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I've seen many questions here about humans being capable of photosynthesis. And I know, humans don't have enough surface area to power themselves full time. However, that's assuming that the energy from the photosynthesis goes towards our calorie intake.

What is a different way to use the energy a photosynthetic human could get from the Sun? Is there a bodily function, like synthesis of unique sugars and amino acids, that could be powered by the energy a human could get from the Sun?

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    $\begingroup$ I think the basic problem is that whatever photosynthetic product is generated ultimately there will be a way of building that product using non-photosynthetic processes or at the very least obtaining it from something else more easily (by eating it). $\endgroup$ – Slarty Oct 7 '17 at 21:37
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What is photosynthesis anyway?

Hmm ... photosynthesis in and of itself is pretty specific: energy from sunlight is used to split water into oxygen, which is discarded as useless junk, and a bunch of hydrogen, with a bonus ATP (energy storage). The energy and hydrogen are then used to turn carbon dioxide into a carbohydrate, aka sugar. Hence photosynthesis ("light" + "putting-together"). Read up on it in detail; chemically it's amazing!

So what do you want to focus on?

Hey it's driven by sunlight!

There's some room to play with here. While not enough to completely support a human metabolism, it sure could slow starvation. Only those wealthy enough to have safe food supplies would go around wastefully wearing shirts.

Hey, it's all about the chloroplasts!

Supposedly chloroplasts were once (the ancestors of) cyanobacteria which got assimilated into a eukaryote. Who cares? We do! Because you can extend this to say that for your world we can do this with any prokaryote. Sky's the limit. There are:

  • Prokaryotes which metabolize sulfur. Now your humans can, too!

  • Prokaryotes which have toxic cell walls; now your humans are safe from predation!

  • Prokaryotes which produce vitamin C; no more scurvy for your mighty sailors!

  • And on and on and on.

Hint: almost all molecules in the entire biological world exist in prokaryotes (I read this a long time ago; I believe that the article said there were a few exotic snake venoms and compounds in flowers which were not found in prokaryota).

You can make these optional, nice-to-have features, or vital to life, such that sunlight-deprivation is a major crime and plot point.

They don't have to be bacteria that exist on Earth; make up your own! They don't even have to be light-driven. They could consume sugar (or other compounds) to provide some service... Maybe there's one which eats sugar and gives off oxygen, making for a race of awesome divers

Okay, now it gets real

So okay, we have humans which have assimilated some wacky bacterium to give them an advantage. Who's to say all humans assimilated the same one? There could be several bloodlines/tribes/clans each of which has a different symbiote. Which could mean:

  • Clan A will pay handsomely for a daughter of Clan B to marry into their line. Clan C may opt for a more sneaky approach.

  • Romeo can never marry Juliet because their symbiotes are incompatible; their children would not survive

  • "Skin color? You expect me to care about skin color? DEATH TO THE SULFUR METABOLIZERS!"

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  • $\begingroup$ Yeah, but everyone had to be literally charcoal-black for it to make a difference. (Aka synthesize on almost all bandwith, and with any possible chemicals) $\endgroup$ – Hobbamok Aug 20 at 11:56
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You don't need to look far.

Our bodies manufacture vitamin D when the sun's ultraviolet B (UVB) rays interact with 7-dehydrocholesterol (7-DHC) present in the skin.

This is a very specific and real kind of photosynthesis.

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    $\begingroup$ Very nice one Z! $\endgroup$ – Willk Oct 7 '17 at 16:02
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Kill parasites, or other unwelcome passengers.

In a competition between host and parasite, the host can take on stresses that it can deal with but which are too much for the parasite to withstand.

An example is the black noddy. It lies in the sun and its black feathers get very very hot. Lice die. Bird does not!

Sunning by Black Noddies (Anous minutus) May Kill Chewing Lice (Quadracepshopkinsi)

sunning black noddy

A somewhat analogous situation is photopheresis. Blood is removed from a human, treated with UV light and then returned. This cripples immune cells, which in this circumstance are probably foreign ones from a bone marrow transplant, attacking the host.

photopheresis schematic http://www.nationwidechildrens.org/photopheresis-clinic

A similar but less technically sophisticated use of light as an immunomudulator is photodynamic therapy. In immune problems confined to the skin (or immune-derived cancers confined to the skin, like Sezary's syndrome) one can attenuate the immune response with UV. You can even do this with sunlight, although it is hard to charge the patient for that I imagine.

Psoriasis is an immune disorder of the skin. Here is a before and after picture showing improvement with phototherapy.

psoriasis treated with photodynamic therapy https://www.pinterest.com/pin/30258628723500161/

So: I could imagine a circumstance where humans either had an overacting immune system (overreacting to some alien antigen?) or perhaps a disease or parasite, and the UV damage produced by sunlight was beneficial in attenuating the immune system, or damaging the parasite.

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It probably couldn't evolve naturally because there is not survival advantage linked to having it, but it would be interesting to have photosynthetic support for short term memory.

That could make the old wives tale of Optography possible.

Optography is the process of viewing or retrieving an optogram, an image on the retina of the eye. A belief that the eye "recorded" the last image seen before death was widespread in the late 19th and early 20th centuries, and was a frequent plot device in fiction of the time, to the extent that police photographed the victims' eyes in several real-life murder investigations, in case the theory was true. The concept has been repeatedly debunked as a forensic method.

In this photosynthetic version, it wouldn't be the eye which records the last moment of life, but part of the brain (supported and maintained by sunlight) which held the last few minutes of a victim's sights and thoughts. This could be used to drive any number of story plots...

  • They killed our spy, but if we can recover her body before sun down,...
  • Ever notice how the murder rate skyrockets during eclipses?
  • Vampire's aren't killed by sunlight directly... the sunlight revives their last mortal thoughts, their human morals and the images of those they loved. No, vampires don't burn in the sunshine... they suicide.
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  • $\begingroup$ "because there is not survival advantage" Are you sure? It would help A LOT with "mapping" the area, where you have already gathered all the food etc. $\endgroup$ – Hobbamok Aug 20 at 11:53
  • $\begingroup$ @Hobbamok. We already have circuitry for that mapping process. It is called "memory". The photosynthetic persistence of memory, optography -as I describe in my answer-, is a supplement to the biochemical/caloric fueled persistence which we already enjoy. It's primary value begins only after that biochemical/caloric fueled persistence ends, as in, after the moment of death. ...which makes it pretty useless in terms of survival advantage. $\endgroup$ – Henry Taylor Aug 20 at 14:01
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Perhaps if the energy balance was different, the chances might improve. If “human” could be stretched to include a cold blooded or hibernating human, there might be sufficient benefit from some form of appendage with an unfurl-able photosynthetic surface. Such a structure could supplement the low level of energy required during dormancy on cold bright days. Over an extended period the meagre supply of energy obtained might just make the difference between life and death.

So to answer your question photosynthesis might supplement emergency survival energy when all other external sources were unavailable.

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  • $\begingroup$ Add to this: Humans can hibernate on purpose (basically turn into a plant) and use this to migrate to other islands (its a giant planet with only small groups of islands) $\endgroup$ – Hobbamok Aug 20 at 11:55

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