The Mitochondria

The purple thing is the mitochondria, you can see the highly folded inner membrane to maximize surface area. The lower picture shows complexes in the Electron Transport Chain which pump protons into the intermembrane space, creating a voltage gradient.

By Kayladanesh – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=79592408

A mitochondria works like a battery. It maintains a voltage gradient across its inner-membrane. The outside of the membrane has a positive charge and the inside has a negative charge. Protons from the outside flow back through the membrane down the gradient, releasing energy, through a large cross-membrane protein complex called ATP synthase, which uses that energy to create ATP.

The energy to maintain the voltage gradient is obtained by oxidizing hydrocarbons – fats and proteins. Whether the caloric source is fat or protein, most of the hydrocarbons are oxidized in the mitochondria. Glucose is broken down to a compound called pyruvate in the cell before entering the mitochondria. Fats go through a process called beta oxidation which breaks them down into Acetyl-Coenzyme A. Both pyruvate and Acetyl-CoA enter into the krebs cycle to be completely oxidized. We are fire in a bottle.

The main product of the oxidation are the reduced electron carriers NADH and FADH2. Remember, oxidation is just electrons flowing from carbon and hydrogen to oxygen. In biological “redox” systems, instead of the electrons flowing through a wire, like from the negative to the positive terminal of a battery, they are transferred from molecule to molecule. Each time they are transferred, a little energy is given off.

By CNX OpenStax - http://cnx.org/contents/GFy_h8cu@10.53:rZudN6XP@2/Introduction, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=49924807
The Electron Transport Chain takes the electrons from NADH and FADH2. The electrons flow from complex I or II to Coenzyme Q to complex III to Cytochrome C to complex IV before returning to the mitochondrial matrix to form water – Oxygen finally gets its electrons back. Along the way, protons are pumped by complexes I, III and IV.

By CNX OpenStax – http://cnx.org/contents/GFy_h8cu@10.53:rZudN6XP@2/Introduction, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=49924807

NADH and FADH2 store the energy from the oxidized hydrocarbons and deliver them to Complex I and II of the electron transport chain. From there the electrons follow the thin black arrows in the diagram. They are passed from Complex I and II to Coenzyme Q to complex III to Cytochrome C to complex IV before returning to the mitochondrial matrix to form water – Oxygen finally gets its electrons back. Along the way, protons are pumped by complexes I, III and IV.

In the final step, protons move through complex V, aka ATP synthase, back into the mitochondria. The released energy from the protons moving down the voltage gradient is used to phosphorylate a molecule of ADP to ATP. ATP is used to power most of our bodily processes like moving our muscles. Here’s an amazing video of the process that is part of the amazing Wikimedia Commons media project.

NADH hands a pair of electrons to Complex I. The electrons moved through the Electron Transport chain, which uses the energy released to pump protons across the membrane. The protons move through ATP synthase which uses the energy to produce ATP. The electrons return to Oxygen.

By Cwallworklegal – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=79116702

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3 thoughts on “The Mitochondria”

  1. Just being pedantic: Mitochondrion is singular if I am not mistaken (non-English speaking person here). Have to work through the rest more diligently.

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