reply to post by klain
Humans don't have anywhere near the surface area it would take to live off of sunlight. You'll note that plants tend to have a lot more surface area
than humans (leaves, needles, etc.), and that they *don't move*. We've got a surface area to volume ratio comparable to some of the least nimble
species of cactus out there. It works fine for stuff like those cacti, but we humans sometimes like to move every once in a while. Not to mention that
sometimes we like to have a heart that's constantly using large amounts of energy to beat, and we sometimes like to have a brain that uses maybe 20W
just to stay alive and ready to think.
You'll also notice that plants have roots, so they can absorb trace nutrients from the ground. We don't. We have to get a lot of essential chemicals
from our food. We don't just eat for calories. Even if you grew a nice head of leafy foliage for hair, you'd still be vulnerable to scurvy. Of
course, if we can put the code for chlorophyll in a person, we can also reactivate the genes that let us make vitamin C. But we need more than that.
We need nitrogen compounds, several amino acids, several other vitamins, several trace minerals, and large quantities of water.
It is concieveable that we could produce most of the organic molecules we need in our bodies, with sufficient modification. Trace elements and water
will always be needed though.
It wouldn't let you heal faster, be smarter, or have any more endurance. photosynthesis creates saccharides from sunlight, water, and carbon dioxide.
And that's all it does. It would simply allow you to go some unknown (short) amount more time between eating. Assuming you aren't starving to death
at the time, it will have no effect on your healing rate. Your blood sugar level isn't the major factor. Endurance is a factor of respiration.
Assuming you aren't malnourished, it won't make a difference. Thought is based on brainpower, which again, would only be impacted if you were
currently starving.
But since humans don't have very much surface area, and are quite massive and energy intense, being green with chlorophyll wouldn't make any
significant difference. Even if the only downside is the social stigma from being the only green person on your block, it's probably not worth it.
That said, if you wanted to do it, it wouldn't be as hard as you suggest. Plant and animal DNA is identical in structure. Genes from plants will code
for the same protein in an animal as it does in a plant. If you could isolate the part of a plant's DNA that codes for chloroplasts, the only hard
part remaining would be putting it in human DNA so that it only expresses in skin cells. A challenge, certainly, because we don't know too much about
gene expression, but I'd peg it as surmountable within a few decades.
There are some other problems with this idea. Photosynthesis costs water in multiple ways. In order to take in carbon dioxide, you need pores in your
outer coating, which allows water to escape through evaporation. Desert plants have various ways of reducing this loss, but you'll notice they're
even less nimble and lithe than regular plants. That and they're waxy all the time. Secondly, water is actually consumed during the photosynthesis
reaction. No matter how efficient you are at keeping water inside your body, you'll be using water faster than without photosynthesis, because
photosynthesis uses water. We also lose a lot of water to breathing.
Now one idea is to ditch the idea of using plant chlorophyll altogether, and build a custom set of genes that let you use carbon dioxide from the
bloodstream, instead of from the air. That would be more science-intensive, and would probably take longer to implement, but at least your could have
skin with a similar texture to normal human skin.
One more problem is that plant DNA does differ from ours in that it's got a lot more redundant copies and repair mechanisms. That and they don't
have a real bloodstream equivalent. They're extremely resistant to getting cancer, and it's very survivable when it happens to plants. Humans don't
have that luxury. We just aren't made for standing around in sun for days at a time. Skin cancer is a real concern.
I'm of the opinion that it could be done (given a few decades), but it would be far from the best use of genetic modification on humans. Wouldn't
hurt to try, except maybe for the ostracization of the test subjects due to their inhuman skin color.
From my rough (and generous) calculations, if you receive full noon day sun on every inch of your body's surface area all the time, and you're
photosynthesizing every incident photon, you'll be receiving just barely enough power to live on.
That's kind of an unrealistic scenario though.
[edit on 12-5-2008 by mdiinican]
