How many plants are needed to keep someone alive?

[Brother Oni]

Wow, thanks! Sounds a reasonable assumption...it hadn't occurred to me that plants would only be absorbing CO2 when they were actively growing, though? Surely they still need to absorb some CO2 in order to make the sugars they live off even when they're not actively growing? Still, 143m^2 of actively growing bamboo is a good baseline, because anything else would require more plant matter.

When people consider plants for carbon capture for environmental management, it's only actively growing forests/plants that are counted, so that's what I based my calculations on.

From this page and this thread, a tree generates 56g of glucose a day. 40% of glucose is carbon, so ostensibly that's (56g x 40%) = 22.4g of carbon captured daily.

Given 50% own use of oxygen for the plant, that leaves 11.2g available daily for human use.
0.84kg / 0.0112kg = 75 trees required to supply the daily oxygen for a single person. Since the link doesn't specify the size of the tree or its footprint, I'm stuck here on how much space is actually required.

A minor point of order though for the bamboo, it's 143 cubic metres per person not square metres. While on a planet you can generally ignore height for growing crops, this is not an option in a spacecraft or other fully enclosed environment.
While there's lots of 'empty' space underneath the foliage, it needs to be clear so that the plant can grow freely to absorb as much sunlight as possible (probably a giant park to help the astronaut's psychological wellbeing).

How can this work? For example, glucose is C₆H₁₂O₆. The carbon:oxygen ratio is 1:1. There isn't enough oxygen in it to fully oxidize all of the carbon to CO₂.

The main thing about biological reactions is that they always occur in aqueous solution, so if you're missing some oxygen or hydrogen from somewhere, it's coming from the water present in the environment.

Taking it from the very start with glycolysis, glucose undergoes a number of reactions to make pyruvate:

C₆H₁₂O₆ → 2 C₃H₃O₃

As you can see we've already lost a number of hydrogen to the water (pyruvate is the conjugated base of pyruvic acid, which is going to happen in an aqueous solution) and to other helper molecules.

The pyruvate then gets chucked into the Krebs Cycle, with the pyruvate getting processed entirely:

Pyruvate ion + 4 NAD⁺ + FAD + GDP + Pi + 2 H₂O → 4 NADH + FADH₂ + 4 H⁺ + GTP + 3 CO₂

Again if you look there's no inhaled oxygen directly involved and the CO₂ has gotten the other oxygen from the water.

Where oxygen is involved, is in oxidative phosphorylation, where oxygen is used to help perform some of the transformative steps in the Krebs Cycle, primarily the final step in the electron transport chain where it acts as an electron sink:

Spoiler: Cytochrome c oxidase electron transport chain

Show

There's a fair few floating hydrogen ions about, but whether that's coming from the Krebs Cycle or just from solution is anybody's guess. Basic chemistry teaches this:

 H₂O ⇌ OH⁻ + H⁺

In actuality, it's a little more complicated (it involves hydronium), so to confirm my initial statement, if you have a biological reaction in solution and there's magically appearing/disappearing oxygen or hydrogen, it's coming from the water.

If you're unhappy with the Wikipedia links, I can dig out my biochem textbooks and cite the book, page and reference.