what does co2 do for your plant?

So I've been hearing a lot of talk about this co2 and was just wandering what exactly it does for your plant? And if there is a way to make it for cheap?

on my last indoor grow i mixed in a one gallon milk jug 4 cups sugar 3 table spoons of yeast and fill half way w/warm water pencil size hole in cap shake daily w/thumb over hole when you realease the pressure dont let it spray ur plant co2 is to plants what 02 is to humans very important in veg. stage the more lumens you use the mor co2 you need

Yes plants need CO2 like people need oxygen.

delta9 said:

on my last indoor grow i mixed in a one gallon milk jug 4 cups sugar 3 table spoons of yeast and fill half way w/warm water pencil size hole in cap shake daily w/thumb over hole when you realease the pressure dont let it spray ur plant

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This does work as a cheap alterntive to create co2. The problem is that your PPM will be all over the place.