What does 13C mean in biology?

Answer 1

Short answer :

It means "carbon 13", that is the (stable) isotope 13 of carbon element. It is used in life sciences to trace carbon-cycle in a cell, or to label some parts of macromolecules.

Longer answer :

Carbon is an element that is present in all of the macromolecules that compose our cells (lipids, carbohydrates, fibres, proteins, vitamins, hormones, etc). For that reason, we are called "organic life", which means "carbon-based life" ; we share this with ALL life forms on this planet (so far...).

Though 99% of carbon in nature is carbon-12 (6 protons + 6 neutrons), other isotopes exist like sable carbon-13 (6p + 7n), and radioactive carbon-14 (6p + 8n). That means we are composed of 99% of carbon-12. Carbon-13 do have exact same chemical behavior as carbon-12, but it has some specific features in its physical properties (e.g. its spin).

As examples, it is possible to "play" with carbon-isotope contents in cells and trace labelled molecules wherever they go ; it is possible as well to solve the structure of macromolecules using carbon-13.

Other elements widely found in life have as well stable isotopes, that are used in a way or another in life sciences [element (isotope)] : hydrogen-1 (deuterium-2), oxygen-16 (oxygen-18), nitrogen-14 (nitrogen-15), ...