Electron transport chain and oxidative phosphorylation

Your heart is constantly working. Whether you’re swimming or taking a nap, your heart is always on the go.

The main form of energy that keeps our heart cells, and really all of our body cells, going is adenosine triphosphate, or ATP.

In most cells, the main ATP producing factory is the mitochondria, which has an inner and an outer membrane, and it’s along the inner membrane where a process called oxidative phosphorylation occurs.

“Oxidative” refers to oxidation - which is when a molecule donates its electron, and “phosphorylation” which refers to the addition of a phosphate group to adenosine diphosphate, or ADP, to form ATP.

So oxidative phosphorylation is the process of making ATP by donating electrons to complexes embedded within the inner mitochondrial membrane.

These complexes are proteins or lipids coupled with metals like iron and copper that facilitate the movement of electrons.

Together, they form the electron transport chain.

During the electron transport chain, electrons are passed on from complex to complex, and finally to oxygen, creating a proton gradient that will be used to make ATP.

The electron transport chain begins with two key molecules that want to donate their electrons: nicotinamide adenine dinucleotide, or NADH, and flavin adenine dinucleotide, or FADH2, both of which get oxidized in the electron transport chain.

NADH and FADH2 are primarily generated in the citric acid cycle which occurs in the mitochondria, but it can also come directly from glycolysis - which is the breakdown of glucose in the cytoplasm, or fatty acid oxidation, which is the breakdown of fats in the mitochondria.

Enzymes called dehydrogenases help generate the electron-rich NADH and FADH2.

And when those molecules are coming from the cytoplasm they can only enter the mitochondria using a specific shuttle.

When using the malate-aspartate shuttle, electrons enter the electron transport chain as NADH.

When using the glycerol-3-phosphate shuttle, electrons enter electron transport chain as FADH2.

There are two entry points into the electron transport chain.

The first point of entry, is where NADH gives its electrons to Complex I.

Complex I contains flavin mononucleotide - a derivative of riboflavin or Vitamin B2 - and iron-sulfur centers called FeS.