The oxygen gas produced during photosynthesis comes from water molecules.

Here's a breakdown:

* Photosynthesis Equation: The overall chemical equation for photosynthesis is:

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

(carbon dioxide + water + light energy → glucose + oxygen)

* Water Splitting: During the light-dependent reactions of photosynthesis, light energy is captured by chlorophyll and used to split water molecules (H₂O). This process is called photolysis.

* Oxygen Release: When water is split, hydrogen ions (H+) and electrons are used in other parts of photosynthesis, while the oxygen atoms combine to form oxygen gas (O₂), which is released into the atmosphere.

So, the oxygen we breathe is ultimately a byproduct of the process that plants use to make their food.