly describes the immediate fate of the molecular oxygen gas generated from this reaction?

  1. It diffuses out of the chloroplast and leaves the leaf via the stomata or is consumed in cellular respiration.Answer
  2. B
    It enters the stroma to react directly with ribulose bisphosphate during carbon dioxide fixation.
  3. C
    It is translocated downward through the phloem vessels to meet the respiratory needs of subterranean root tissues.
  4. D
    It combines with carbon dioxide in the light-independent stage to synthesize 3-phosphoglyceric acid.

Answer

The oxygen gas diffuses out of the chloroplast and exits the plant through stomata or is used internally for cellular respiration.
During photolysis of water in the light-dependent phase of photosynthesis, water molecules are split into hydrogen ions, electrons, and molecular oxygen gas. Since oxygen is a byproduct rather than a reactant for subsequent light-independent reactions, it diffuses out of the chloroplast into mesophyll air spaces and exits through stomatal pores, or is used by the cell's mitochondria during aerobic respiration.

Step-by-Step Solution

1
Identify the site and products of photolysis of water.
Photolysis occurs in the light-dependent stage within thylakoids, splitting water molecules (2H2O→4H++4e−+O22H_2O \rightarrow 4H^+ + 4e^- + O_2).
Water molecules supply replacement electrons for Photosystem II and generate protons for ATP synthesis.
2
Determine the metabolic destination of the released oxygen gas.
Oxygen gas is a byproduct of light reactions and is not used as a substrate in the Calvin cycle.
The light-independent reactions require CO2CO_2, ATP, and NADPH, but not oxygen gas.
3
Trace the physical exit path of oxygen from the photosynthetic cell.
Oxygen diffuses down its concentration gradient out of the chloroplast into mesophyll air spaces, exiting via stomata or entering mitochondria.
Gaseous exchange in leaves occurs primarily by simple diffusion through stomatal pores.

Key Concept

Photolysis of water and destination of photosynthetic byproducts
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Topics

Plant Nutrition and Photosynthesis

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