KP posted above a link to a good cartoon of the carbon cycle.
Very true that plants use a lot of CO2 -- land plants absorb about 440 Gt of CO2 per year. Various biological and chemical processes in the ocean absorb another 330 Gt. This is where it is "going".
As to where it is "coming from": Consumption of vegetation by other organisms produces 220 Gt per year. Respiration by plants produces another 220 Gt, and the ocean releases about as much as it absorbs, or 330 Gt. Humans are producing a paltry 26.4 Gt per year in emissions.
But see how perfect the balance is without human emissions?
And what happens to these human CO2 emissions?
About 40% are absorbed (mostly into an increasingly carbonized = acidifying ocean...a potentially huge problem in its own right). The rest remains in the atmosphere for a net annual gain. Natural changes of about 100 ppm in the past have occurred over thousands of years. Atmospheric CO2 is now at its highest level in 800,000 years, and the latest increase of 100 ppm took only 120 years! Something unatural is definitely going on here.