How much plant biomass is needed per human?

In the first place, someone will (correctly) respond that the question
is poorly conceived, hopelessly vague, dependent on too many unknowns,
etc.

Granted. Cleaning up the question will probably be the first step
towards finding a meaningful answer.

So let's start with these:
* I'm referring to O2 and CO2 exchange only; not using plants as
food, fuel, building materials, etc.
* We can all recognize that humans have varying needs. One size does
not fit all.
* The O2 output per gram of one plant is probably wildly different
than that of another species.

Asked another way:
Assume that one normal healthy adult human is placed in a sealed
environment with everything needed, except a renewable oxygen supply,
and he wants to live there indefinitely. How much plant life should
be included in this environment? (Ignore the fact that the artificial
environment will almost certainly degrade quickly due to other
causes.)

Thank you!

Ted Shoemaker

P.S. No, this isn't a homework assignment. I'm just trying to learn.