Q&A

If an ET has caramel blood, what it implies?

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I want to write a SF story where humans discover that a species of ET has caramel instead of blood and I need some biological information in order to develop it in a realistic way.

For my story, i want to know If these creatures have caramel instead of hemoglobin, what does it implies about their physiology?

Other factors that would be nice include can they have an awful tasting flesh? Can caramel be made addictive? - High sugar concentrations or a special addictive molecule?

Thanks for yours future answers (and sorry for the language errors I'm French)

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Interpretation:

ET's blood equivalent is primarily composed of caramel, with some additional components, but without hemoglobin. The flesh tastes terrible, without rendering the caramel blood inedible. The caramel should be addictive.

Most animals with blood have a very similar blood composition. This is likely to be true with ET's homeworld, too.

Caramel a mix of dehydrated glucose and fructose, alongside polymerised carbohydrates like caramelans ($C_{24}H_{36}O_{18}$), caramelens ($C_{36}H_{50}O_{25}$), caramelins ($C_{125}H_{188}O_{80}$) and some chemicals such as diacetyl. Having blood composed primarily of caramel implies a great deal of food availability in the ancestral environment of ET, to the point that so much caramel can be easily synthesised inside ET-esque organisms.

However, there must have been some evolutionary pressure to keep force ET to have caramel blood, rather than have pockets of sucrose, fructose, and glucose, or even just a large stomach holding undigested food.

Having pockets of fructose is actually done by some organisms on Earth (fruits tend to have lots of fructose), and the fructose is mostly held in solution, so being liquid is not sufficient to distinguish caramel from having sugar solutions inside the body.

If we relax the definition of caramel to only require a combination of sugars and diacetyl (which are the main portions of the flavor), then a solution becomes more viable. Diacetyl is a by-product of fermentation, which is one of the two main routes of energy.

A quick step back: two main sources of energy are respiration and fermentation. On Earth, due to an abundance of available oxygen, respiration is very common. However, some species switch to fermentation, if there is not enough oxygen. Such species include yeasts, including the yeasts used to make bread, and the yeasts used in brewing alcoholic drinks. However, it may be possible for ET to use anaerobic respiration, using fumaric acid in place of oxygen.

If oxidisers, like oxygen, are rare in ET's environment, then they may primarily use fermentation and anaerobic respiration, and so as a by-product, create diacetyl, ethanol, and lactic acid. These waste products would then put in the blood stream.

Fumaric acid is found in many Earth-based organisms, and is a common food additive (E297). It is also used in the respiration of E. coli, so this respiratory route is evidently possible.

Of course, main constraint in this ET's environment is not a lack of food (food is easily available) but a lack of oxidisers. Thus, ET's ancestors will not be concerned with finding sources of food, but with finding plants that are able to make fumaric acid.

With this, I think we have the building blocks to make an ET to the specifications you've requested.

1. ET comes from a planet with abundant fuels, but with little-to-no oxidisers. Perhaps they have a methane ocean or atmosphere.
2. ET uses anaerobic respiration and fermentation for energy, with cells consuming the sugars from the "bloodstream".
3. The by-products of anaerobic respiration and fermentation include hydrogen, ethanol, diacetyl, and lactic acid. (Ethanol, lactic acid, and hydrogen, and will likely be filtered off from the circulatory system to prevent damage; yes, this means ET will likely urinate alcohol and fart hydrogen gas).
4. ET's blood, more importantly to ET, contains amounts of fumaric acid, which is essential to it as oxygen in hemoglobin is to us since fermentation alone is not likely sufficient to sustain it (though due to its rarity, fumaric acid will likely be concentrated in ET).
5. ET's fat equivalent will likely be concentrated fumaric acid, and thus unpalatable (though it can be processed to something edible).

This means, anatomically and in terms of taste:

1. ET has glucose, diacetyl, fumaric acid, and ethanol in its blood. Glucose is sugar, diacetyl tastes buttery, fumaric acid tastes fruity and sour, and ethanol is drinking alcohol. It also has lactic acid, which is used to pickle things, but lactic acid will likely be filtered off by their equivalents of the kidney.
2. ET's anatomy concentrates fumaric acid in fat equivalents, which would be unpalatable to us due to being very sour.
3. ET's muscles are not particularly constrained by the setup, but we can assume that they also hold fumaric acid in unpalatable quantities.

Let's compare this against our requirements.

1. ET's blood equivalent is primarily composed of caramel. Done: it's a sugar solution with the caramel coloring (diacetyl) included; this is similar enough in my book.
2. ET's blood has no hemoglobin. Done.
3. The flesh tastes terrible. Done: it likely has an unpalatable amount of fumaric acid in it (and if there's an outbreak of ET obesity, then there's likely even more fumaric acid there).
4. The caramel should be addictive. It has loads of sugar, some natural flavorings, and alcohol. I think it's plenty addictive (though ET's urine might be more so, due to having more ethanol).