Question for experts: what if humans had green skin?

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If humans had chloroplasts in our skin cells, which would enable us to photosynthesize how much would that reduce our need for food intake?

If our skin were as efficient per unit area as the leaves of say trees, how much energy could we take in from sunlight?

And if you subtracted that from the daily average calorie need of an adult human how much would remain (that we would need to get from food)?

Humans move around a lot more, and do a lot more thinking and emoting than do plants. So I doubt it would be a 100 percent. But it might significant.

Any experts care to try to calculate?

Others have pondered the same.

Here's one attempt:
https://hplusbiopolitics.wordpress.com/ ... ti-people/

So the energy produced by a human being lying in the sun for an hour (3600 seconds) at midday would be:

400 J/s/m2 x (0.5 x 1.8m2) x 0.05 x 3600s = 64800J = 64.8kJ (or 15.43 kcal)

By comparison, an apple has about 400kJ of usable food energy. So an hour in the sun is about the same as a sixth of an apple. The daily energy requirements for a human being sit around 10,000 kJ per day, so that’s going to require 150 hours per day of sitting in the sun. Needless to say, that’s impossible.

Here's another for cows:

https://what-if.xkcd.com/17/

Both imply the calories would be negligible compared to our normal needs.

Interesting.

I see a minor flaw. Photosynthesis in plants uses carbon dioxide and forms oxygen in the process. While that seems like a great way to get rid of excess carbon dioxide in the atmosphere, it would in turn come back to bite us if that is the only way that we could obtain energy to live. There are more than seven billion people on this planet. It would convert so much carbon dioxide into oxygen that the atmosphere would likely become carbon dioxide deficient. Plant (and then people) would struggle with the little left to survive on in the atmosphere. We would then be forced to pollute the atmosphere by burning fuels just to be able to do photosynthesis. If humans could do both photosynthesis and normal respiration, it might be a much better choice.

I dont think so.

Humans are just one species. Granted we are large animals, and we are mammals (that do more respiration than do cold blooded animals the same size), and granted humans have doubled in number a couple times in the last century, so we as a species constitutute more biomass than does any other single species. But we as a species still only make a tiny part of the biomass of the planet. So if humans were to suddenly start doing like euglena (one celled pond critters that have chloroplasts but also hunt and eat like animals), or like those snails in the article posted above, and started to exhale CO2 it wouldnt effect the planet as much as our vehicles and our industry does as far as greenhouse gases.

In fact there might be a net decrease in greenhouse gases because we would need less food, ergo less of agrobusiness infrastructure, less fertilizer, etc, which would reduce greenhouse gases -probably far more.

The calculations are a bit beyond me, but I think of it this way:

Various animals use photosynthesis by keeping single-celled algae or dinoflagelates in their flesh, to which they give nutrients in exchange for energy. But most of these animals seem to be much less energy-hungry than humans are. As mammals, we burn through ridiculously large amounts of energy compared to reptiles or invertebrates. It's the cost of our high activity levels and ability to warm our own blood. And advanced brains take a lot of energy to run, too- our large brain is a power drain.

The animal groups that use this kind of photosynthesis the most are:
Sponges (rooted to the spot, no nervous system, no blood)
Corals and sea anenomes (rooted to the spot, no brain, no blood).
The symbiosis is very common in both groups, and the host gets a large percentage of its energy from the microbe.

It also occurs in:
Several species of giant clams (tiny brain, rooted to the spot, ectothermic)
One species of jellyfish (lazily mobile, no brain, no blood)
One species of salamander (very mobile, ectothermic, a positive genius compared to everything else on the list).

A few species of sea slug (sluggishly mobile, slightly larger brain than a clam, ectothermic) don't use live algae: instead they use DNA and chloroplasts taken from algae they've digested.

A handful of arthropods seem to be able to collect solar energy directly using pigments in their exoskeletons:
Several species of aphid (small, very mobile, ectothermic, small but fairly complex brain)
One species of wasp (ditto).

All of this suggests that photosynthesis just isn't worth it for most animals that are mobile, intelligent and/or endothermic. Particularly as the two groups that use it most are the oldest and most anatomically primitive. Also, jellyfish and corals are closely related. So it's striking that mobile jellyfish practically never photosynthesise, whilst immobile corals very often do.

...damn, I just lost a couple of hours there.

Interesting.

So...it doesnt pay to act like a plant chemically unless you ...act like a plant physically (are rooted in one spot like a choral or a sponge)like a plant. And the more your intellect is beyond that of plant the worse the payoff is.

Though there is that salamander.

Thanks.

Interesting.

So...it doesnt pay to act like a plant chemically unless you ...act like a plant physically (are rooted in one spot like a choral or a sponge)like a plant. And the more your intellect is beyond that of plant the worse the payoff is.

Though there is that salamander.

What got me on this train of thought was stumbling upon vids about "the Breatharian Movement". Folks claiming that humans can live without food, and live off of breath and sunlight. Which is nonsense. But it got me wondering about what if we humans were properly equipped, with green chloroplasts, could we at least be partially solar powered? Apparently even then it wouldnt be feasible.

Here are two vids. One is put-on by a comedian. The other is serious. I will let you figure out which is which.

Aye, I'd like to know more about that salamander, and the wasp too. Wasps are very smart by insect standards. Why is it worth it for those two, and not for the thousands of other amphibians and wasps?

Yes. And though wasps are not "warm blooded"(endothermic) they do move around a lot and use a lot of energy. And even build nests.