Re: When an organism dies, are there more C12 or C13 deposits left over?

Hello! I know that certain enzymes in living organisms "prefer" C12 because of its lighter mass. Using this logic, I decided that when an organism dies, more C13 is left over since the body reacts with C12 faster.

However, my tutor said otherwise (with uncertainty) and my teacher just re-explained that c12 reacts quicker w/o answering my question. So, when an organism dies, is more c12 or c13 left over? Thanks!

Hi Rachel,

It sounds like you are asking if an organism is "enriched" for carbon 13 (¹³C) relative to carbon 12 (¹²C), so that there are more ¹³C atoms than ¹²C atoms in the body after the organism dies. However, I may be interpreting your question incorrectly (more on that below), so bear with me.

First, take a look at this answer (1055532737.Bc) about the enrichment (isotopic fractionation) of ¹²C over ¹³C in biological systems.

From that answer, you can see that while it is true that "certain enzymes" have a preference for ¹²C over ¹³C, the group of enzymes involved is rather small. For the most part, the isotopic fractionation occurs when carbon is fixed by photosynthetic and chemosynthetic organisms; the δ ¹³C value for a given carbon fixing organism is the same as it is for organisms that eat those carbon fixers. So, that means that most biological processes in most organisms (especially heterotrophs) are "agnostic" with respect to ¹²C vs ¹³C.

In addition, your question seems to assume that biological processes will act preferentially toward every individual ¹²C atom relative to every ¹³C atom. However, as you can also see in my earlier answer, enzymatic isotopic fractionation only occurs with specific atoms in specific substrate molecules (like pyruvate's keto carbon). So most of an organism's carbon atoms are unaffected.

With that having been said, it is not clear to me what you mean when you describe ¹²C or ¹³C "deposits" as being "left over." Left over from what? When an organism dies, the remains have the same composition that they had when it was alive (at least for a little while). So an organism that consumed C4 plants all its life would have a δ ¹³C value of -13 both while it was alive and when it was dead.

In absolute terms, all biological systems are composed of a greater number of ¹²C atoms than ¹³C atoms, because the environmental ratio of ¹²C to ¹³C is very large to start (9893:107). In organisms, that ratio is even greater relative to the environment. It doesn't make a difference if an organism is alive or dead, there's always more ¹²C than ¹³C.

I hope this helps address your question!

Keep asking questions!