In this post, we will discuss how the components of the mitochondria come together to produce energy or ATP for the cell.

Mitochondria uses, both, pyruvate (mainly glucose plus other sugars) and fatty acids (fat) as fuels. As they enter the mitochondrial matrix, they are converted into acetylCoA (via the enzyme acetyl-coenzyme-A) which is a metabolic intermediate used in the citric acid cycle.

Pyruvate; a component of sugar molecules that enter CAC. Click on image for credit.
Acetyl CoA. It’s many carbon atoms are important for the cycle. Click on image for credit.
Citric Acid Cycle. Click on image for credit.

The above images are for perspective only. Please focus on the text and eye the images as reference. You do not have to memorize everything.

The citric acid cycle converts the carbon atoms in acetylCoA to CO2, which is released from the cell as a waste product.

During this conversion process, the cycle generates high energy electrons carried by carrier molecules NAD+ (Nicotinamide adenine dinucleotide which then becomes NADH) and FAD (flavin adenine dinucleotide which then becomes FADH2). These high energy electrons are then transferred to the inner mitochondrion membrane, where they enter the electron-transport chain.

Electron transport chain on the cristae of the inner mitochondrial membrane now begins.

A hydride ion (H) is removed from NADH by the enzyme NADH dehydrogenase complex, and is converted into a proton and two high energy electrons (H -> H+ + 2e). NADH dehydrogenase complex accepts the electrons.

The electrons are then passed along the chain to the second enzyme Cytochrome b-c1 complex with the help of ubiquinone (a small hydrophobic molecule) that picks the electron up from NADH dehydrogenase and delivers it to cytochrome b-c1.

Electron transport chain. Click on image for credit.

The third and final enzyme which receives the electron (this time with the help of cytochrome c) is Cytochrome oxidase complexand as its name indicates, it will finally donate it’s electrons to oxygen.

Actually, four electrons from cytochrome c are added to four protons from the aqueous solution to form two hydrogen molecules (2H2) which then combine with O2 to form water (4e + 4H+ + O2 -> 2H2O). It is here that almost all oxygen we breathe is used, serving as the final receiver for the electrons that NADH donated in the beginning of electron transport chain.

This is also sometimes called the respiratory chain. But what is the point of all this?

Each of the above three respiratory enzyme complexes uses the energy of the electron transfer to take protons from water (present in the aqueous solution H2O dissociating into H+ + OH) and pumps the protons out into the intermembrane space of the mitochondria (explained how later). This creates a build-up of protons in the intermembrane space, building up the electrochemical proton gradient.

Electron transport chain showing clearly the ATP synthase at work. Click on image for credit.

A large inner membrane-bound (keep in mind all this is happening in the mitochondria’s cristae, which is the membrane infolding) enzyme called ATP synthase creates a hydrophilic pathway that allows protons to flow down their electrochemical gradient back into the mitochondrial matrix. As the protons rush and move back into the mitochondrial matrix from the intermembrane space, the ATP synthase uses this energy to drive the energetically unfavourable reaction between ADP (Adenosine Diphosphate) and Pi (Phosphate group) to make ATP. This process is called oxidative phosphorylation.

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