Electron Transport Chain

The electron transport chain, in cellular biology is one of the steps that your cells use to make energy from the food you eat.

It is the third stage of aerobic cellular respiration. Cellular respiration refers to the process by which your cells use food to generate energy. The electron transport chain is the place where most of energy required to run cells is generated. This “chain” actually refers to a collection of protein complexes as well as electron carrier molecules in the inner membrane cell mitochondria. Also known as the cell’s powerhouse,

The electron transport chain is a collection of protein complexes as well as electron carrier molecules that are found within the inner membrane mitochondrial. They generate ATP.

The electrons travel along the chain, passing from one protein complex to another until they reach oxygen. Protons are pumped from the mitochondrial matrix across to the inner membrane and into the intermembrane space.

An electrochemical gradient is created when protons accumulate in the intermembrane area. This causes protons to flow downwards through ATP synthase and back into the matrix. This proton’s movement provides energy for the production ATP.

The Electron Transport chain is occurs within the Mitochondria

Components/ Electron carriers of Electron Transport Chain

FMN (Flavin Mononucleotide)



Iron-sulfur proteins

Electron Transport Chain Complexes

Complex I (Mitochondrial complex I)

Complex II (Mitochondrial complex II)

Complex III (Mitochondrial complex III)

Complex IV (Mitochondrial complex IV)

The end products of the ETC or electron transport chain are the formation of ATP and the formation of water by the reduction of O2.

Mitochondrial NADH+H+ arrives directly at the ETC from the TCA cycle and immediately oxidizes to NAD+, with its protons (hydrogen ions) remaining in the matrix, and its electrons (e-) going to complex I. As the electrons arrive on complex I, the complex immediately goes through a series of redox (reduction and oxidation) reactions. These reactions create a proton pump within complex I, pumping (or translocating) 4 protons from the matrix through the protein into the intermembrane space. The electrons now transfer to mobile carrier Q, and NAD+ returns to its original source to pick up more hydrogen ions.

FADH2 arrives to the ETC from the TCA cycle. FADH2 then oxidizes to FAD, with its electrons and protons going to complex II. FAD then returns to the TCA to pick up more electrons and protons. Complex II goes through redox but it does NOT create a proton pump.

Mobile carrier Q also picks up all of the electrons on complex II, and shuttles the electrons it collects to cytochrome complex III. The electrons are then transferred to complex III, which also immediately goes through redox (reduction and oxidation) reactions. This again creates a proton pump, pumping 4 protons from the matrix through complex III, directly into the intermembrane space of the mitochondrion. (NOTE: there are many mobile carrier Q coenzymes present in the ETC, and these molecules are also called ubiquinone or ubiquinol).

The electrons are now shuttled from complex III to complex IV by mobile carrier C. As the electrons transfer onto cytochrome complex IV, it immediately goes through another redox reaction. This creates a final proton pump, pumping 2 protons from the matrix through cytochrome complex IV, directly into the intermembrane space of the mitochondrion.

Complex IV is the last step in the ETC, and the electrons that have been driving these reactions now need another place to go. To solve this problem, an oxygen atom, which has a very strong attraction for electrons, picks up two electrons from complex IV, along with two some free protons from the mitochondrial matrix, to simply form water (H20). In essence, the FINAL acceptor of the electrons at complex IV is oxygen (which forms water with the protons and electrons it accepts into its structure). This is referred to as 'metabolic water' (because it is made in metabolism), and is actually makes up for 10% - 20% of total daily fluid losses (the rest we must replenish from the fluids and foods we intake).

The many protons that this process pumps in the intermembrane space create an imbalance in hydrogen ion concentration (charge) that the cell does not like. All cells prefer homeostasis, so to help relieve this imbalance, ATP synthase (a special protein embedded next to the ETC) pumps several protons at a time back into the matrix. As these protons move into the matrix, enough energy is liberated to phosphorylate (or add another Pi to) ADP, thus synthesizing ATP. The ATP will then leave the mitochondrion and go where energy it is needed in the cells for life processes.