Here's the breakdown of the gases involved in photosynthesis:

* Reactant: Carbon dioxide (CO2) is the gas that plants take in from the atmosphere.

* Product: Oxygen (O2) is the gas that plants release into the atmosphere.

Here's how it works:

Photosynthesis is the process by which plants convert light energy from the sun into chemical energy (in the form of sugars) that they use for growth and other functions. This process requires carbon dioxide and water as inputs:

* Carbon dioxide provides the carbon atoms needed to build sugars.

* Water provides the hydrogen atoms needed to build sugars and also plays a role in the chemical reactions.

During photosynthesis, plants use sunlight to split water molecules. This releases oxygen as a byproduct and leaves behind hydrogen. The hydrogen is then combined with carbon dioxide to form glucose (sugar).

So, in summary:

CO2 + H2O + Sunlight → Glucose + O2