Question

With regard to metabolic reactions, what does it mean when someone says a molecule has been "reduced"

Answer 1

Example is conversion of NAD+ to NADH+ is an example of reduced in metabolism.

Explanation:

Oxidation and reduction or redox reaction means transfer of electron from one atom to another in a compound.

In photosynthesis and respiration redox reaction are seen.

The substance that gain the electrons in  metabolic reaction is said to be reduced.

In respiration process oxygen is being reduced. The NAD+ accepts hydrogen ion  and 2 e- to become NADH+H+. These are electron carriers.

The reactions can be seen in glycolysis, Kreb's cycle.