In photosynthesis, the oxygen gas released into the atmosphere comes from the splitting of which reactant molecule during the light dependent reactions?

Introduction / Context:
During school level biology, many students learn that photosynthesis produces oxygen, but it is easy to be confused about where that oxygen actually comes from. Does it come from carbon dioxide, from water, or from the sugar that is produced? This question specifically tests your understanding of the light dependent reactions of photosynthesis and the origin of molecular oxygen (O2).

Given Data / Assumptions:
- Photosynthesis has light dependent reactions and light independent reactions (Calvin cycle).
- Water (H2O) and carbon dioxide (CO2) are reactants, while sugar and oxygen are products.
- Oxygen gas is released into the atmosphere from green plants, algae, and some bacteria.
- The question asks which molecule is split to release that oxygen gas.

Concept / Approach:
In the light dependent reactions of photosynthesis, which occur in the thylakoid membranes of chloroplasts, water molecules are split in a process called photolysis. Light energy powers photosystem II, and this energy is used to split water into electrons, protons (hydrogen ions), and oxygen gas. The electrons enter the electron transport chain, the protons contribute to a proton gradient for ATP synthesis, and the oxygen atoms combine to form O2, which is released as a by product. Carbon dioxide is not split to form oxygen gas; instead, its carbon atoms are incorporated into sugars in the Calvin cycle.

Step-by-Step Solution:
Step 1: Recall the overall photosynthesis equation: 6 CO2 + 6 H2O, using light energy, produce C6H12O6 (glucose) + 6 O2. Step 2: Recognize that water molecules are the source of the oxygen atoms that end up in O2 gas. Step 3: Understand that in photolysis, 2 H2O are split to yield 4 H+, 4 electrons, and O2. Step 4: Note that carbon dioxide is used later in the Calvin cycle to build sugar and is not split into oxygen gas. Step 5: Conclude that oxygen released in photosynthesis comes from water molecules split during the light dependent reactions.

Verification / Alternative check:
Classic isotope tracing experiments used water labeled with a heavy oxygen isotope to show that the oxygen gas released by photosynthesizing plants came from water, not carbon dioxide. When water was labeled, the released O2 was labeled; when carbon dioxide was labeled, the sugar was labeled but the O2 was not. This clear experimental result confirms that water is the source of the oxygen gas.

Why Other Options Are Wrong:
Option A: Sugar is a product of photosynthesis, not the source of the oxygen gas, and oxygen is not released by splitting glucose during photosynthesis.
Option C: Carbon dioxide contributes carbon and oxygen atoms to sugars, but it is not the molecule that is split to produce O2 gas in the light reactions.
Option D: Pyruvic acid is an intermediate of cellular respiration, not a reactant in the photosynthetic light reactions, and it is not involved in releasing O2.

Common Pitfalls:
A common misconception is thinking that because carbon dioxide contains oxygen, the released oxygen gas must come from CO2. Another pitfall is not distinguishing between the light dependent reactions (which split water) and the Calvin cycle (which fixes carbon). Remembering that photolysis literally means splitting by light and that this applies to water helps keep the source of oxygen clear.

Final Answer:
The correct answer is water molecules split in the light dependent reactions because the oxygen released during photosynthesis is produced when water is broken down in photolysis, and the resulting oxygen atoms form O2 gas.