Animals respire, right? They take oxygen from the air, glucose from their food and gain the chemical energy from it to make Adenosine Triphosphate, and Carbon Dioxide and water are produced as waste products. The simplified equation is:
Oxygen + Glucose --> Carbon Dioxide + water + energy
6O2 + C6H12O6 --> 6CO2 + 6H2O + energy
And animals get Glucose ultimate from plants, who produce it using the energy from the sun by photosynthesis. The simplified equation is:
Carbon Dioxide + water -->Sunlight--> Oxygen + Glucose
6CO2 + 6H2O -->Sunlight--> 6O2 + C6H12O6
It's respiration in reverse, isn't it? Plants use this glucose in many ways, for example, the cellulose that makes up their cell walls are massively long chains of alpha-Glucose. With Nitrates they can make amino acids with them, which they use for proteins. All animals get their proteins from eating plants, we are very much parasites of them.
But carbon is only sequestered into cellulose and amino acids and other products when there is actual plant growth, otherwise there is always an equilibrium. The other important use they have for their glucose is the same as us: respiration.
6O2 + C6H12O6 --> 6CO2 + 6H2O + energy
So yes, damn, plants use oxygen and produce Carbon Dioxide.
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