An electron carrier is a molecule that accepts an electron from one molecule and transports it into another, this movement of electrons by carriers is known as the electron transport chain. Carriers can exist in either an oxidized or reduced form, in an oxidized form the carrier is available to receive additional electrons, in a reduced form the carrier is actively carrying electrons. Although there are many different electron carrier, the two most common within a human body are NADH and FAD. NADH is the reduced form of NAD⁺ that has accepted two electrons and a hydrogen ion, furthermore, FAD is the reduced form of FADH2 that has accepted two electrons and a hydrogen ion. These electron carriers are essential in cellular respiration where they harvest electrons from glucose to produce vital ATP, redox reactions during cellular respiration add or remove electrons from the electron carriers. Cellular respiration generally occurs in three processes but can vary depending on environment and the type of cell, these processes are glycolysis, the citric acid cycle, and oxidative phosphorylation. In glycolysis, the six-carbon molecule glucose undergoes a range of chemical transformations, the electron carrier NAD⁺ is reduced by accepts two electrons and a hydrogen ion producing NADH.  Additionally, two molecules of pyruvate, a three-carbon organic molecule, and ATP is produced. The pyruvate molecules then enter the mitochondria where they are processed into acetyl CoA, NADH is also produced while Co₂ is released. The processes then continue into the citric acid cycle, acetyl CoA is processed with a four-carbon molecule to produce ATP, NADH, and FADH₂. Additionally, the four-carbon molecule is regenerated to be used again and CO₂ is released. The final process of oxidative phosphorylation is now initiated, NADH and FADH₂ place their electrons into the electron transport chain to become oxidized. With these electrons, the electron transport chain expels protons from the matrix to create an electrical gradient. Protons reenter the matrix with the assistance of an enzyme called ATP synthase and produce ATP, during this process the majority of ATP produced in cellular respiration is made. With the process of cellular respiration explained, it’s easy to see how vital electron carriers are in the production of ATP.

References:

https://www.khanacademy.org/test-prep/mcat/biomolecules/overview-metabolism/v/electron-carrier-molecules