The gaining of electrons by an atom during metabolism is called reduction.

Here's why:

* Reduction is a chemical process where an atom or molecule gains electrons. This results in a decrease in the oxidation state of the atom.

* Oxidation is the opposite process, where an atom or molecule loses electrons.

In metabolism, these processes work in tandem. The gain of electrons in one molecule (reduction) is often coupled with the loss of electrons in another molecule (oxidation). This is called a redox reaction.

For example, in cellular respiration, glucose is oxidized (loses electrons) to generate energy, while oxygen is reduced (gains electrons).