There are a couple of stories where humans gain chloroplasts.

Here's a clip from one story:

They are also infused with bacteriorhodopsin, allowing photosynthesis. It is now possible for a human being to lie on the ground, in the sunlight, for thirty minutes, and absorb all the energy and nutrients they need for a 24-hour period.

But, this strikes me as unlikely. The other story had humans being powered by sunlight.

While working with this idea we happened across an interesting and rather common tittle freshwater coelenterate named Hydra viridis. This particular hydra, unlike its relatives, exists in a symbiotic relationship with a wonderful alga called chlorella.

In addition to producing its own food, it will also produce enough to keep its host alive. "When chlorella is not present, the hydra is forced to eat solid food just like its white relatives."

...will have to believe you after you've done nothing but sit in the sun and starve yourself tor weeks on end without getting hungry or losing any weight.

EDIT: removed 3 hours a day for power/meals, since the story didn't clearly say that, only implied it.

It sees to me that an animal running around needs a lot of sugar/energy. Even if we take out the energy needed to process food (a non-trivial amount), it seems to me that a human would require more power than could be provided by merely having a green skin. Square-cube law, and more mass behind a smaller surface area than a microscopic animal.

IIRC, humans need about 2,500-3,000 calories a day.

Working skin is a under a couple of layers of dead skin, which should impact how effective the chlorophyll would be (how much?)

Assuming there was a way to prevent them from being liquidated and absorbed; how much energy would a bunch of chloroplasts within vacuoles in the human skin provide?

(an aside, too bad the fortnightly challenges don't say when they expire)

Hmm, the calorie consumption numbers are for food, which humans are not good at digesting, does anyone have glucose/ATP consumption figures for human beings? Maybe how much are people who're on IV drip fed?

  • $\begingroup$ Each challenge lasts for two weeks from the time it is posted on Meta. $\endgroup$
    – James
    Mar 2, 2015 at 14:50
  • $\begingroup$ Another question is why people would engineer humans to use photosynthesis. Anyone who could afford the procedure could afford food anyway, and most people like eating. $\endgroup$ Mar 3, 2015 at 6:27
  • $\begingroup$ @TomAnderson As an example: In the Old Man's War trilogy, the genetically engineered bodies of the soldiers feature chloroplasts in order to supplement, rather than replace energy intake from food. This is done in order to provide extra or emergency energy for fighting. $\endgroup$
    – Timpanus
    May 28, 2016 at 23:50

1 Answer 1


Well, there's a reason animals run around and eat plants, and not the other way around. Let's find out why:

Step 1: Inbound solar radiation in the vacuum at Earth orbit is 1,413 $W/m^2$, or about 1500 $W/m^2$

Step 2: Photosynthetic efficiency is 2% to 5% at best, under optimal conditions. (I've seen estimates as low as 0.1%) Let's use a wildly optimistic 5%.

Step 3: Estimate humanoid body area to 1.5-2 $m^2$. Let's round up to 2, and say half of it, or $1 m^2$, can face the sun at a given time.

Step 4: Estimate average daily food intake of a moderately active man to be around 2600 Calories, or $1.1\times10^7 J$, so round down to around 10 million Joules. Keep in mind that a Joule is a Watt/second.

Step 5: (My favorite) Calculate the energy capture formula as a function of energy inflow, area, efficiency and time: $1500 W/m^2 \times 1m^2 \times 0.05 \times 1s = 75 W/s$

So at that wildly optimistic conversion rate under perfect sunlight you'd need, um, 133,333 seconds to power up for the day. That would be not half an hour, not 3 hours, but 37 hours.

Keep in mind that in reality, even at noon, only about 1000-1100 Watts/sq. meter actually reach the ground (some get dissipated in the atmosphere), far less than half the body area will have direct solar incidence, and typical photosynthetic efficiency might actually be closer to 1%.

So unless you come up with a way to increase the surface area about 10-fold at least, it's looking a little difficult. You might consider placing your green men in the focal center of a set of reflecting mirrors. Might get a tad hot, though, and photosynthesis has an optimal temperature range.

  • $\begingroup$ There's room for improved efficiency with artificial / engineered photosynthesis (I mean, theoretically). That could be where that 3-hour figure originally quoted comes from - if you had 25% efficiency say (you get some the rest of the day too). $\endgroup$ Feb 28, 2015 at 17:51
  • $\begingroup$ Man, that would totally have to be engineered - the numbers for natural efficiency are actually like .3% $\endgroup$
    – user3082
    Feb 28, 2015 at 18:34
  • $\begingroup$ @Dan Smolinske: Is there really room for improved efficiency? Plants have been experimenting with photosynthesis for what, a couple billion years? With strong pressure (I can make more seeds, outcompete all my neighbors, and dominate the world!) for improvement, yet 3-6% (per en.wikipedia.org/wiki/Photosynthetic_efficiency ) is the best they've come up with. Maybe that's about the best sunlight->food conversion possible? $\endgroup$
    – jamesqf
    Feb 28, 2015 at 18:47
  • $\begingroup$ @jamesfq One problem with genetic evolution in general is it can reach what is called a "false peak" - in other words, maybe to get really good efficiency you'd first need a series of mutations that would reduce efficiency, before finally arriving at a much better overall solution. That means that it gets stuck - the necessary mutations are being selected against, unless all of them happen at once (which is very unlikely). Artificial genetic manipulation can cut across that potential gap. $\endgroup$ Feb 28, 2015 at 21:33
  • 1
    $\begingroup$ @Dan Smolinske: Certainly that's a theoretical possibility, but AFAIK no one has come close to demonstrating a more efficient sunlight->food conversion pathway, or even suggesting how one might work. $\endgroup$
    – jamesqf
    Mar 1, 2015 at 5:02

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