system
Given the following post, respond as if you're commenting on Reddit:
human
Specifically, what about heavy water is it that makes it harmful to organisms?

So it has some extra neutrons. So what?

According to its Wikipedia article,

> Experiments in mice, rats, and dogs have shown that a degree of 25% deuteration causes (sometimes irreversible) sterility, because neither gametes nor zygotes can develop

> High concentrations of heavy water (90%) rapidly kill fish, tadpoles, flatworms, and Drosophila

> deuterium-poisoned mammals die of a failure of bone marrow (bleeding and infection) and intestinal-barrier functions (diarrhea and fluid loss)

But, at the chemical level, what specifically is it about the added neutrons that means that gametes and zygotes can't develop, or that the bone marrow fails, or any of the other effects it has?

My understanding of chemistry admittedly isn't great, but I didn't think the number of neutrons affected how elements react with each other in any way, so it can't be because of some adverse chemical reactions taking place (unless I'm wrong, of course).

So what exactly is it that makes it harmful to organisms?
gpt
The difference is due to something called kinetic isotope effects. Bonds to deuterium are just a tiny bit stronger and shorter than bonds to hydrogen. This is a result of the deuterium being twice as heavy.

The toxicity stems from the fact that enzymes are very finely tuned systems and the small change in bond strength is enough to really mess with how fast some reactions proceed. Do this on a large enough scale by drinking a bunch of D2O and your whole biochemistry goes out of whack leading to death.