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Comment by jacquesm | Hacker News Reader

You could breathe your homegrown oxygen too that way!

On a more serious note, the sun puts out about 1KW per square meter of receiver that is exactly perpendicular to the incident rays, in full sunlight. So theoretically there is enough power there, but in practice photo-synthesis is not efficient enough to make this happen, typically somewhere around 5% of the energy gets captured, and that's for a very dark plant. So if your cross section towards the sun is .2 m^2 and you convert at 5% efficiency then you can expect about 10W from that in usable energy. There is a reason leaves are relatively flat and can change orientation.

http://en.wikipedia.org/wiki/Photosynthetic_efficiency

It's not easy, being green.

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This brings up a somewhat more scary discussion: It's more efficient to be a cybernetic organism than a gene-modified human with genes from plants.

If I get this correct, this (cited) Wikipedia-article says that solar cells with a 0.01 m^2 surface area would produce approximately 1.4 watt. Multiplying 1.4 watt by 20, to get a 0.2 m^2 surface area, gives us 28 watts.

http://en.wikipedia.org/wiki/Solar_cells#Energy_conversion_e...

For reference, a 2000 Calorie per day diet translates to about 100 Watts.

2K Calories of energy per day translates in to 1163*2 = 2326 Watt-hours, which you could interpret as an average consumption of 96.91 Watts over the course of a day.

Also, when you're the only guy on the block that's able to turn energy into a useful form, you tend to get eaten a lot.

Of course, you wouldn't need oxygen. You need oxygen to break down compounds; you need carbon dioxide to build them. Your lungs would have to switch to absorbing CO2 and expelling O2.

Now, on the other hand, if you continued to eat food AND synthesized, that could get pretty cool. I'm not really sure what you'd wind up expelling in that case.

You still need oxygen. Plants breath in oxygen too. Photosynthesis does not magically create "bio energy". Or rather, photosynthesis does not directly regenerate ATP. It creates glucose which then has to be broken down for ATP via Krebs cycle (same thing you do).

I guess in theory, you COULD do everything anaerobically. But then your efficiency drops even more.

A plant still produces a net positive of oxygen though, correct?

Only during the day from what I've read. At night they consume a net amount of oxygen.

The amount consumed is smaller than the amount produced, so there is a net production over the whole cycle.

Sort of, a plant converts a net positive of CO_2 to O_2.

This is also another one of those cases where some folks can misunderstand the statement to believe that plants somehow take in all this CO_2 and scrub it of Carbon. Not really, that Carbon becomes the plant itself. The amount of C's that a plant pulls out of the CO_2's is nearly equivalent to the plant's dead weight. This is important to understand, because old growth forests don't actually remove net CO_2 from the atmosphere, they just fail to release it. If you cut down the forest, the dead plant matter starts decomposing and/or burning and then CO_2 and/or methane goes back into the atmosphere.

Or at least, that's how I understand it.

Essentially correct. There may be some second order effects that help established forests pick up some more carbon out of the atmosphere, especially if the atmosphere gets more carbon laden than it used to be when the forest was formed. The Economist had a special report on forests last week that had some details on the climate benefits of old forests.

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