What is Electron carriers?

$\mathrm{NAD^+}$ (nicotinamide adenine dinucleotide) accepts two electrons and one proton from a substrate to become NADH; FAD accepts two electrons and two protons to become $\mathrm{FADH_2}$. Both are coenzymes of dehydrogenases, and both are recycled: reduced by catabolism, re-oxidised by the respiratory chain. A cell holds only micromoles of them, turning over thousands of times an hour. Their redox potentials, $E'_0 = -0.32\,\mathrm{V}$ for $\mathrm{NAD^+}/\mathrm{NADH}$, place them far below oxygen ($+0.82\,\mathrm{V}$): each pair of electrons NADH hands to oxygen releases $\Delta G^{\circ\prime} = -2F\Delta E = -220\,\mathrm{kJ}/\mathrm{mol}$, enough for more than four ATP.