As you point out, plants break down the water molecules to produce the oxygen, but the CO2 is the fuel used for that process, just as my engine produces torque by using a combination of fuel (gasoline or diesel) and air.
You can choose a DNA without a subscription to the Ancestry.com service, a DNA test with a subscription to U.S. discovery service or a DNA test with World explorer. Once you choose an option, Ancestry.com will ship you a test kit. The test kit has a small mouth swab which you wipe on the inside of your mouth.
Y & Mitochondrial are totally useless in giving a person an overall view of their ancestry. Understand that Y & Mitochondrial testing is very exact but represent a tiny part of your total ancestry. Most of your DNA is Autosomal. It is what is used in the overall ancestral testing. However Autosomal does not come down in a straight line unchanged. There are no Haplogroups with Autosomal.