# Photosynthetic humans. **URL:** \ **Category:** Factual Questions\ **Created:** [July 9, 2009, 10:56am UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438 "2009-07-09T10:56:04Z")\ **Posts on this page:** 12\ **Page:** 2
**Author:** ![smithsb](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smithsb/32/12107_2.png) [@smithsb](https://boards.straightdope.com/u/smithsb)\ **Post date:** [July 10, 2009, 7:29am UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/21 "2009-07-10T07:29:09Z")
SEE! That’s why aliens have the big heads (more surface area) and are green. ---
**Author:** ![SmashTheState](https://avatars.discourse-cdn.com/v4/letter/s/e36b37/32.png) [@SmashTheState](https://boards.straightdope.com/u/SmashTheState)\ **Post date:** [July 10, 2009, 8:51am UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/22 "2009-07-10T08:51:25Z")
Lungs evolved as a way of maximizing surface area to exchange gasses. There’s no reason to believe it’s impossible to design the equivalent to trap light. Picture specialized cells covering the skin which are shaped like empty barrels with reflective inner sides, so that light which enters bounces around until it strikes a chloroplast at the bottom of the barrel. That would certainly improve efficiency. Likewise, as with any animal built on our scale, the square/cube law means the largest problem isn’t conserving energy, but finding ways to radiate it. Develop a biological Stirling engine to utilize this “waste heat” and 200-300 calories a day from sunlight would probably be enough to keep us going. As for micronutrients, there’s no reason we couldn’t get it the same way plants do: eat dirt. ---
**Author:** ![The\_Tao\_s\_Revenge](https://avatars.discourse-cdn.com/v4/letter/t/73ab20/32.png) [@The\_Tao\_s\_Revenge](https://boards.straightdope.com/u/The_Tao_s_Revenge)\ **Post date:** [July 10, 2009, 1:37pm UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/23 "2009-07-10T13:37:12Z")
> [@dracoi](#): > > More important than vitamins and trace minerals, we’d need a hefty source of protein, since we can’t synthesize our own amino acids. > > Of course, if genetic engineering gets to the point of photosynthetic skin, I’m sure we can add little things like protein synthesis and nitrogen fixing. Imagine setting down to a nice meal of peat moss, and volcanic ash soup. Mmm mmm good. ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [July 10, 2009, 1:48pm UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/24 "2009-07-10T13:48:22Z")
> [@The\_Tao\_s\_Revenge](#): > > Imagine setting down to a nice meal of peat moss, and volcanic ash soup. Mmm mmm good. GNC ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [July 10, 2009, 2:06pm UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/25 "2009-07-10T14:06:01Z")
Fish, plankton, sea greens, and proteins from the sea! ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 10, 2009, 4:41pm UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/26 "2009-07-10T16:41:02Z")
> [@](#): > > Lungs evolved as a way of maximizing surface area to exchange gasses. There’s no reason to believe it’s impossible to design the equivalent to trap light. But you’d still be limited by the amount of light that hits you at all. A better strategy would be to have a huge “leaf” that can unfold like a parasol to face the Sun. > [@](#): > > Likewise, as with any animal built on our scale, the square/cube law means the largest problem isn’t conserving energy, but finding ways to radiate it. Develop a biological Stirling engine to utilize this “waste heat” and 200-300 calories a day from sunlight would probably be enough to keep us going. Assuming a human body temperature of 310 K and an ambient temperature of 298 K, that’s going to be horribly inefficient at best. Stirling engines depend on having a large temperature difference between the heat source and the cold reservoir. ---
**Author:** ![Satchmo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/satchmo/32/4223_2.png) [@Satchmo](https://boards.straightdope.com/u/Satchmo)\ **Post date:** [July 10, 2009, 8:52pm UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/27 "2009-07-10T20:52:12Z")
> [@Ludovic](#): > > Fish, plankton, sea greens, and proteins from the sea! Yes, but are they fresh as harvest day? Have they been frozen for storage? Do your birds sing when the wind blows? 😉 ---
**Author:** ![SmashTheState](https://avatars.discourse-cdn.com/v4/letter/s/e36b37/32.png) [@SmashTheState](https://boards.straightdope.com/u/SmashTheState)\ **Post date:** [July 10, 2009, 11:13pm UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/28 "2009-07-10T23:13:49Z")
> [@Chronos](#): > > But you’d still be limited by the amount of light that hits you at all. A better strategy would be to have a huge “leaf” that can unfold like a parasol to face the Sun. We already have the equivalent of “leaves”: hair. Hair resembles the alveoli in the lungs, and probably has a much, much larger surface area than skin. Fill a thick, fuzzy, green pelt with light-absorbing chloroplasts – a sort of human Chia Pet – with, say, elephant-like ears which can be spread to gather even more sunlight and you’re probably doubling or tripling the amount of sunlight exposure. > [@Chronos](#): > > Assuming a human body temperature of 310 K and an ambient temperature of 298 K, that’s going to be horribly inefficient at best. Stirling engines depend on having a large temperature difference between the heat source and the cold reservoir. You’re assuming we remain warm-blooded. If we’re no longer burning ATP for energy, heat differential will be based more on which side of the body is in sunlight, which means you may well get a more extreme difference between the side in sunlight and the side in shade – although I think it’s probably more realistic to imagine a hybrid approach with the body using standard ATP energy transfer as a back-up when no sunlight is available. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 10, 2009, 11:55pm UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/29 "2009-07-10T23:55:28Z")
> [@dracoi](#): > > Breakdown of glucose converts 56% of the potential energy into heat. (That may sound bad, but it’s actually pretty close to the theoretical maximum.) But isn’t the question: How much of the energy of incident sunlight ends up stored as potential energy in glucose through photosynthesis? > [@](#): > > Thus, even if you could convert light energy directly to ATP, you’d only double the effective energy obtained from photosynthesis. We’re still only talking about 500 calories or so. I’m not sure if this answers the above question or not - and I’m not entirely sure how to do the conversion, but sunlight is about 700 to 1000 watts per square metre, isn’t it? What’s that in (food)calories per hour? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 11, 2009, 1:28am UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/30 "2009-07-11T01:28:10Z")
> [@](#): > > We already have the equivalent of “leaves”: hair. Hair resembles the alveoli in the lungs, and probably has a much, much larger surface area than skin. Fill a thick, fuzzy, green pelt with light-absorbing chloroplasts – a sort of human Chia Pet – with, say, elephant-like ears which can be spread to gather even more sunlight and you’re probably doubling or tripling the amount of sunlight exposure. No, you don’t get it. Surface area isn’t enough-- You need cross-sectional area. Unless you’ve got a really big Afro, your hair’s cross-sectional area isn’t anywhere near as big as your skin’s. Stand outside on a sunny day and look at your shadow, and compare the portion of shadow that your hair accounts for to the portion of shadow from everything else. > [@](#): > > You’re assuming we remain warm-blooded. If we’re no longer burning ATP for energy, heat differential will be based more on which side of the body is in sunlight, which means you may well get a more extreme difference between the side in sunlight and the side in shade – although I think it’s probably more realistic to imagine a hybrid approach with the body using standard ATP energy transfer as a back-up when no sunlight is available. Just how much of a heat differential do you think you can get that way? You’d need to be up in the vicinity of several hundred Celsius before you’d start seeing any real efficiency from a heat engine. ---
**Author:** ![The\_Tao\_s\_Revenge](https://avatars.discourse-cdn.com/v4/letter/t/73ab20/32.png) [@The\_Tao\_s\_Revenge](https://boards.straightdope.com/u/The_Tao_s_Revenge)\ **Post date:** [July 11, 2009, 3:41pm UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/31 "2009-07-11T15:41:33Z")
Another potential problem is skin is airtight. Either lungs and blood would need to be modified to process nitrogen (for nitrogen fixing) and intake or output of CO2 or oxygen depending on needs or skin modified to breath amphibian style. Depending on what you’re doing you’d need or the other at different times. Sunning your self during a nap? If you’re energy self sufficient you’ll be making more oxygen then you burn when in inactive in the sun. You’ll have to get rid of the excess oxygen, and breath in some CO2 somehow. Likewise running at night would pretty much be pure animal mode and need the classic oxygen in and CO2 out cycle. Amphibians can only beath through their skin in the water IIRC, how hard would human skin be to make air breathable? Harder then modifying blood to carry nitrogen, and lungs to reverse their function at the drop of a hat? ---
**Author:** ![snailboy](https://avatars.discourse-cdn.com/v4/letter/s/97f17d/32.png) [@snailboy](https://boards.straightdope.com/u/snailboy)\ **Post date:** [July 11, 2009, 4:17pm UTC](https://boards.straightdope.com/t/photosynthetic-humans/502438/32 "2009-07-11T16:17:48Z")
> [@SmashTheState](#): > > We already have the equivalent of “leaves”: hair. Hair resembles the alveoli in the lungs, and probably has a much, much larger surface area than skin. Fill a thick, fuzzy, green pelt with light-absorbing chloroplasts – a sort of human Chia Pet – with, say, elephant-like ears which can be spread to gather even more sunlight and you’re probably doubling or tripling the amount of sunlight exposure. This works for lungs because the air is exerting the same pressure across the entire surface area, thus more surface area means more force. If you did such a thing with the skin, most of the sunlight would be hitting a part of the skin at an angle and wouldn’t be as dense. You could fit more chloroplasts, but each one would be getting less light. [Previous page](https://boards.straightdope.com/t/photosynthetic-humans/502438.md?page=1)