Stoichiometry of oxygen evolution — photosynthesis gas ratios Under the overall light-driven reaction of oxygenic photosynthesis, what is the volume ratio of O2 liberated to CO2 fixed?

Introduction / Context:
Understanding the stoichiometry of photosynthesis is key to plant physiology and global carbon–oxygen cycling. The classic balanced equation links carbon dioxide uptake to oxygen evolution, guiding both conceptual questions and quantitative problems.

Given Data / Assumptions:

• We consider the net overall oxygenic photosynthesis reaction.
• Idealized equation: 6 CO2 + 6 H2O (light) → C6H12O6 + 6 O2.
• Volume ratios of gases under identical conditions mirror mole ratios.

Concept / Approach:

The stoichiometric coefficients in the overall reaction indicate that 6 moles of CO2 are fixed while 6 moles of O2 are released. Therefore, the molar (and thus volume) ratio of O2 produced to CO2 consumed is 1:1. Although various pathways (C3, C4, CAM) differ in biochemical details, the global stoichiometry for oxygen evolution relative to CO2 fixation remains 1:1 under the simplified overall equation.

Step-by-Step Solution:

Verification / Alternative check:

Introductory biochemistry and plant physiology texts use the 1:1 relation when discussing gas exchange in photosynthetic organisms, barring side reactions like photorespiration adjustments.

Why Other Options Are Wrong:

2:1, 1:2, 3:1, and 1:3 do not match the balanced overall equation of oxygenic photosynthesis.

Common Pitfalls:

Confusing net oxygen evolution with water-splitting details, or overcomplicating with photorespiration; at the overall level, 1:1 applies.

Final Answer:

1:1