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I have been brainstorming an idea for a kind of human-plant symbiosis, in which a plant has either evolved or been modified to photosynthesize with heat instead of light. This plant may have evolved in an ocean or a cave with very little light, but plentiful heat.

My thought is that a human could grow this plant in their veins to replace their blood, and provide the plant with heat, carbon dioxide, and water in exchange for glucose/other energy molecules and oxygen.

Is this possible? What modifications would need to be made to the human or the plant to make this possible? What would potential consequences of this paring be?

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    $\begingroup$ Hello Kirby, welcome to Worldbuilding. People are going to comment about the fact that solid plant material won't do as well as liquid to transport nutrients (plants transport nutrients using water, but it's much slower than blood). But before that, if you take our tour and read the first to bullets of our help center you'll discover you're expected to ask only one, specific question. It' might even be better to edit your Q to ask how to change photosynthesis to thermalsynthesis and then follow-up with a second question. $\endgroup$
    – JBH
    Commented Oct 7, 2020 at 3:53
  • $\begingroup$ So you basically suggest to replace blood (erythrocytes) cells with symbiotic plant cells, am I right? $\endgroup$
    – ksbes
    Commented Oct 7, 2020 at 6:55
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    $\begingroup$ Blood doesn't provide nutrients, it transports them. Even plants have something vaguely similar to blood: sap. $\endgroup$
    – chepner
    Commented Oct 7, 2020 at 11:49
  • $\begingroup$ Plants make their own food with carbon dioxide, but then they also use that food in the same way people do: they "burn" it using oxygen. They just happen to be a net producer of oxygen rather than a net consumer. $\endgroup$
    – chepner
    Commented Oct 7, 2020 at 11:51
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    $\begingroup$ @chepner There's no particular reason why a plant can't produce more energy than it uses. Many trees, for example, store excess sugars in the form of sap, and people have been harvesting sap without killing the tree for hundreds, if not thousands of years. Clearly, these trees produce more food than they use - they produce enough to feed themselves and waffle lovers. $\endgroup$ Commented Oct 7, 2020 at 14:41

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No, it's not possible because it would be a sort of perpetuum mobile: the human supplying the plant with heat produced by consuming what the plant supplies and the plant using that heat to provide food to the human.

For the same reason you can't survive eating only your own flesh.

Thermodynamics is a harsh mistress.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Oct 7, 2020 at 19:30
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We (science) don't yet have a way of converting heat into energy directly, a gradient from hot to cold is needed where the flow of heat from hot to cold can be tapped to create energy. In a human body, there isn't really the needed heat gradient to tap for energy.

If however we disregard that, and assume some "magical" feature has been found by the plant. We would probably have to see some kind of algae, that would be able to substitute the function of the red blood cells (oxygen transporters). but still leave the flow of "blood" to transport nutrients around.

It's likely the immune system would be less fond of foreign algae floating around the body, but there are ways around that (symbiotic relationship, immuno-suppressant drugs or other such features)

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  • $\begingroup$ Note that the question is tagged with science based. Calling in magic doesn't seem like an appropriate answer $\endgroup$
    – L.Dutch
    Commented Oct 7, 2020 at 9:21
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    $\begingroup$ I was trying to refer to "magic" as in "any science advanced enough, is indistinguishable from magic..." kind of magic, so not really magic, but we don't even have any theoretic models for how such a thing could be done, so... $\endgroup$ Commented Oct 7, 2020 at 10:40
  • $\begingroup$ One thing to consider: if we read "heat" as a common / perhaps slightly unscientific term for "infrared light", then perhaps we get around the conversion of heat into (a more useful form of) energy? Warm things, like people, tend to emit IR light, which is we night vision cameras work. Maybe these plants will do the same? $\endgroup$
    – elemtilas
    Commented Oct 7, 2020 at 15:23
  • $\begingroup$ @MichaelMortensen wouldn't you call Maxwell's demon a theoretical model for how such thing could be done? $\endgroup$
    – lvella
    Commented Oct 7, 2020 at 16:55
  • $\begingroup$ @Ivella I would call that a thought experiment, same as the wiki does, not a theory $\endgroup$ Commented Oct 19, 2020 at 13:37
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Blood has several functions, only one of which is the transport of oxygen, carbon dioxide, and nutrients. This is primarily the job of red blood cells, but your blood also consists of white blood cells and platelets. White blood cells are a critical component of the immune system, and without them one will be highly susceptible to infection. Platelets are needed for blood clotting, and without them, even small cuts can be dangerous as bleeding simply won't stop.

Replacing just red blood cells with this plant that can process oxygen, carbon dioxide, nutrients, and waste might work, but the plant as described does not replace any functions of white blood cells or platelets. Replacing the entirety of one's blood with this plant will be a bad idea, since you will lose two very important functions of blood, and have very little ability to fight infection or stop bleeding.

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  • $\begingroup$ About platelets: as you say, they prevent blood from leaking out. But why would they be needed if the blood is replaced by a plant ? There's nothing to leak. About white cells, one could imagine that the plant is also the one handling the defenses and is able to produce white cells externally (to itself) so that it protects the host. $\endgroup$
    – Jemox
    Commented Oct 7, 2020 at 15:10
  • $\begingroup$ @Echox I had imagined the blood would be something like an algae-filled fluid, rather than a single plant that grows tendrils through the vasculature, so you'd still want to keep that inside. Having a symbiotic species provide immune defense to a different species will be pretty tricky - immune cells go through positive and negative selection to ensure that they can attack foreign cell, but will not attack the body's own cells. The plant species' immune cells would need to recognize a completely different species as "self", which isn't entirely impossible, but seems tough. $\endgroup$ Commented Oct 7, 2020 at 15:34
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Reality Check = POSSIBLE
As a side note, we've actually already done this rather successfully. This is essentially no different than our how prokaryiotic ancestors allowed some kind of archeobacterium to associate with and eventually inhabit their cells. We now call these bacteria "mitochondria" and we really can't live without them! Their function is already to do at least one thing you want the symbio-plant to do, namely, it provides the energy our complex cells require to not only live but thrive. Mitochondria, among other functions, break down glucose into ATP, which is used to fuel other cellular functions.

As to your specific scenario, I think such a relationship could be engineered if not naturally evolved. I don't think we could do it with our present understanding or technology, but I wouldn't dismiss the possibility that, maybe sooner than later, we could actually do something like this.

I would suggest that blood itself not be replaced with a plant. As if all the blood vessels had tiny vinca growing in them or something. We do actually need blood, and reengineering a human to exist without blood would be far more troublesome than merely integrating a symbiotic plant!

I'd suggest creating a plant-origin organ structure of some kind within the body. It would have to in some way communicate with the circulatory system in order to get the oxygen attached to the person's hemoglobin (that's what we use for oxygen transport, it doesn't just float around in the blood). Very likely this plant organ system would dump waste oxygen into a kind of vessel that connects with the lungs. As the symbiotic human breathes in, free O2 from the air and waste O2 from the plant organ would mix.

As we already know, plants need some kind of light, and water, and CO2 of course. But they also need O2. It's just that the net result of plant metabolism is an overall excess of O2, which animals take advantage of. Happily, animals don't use all the O2 they breathe in, which will leave plenty for the plant organ.

You would need some serious modifications to both the plant and the animal for this to work. On the animal side, you'd need to fully understand how symbiosis works in order to understand how to get it to happen again. You'd need to alter the animal's immune system to accept the plant organ & all its associated structures as Self. You'd probably want to expend considerable research into autoimmune disease states as well in order to prevent the animal body from attacking its plant symbiont and also possibly itself in consequence. Autoimmune diseases are already horrible enough.

On the plant side, you'd need to engineer a plant that produced photosynthetic pigments that react to far red and infrared wavelengths. Very little human visible light penetrates more than a few mm into the body (this is why, when you close your eyes and face a bright light, you can see red -- red wavelengths of light do indeed penetrate). But near infrared can penetrate much deeper. You'll want your plant organ to take advantage of what light can be transmitted through body tissues. (You'll also want to condition your host humans to spend as much time naked and exposed to IR light as possible.) You'll also need to engineer this plant to grow in specific ways: it will need to be supported in some way by the host's skeleton (you don't want a woody plant competing the host animal's flexible body plan); and it will need to integrate with the host's respiratory tract.

Lastly, you'll need to understand how symbiote genetic material gets passed passed on so that the system can be perpetuated without technological intervention.

The long and short of it: probably a negligible net effect for whole lot of effort.

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Maybe.

Humans need a lot more energy to run than a plant does, to the point that an order of magnitude that'd be marginally helpful to a human would be a product of several months of growth for a plant. An amount of "plant" that will produce yields that make a dent in an individual human's diet will take a fair amount of space, definitely much more space than that human typically occupies.

So, unless your man has a couple 20ft trees growing out of his back, he's probably not going to be sustained by plant alone.

It seems reasonable that a plant could make a store of this chemical energy, and provide the human with a temporary burst of sugars or similar chemical energy - but this seems less like symbiosis and more like a parasite making an effort to save it's host in an emergency.

A plant could of course drip feed that product into the host and supplement their metabolism, but again, the effect would be relatively minor because the difference in 'maintenance costs' for the two biological systems is so high.

A much better paradigm might be the plant helping the human to digest things they might otherwise gain little or no benefit from - plants have much different chemistry than humans do and so may produce digestive enzymes that humans do not, allowing them to break down food more efficiently. This would probably manifest more as an expanded dietary range than enabling eating dirt or something.

A plant does produce oxygen, a beneficial byproduct, that it can afford to share with the human - but the logistics of getting that oxygen into the human are strained. To strip carbon from CO2 the plant needs sunlight (so it can't just hang out in your lungs), and plants don't have active respiratory or circulatory systems to move their waste O2 somewhere else. You'd need a transfer mechanism to put that O2 in the human's bloodstream or lungs.

You'd also need a pretty noticeable surface area involved in that photosynthesis to produce a helpful amount of O2 - More like swamp thing than just a dude with greenish looking skin.

If you solved those problem this could be of value to the human - it's effectively a blood-doping system. It's not likely that the plant could completely replace normal respiration, but augmenting it for noticeable benefit is definitely feasible, assuming you solved the logistics.

That'd likely manifest as a human with above average physical endurance, but mostly normal strength. They're still bound by lactic acid buildup and other things, so, this isn't a super power, just like a really fit person.

Your human is going to need to drink a lot of water. That plant will suck up a considerable amount of moisture. This would be a noticeable downside to this relationship - manageable, but potentially dangerous as they'd be much more vulnerable to dehydration.

Overall, I think this synthesis could be of benefit in the right environment, perhaps even allowing a human to survive or thrive in an environment they otherwise might (very) slowly die in (without the aid of technology), but probably not something beneficial enough that this symbiosis would be attractive in an earth-like environment.

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I'll look at this from a different angle.

Blood is one of the most mission-critical parts of your body. If you lose too much blood, your body will produce more to replace it and ensure that you stay alive. If the blood was replaced with a plant, the body would be unable to recover from such a situation on its own. It would be completely dependent on this separate organism to somehow discover the problem and resolve it in a timely manner. Even if the body and the plant could communicate effectively, it's unlikely that the plant could regenerate as fast as the body can replace lost blood (when you donate blood plasma, your body replaces it within several hours). This means that any creature that evolved to use the plant instead of blood would have a significant evolutionary disadvantage and would be unlikely to survive over those with regular blood. Blood plays far too important of a role to outsource to another organism.

If instead you're arguing that a normal, blood-filled human undergoes a procedure to replace their blood with this plant, then you run into all sorts of new problems. Your body wasn't designed for that, and it will continue to produce blood even though your veins are full of plant matter. Your immune system will correctly identify the plant matter as foreign DNA and attack it. If the plant matter doesn't have the same physical properties (density, compressibility, coefficient of thermal expansion, etc) as blood, then it could be completely incompatible with the mechanics of your circulatory system. Your heart might not be able to pump it effectively, the valves in your heart and blood vessels might not be able to enforce one-way traffic, your body's mechanisms for regulating blood pressure might no longer work, etc. There are a lot on knock-on effects for swapping out blood for plant matter, and it's unlikely that you'll be able to find anything other than blood that will work. If you go the evolutionary route from the first paragraph, you'll need to almost completely re-design the circulatory system.

Don't forget that plants don't (generally) live as long as humans. That means that those plants in your veins will die at some point in your life. Plant matter can decay into all sorts of substances that are toxic to humans. You would need to evolve some sort of mechanism that could locate dying plant matter preemptively and dispose of it safely before it began to decompose.

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