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regular_modern_girl t1_is6e6zn wrote

Reply to comment by BlazeObsidian in Why do people take iodine pills for radiation exposure? by Furrypocketpussy

From my knowledge of biochemistry (and chemistry in general), this is correct, there isn’t any difference in how biological systems handle different isotopes of a given element, with the notable exception of hydrogen isotopes, which are apparently different enough from each other atomically that they have notably different effects on biochemistry (in particular, an organism that is given only heavy water in lieu of regular water will eventually become “deuterated”—meaning that the majority of its hydrogen-1 is replaced by hydrogen-2 or deuterium—and suffer a variety of symptoms strikingly reminiscent of radiation poisoning, even though deuterium is not itself radioactive, before eventually dying, apparently because the added mass of deuterium just throws off a bunch of key cellular processes enough on a molecular level that basic enzymes don’t work correctly; note that this doesn’t happen just from consuming small amounts of heavy water, which is actually considered non-toxic on its own, just so long as most of the water you’re consuming has H-1), but this is the one unique exception to my knowledge (at least this is what my chemistry professor claimed).

EDIT: I looked more into this, and it does indeed appear to be the case, as there have actually been studies with oxygen-18 (which like hydrogen-2 is to hydrogen-1, is a rarer, heavier, but non-radioactive isotope of oxygen that makes up something like 0.2% of Earth’s atmosphere) which have shown that aerobic organisms can safely breathe it exclusively with no ill-effects, unlike replacing an organism’s water intake with heavy water. So essentially, biology really doesn’t distinguish between isotopes, and it usually doesn’t matter unless it’s a heavier isotope of hydrogen, or a given isotope is giving off ionizing radiation.

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BlazeObsidian t1_is6ggpz wrote

Thank you for confirming. Interesting to know about Hydrogen isotopes though. Knew that H2O and D2O differ quite a bit in chemical properties but never knew how much magnified it is at the biochemical scale.

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newappeal t1_isb99lc wrote

>So essentially, biology really doesn’t distinguish between isotopes, and it usually doesn’t matter unless it’s a heavier isotope of hydrogen, or a given isotope is giving off ionizing radiation.

It's true that biology doesn't distinguish between isotopes on a biochemically-relevant level (except, as you mentioned, in extreme examples like if an organism was exposed solely to heavy water), but isotopic discrimination is strong enough that it can be used to track large-scale, long-term changes in biogeochemical cycling. One prominent example is the use of isotopic signatures to differentiate between CO2 of organic vs nonorganic origin, which is a crucial piece of evidence in showing that modern CO2 concentrations are rising due to the burning of fossil fuels and not volcanic activity. Plants which use C3 photosynthesis also have a different 13C:12C ratios than those which use C4 photosynthesis, since the former's carboxylation reaction takes gaseous carbon dioxide as an input and the latter's aqueous carbonate.

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regular_modern_girl t1_isbb3dq wrote

Is that really biological systems distinguishing, though, or is that just human researchers looking at isotope ratios and using them to determine where a given element in a biochemical context came from? Like do plants that use C3 photosynthesis have a different ratio than C4 plants because something about the process causes them to preferentially take in one carbon isotope over another, or is it just because (as you said) they’re getting their carbon from different sources?

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newappeal t1_isbbnbh wrote

>Is that really biological systems distinguishing, though, or is that just human researchers looking at isotope ratios and using them to determine where a given element in a biochemical context came from?

Those are the same thing. The biochemical discrimination is the mechanism that causes the difference in isotope ratios.

However, if you're asking whether the discrimination is teleological in nature - i.e. that it has a biological "purpose" that has been acted upon by selection pressure - then the answer is no, it is not "intentional", but rather a correlate of the differences in photosynthetic strategies that were directly selected for.

Edit: To specifically address the last bit: they're both getting the same atmospheric carbon, just different isotope fractionations of it.

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regular_modern_girl t1_isbetxk wrote

To clarify, I guess what I meant by “distinguish” is whether or not different isotopes behave fundamentally differently as far as biochemistry is concerned (apart from the obvious difference that some are radioactive and give off ionizing radiation that destroys biological macromolecules, etc.), like in the way that different elements do, and the only specific example of that it seems is with different hydrogen isotopes. It seems that basically apart from deuterium, as long as an isotope isn’t radioactive, it can be used biologically just the same as the more common isotope of that element (assuming a biochemically-relevant element here, obviously) without causing any problems.

Although tbf, I guess the comment that started this thread was asking if the human body (in particular the thyroid gland) “knew the difference” between different isotopes of iodine, and I guess sort of what I’m asking here is, do C3 plants take in more or less of a given isotope than C4 plants because something is making a distinction between the isotopes (regardless whether or not there’s a specific “purpose” to taking in more of one than the other), or is it just a side effect of getting carbon from CO2 in the air versus carbonic acid/carbonate in the water?

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newappeal t1_isbi1rp wrote

>To clarify, I guess what I meant by “distinguish” is whether or not different isotopes behave fundamentally differently as far as biochemistry is concerned

I would maintain that if the isotopes are incorporated at different rates (as they indeed are), then they behave differently by definition. I'm not sure what "fundamentally" means here - if you mean "substantially" in the sense of having biological relevance, then I would say no, they do not. But "biological relevance" itself has no objective definition. I could say that they form the same sort of chemical bonds, but that's not actually entirely true, just mostly true, of isotopes.

>or is it just a side effect of getting carbon from CO2 in the air versus carbonic acid/carbonate in the water?

I should specify that CO2 ultimately comes from the atmosphere in both cases. The difference is the carboxylation reaction in C3 plants uses carbon dioxide directly, whereas in C4 plants, CO2 first reacts with water to form bicarbonate before being conjugated to an organic molecule. The relevant factors for fractionation are therefore the diffusion rate of 13CO2 and 12CO2 in the gaseous state, and the preferences for the relevant enzymes for each carbon isotope.

The underlying physical principles are the same here as in the case of neutron-free hydrogen vs. deuterium. The difference is just one of degree. Carbon-13 is 8.3% heavier than Carbon-12, while deuterium is twice as heavy as hydrogen. Moreover, hydrogen atoms (of all isotopes) are commonly transferred between compounds individually, whereas single carbon atoms do not appear in biological reactions. (In the specific case of carbon fixation, the carbon makes up a minority of the mass of the molecule that actually participates in the reaction.) The discrimination between hydrogen and deuterium in chemical and physical processes is therefore as high as it could possibly be for stable isotopes, and the differences in rates between them is therefore maximal compared to other elements. These discrepancies in rates, which differ in relative magnitude direction for different processes, are enough to upset the balance of biological systems if they are supplied with too much deuterated water. However, I also can't say for certain that a biological system supplied with only 13C wouldn't suffer a similar fate. After all, we're comparing the partitioning of naturally-occurring isotopic ratios of stable carbon isotopes to the extreme hypothetical of exposing an organism to pure heavy water.

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