Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologyhardmcq1 pt

Researchers supply bacteria labeled with the isotope nitrogen-15 instead of common nitrogen-14 to trace where plants take up bacterial nitrogen. Why is nitrogen-15 suitable as a tracer?

A.An extra proton gives it a positive charge that root membranes admit easily.
B.An extra electron makes it radioactive and easy to image in tissue.
C.Identical proton and electron counts give it essentially the same chemical behavior as nitrogen-14.
D.It forms distinctive covalent bonds that plant enzymes preferentially absorb.

Explanation

Core Concept

Isotopes of an element have the same number of protons and, when neutral, the same number of electrons, so they have very similar valence chemistry. Nitrogen-15 differs from nitrogen-14 by one neutron and can therefore be detected by its mass while participating in most biological reactions in nearly the same way. Small isotope effects can exist, so 'essentially the same' is more accurate than 'exactly the same.'

Correct Answer

CIdentical proton and electron counts give it essentially the same chemical behavior as nitrogen-14.

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