What is radioactive?? And which one, xylem or phloem will become radioactive first when exposed to CO2?

Answer 1

Well, generally speaking neither is radioactive in any vast sense. When you talk about picking up radioactivity with exposure to CO2, you can only really mean carbon-14.

Carbon-14 occurs naturally... it's produced in the upper atmosphere and trickles down. Even then it's not too common, relatively speaking, occurring in only about one of every trillion carbon atoms. Still, given that there are many trillions of carbon atoms in a typical gram of organic stuff, it's pretty much ubiquitous among living things.

And I qualify that with living things for a good reason. Living things are constantly exchanging carbon with the environment, either by making sugars or breaking them down. So living things will have an amount of carbon-14 similar to that in the atmosphere. Because carbon-14 is radioactive, it will slowly break down in dead things until it is all eventually gone. It has a half-life of about 5700 years, so that can take a long time for you and me, but not a very long time geologically.

Now this is all important to answer your question - in an adult plant, xylem is often dead tissue. It is just a straw through which water and nutrients are transferred. Phloem is the actual living stuff. So if you exposed a plant to a lot of carbon-14, the amount would increase in the phloem, but would decrease as usual in the xylem.

Hope that helps!

PS: Don't worry about being irradiated by your own carbon-14... like I said, there's really so little of it there that you have far more to worry about on a sunny day.