What do plants do to atmospheric gases?

• A. Absorb helium
• B. Remove CO2 and increase oxygen
• C. Increase nitrogen
• D. Produce methane

Answer: (B)

Photosynthesis is how plants change atmospheric gases: they remove carbon dioxide from the air and release oxygen. In the presence of light, plants use CO2 and water to build sugars, and oxygen is released as a byproduct through the stomata in the leaves. This means that, during the day, the atmosphere becomes lower in CO2 and higher in O2 due to the activity of plants (and other photosynthesizing organisms). At night, plants still respire and use some oxygen, but this doesn’t usually offset the overall daytime production of oxygen and consumption of CO2, especially in forests and oceans where photosynthesis is widespread. So the net effect is a decrease in atmospheric CO2 and an increase in O2. The other options don’t reflect this primary process—helium isn’t biologically absorbed by plants, atmospheric nitrogen isn’t produced or increased by plant activity, and methane isn’t generated by plants as part of normal photosynthesis.