This MCAT Basics Podcast continues its series on metabolism, and in today’s episode, Alex Starks talks about the Electron Transport Chain and ATP synthesis. The story continues with a look at how electrons flow through complexes I,II,III, and IV, as well as how it is coupled to the synthesis of ATP. Everything you need to know about this stage of metabolism is made clear and easy to understand so you can ace any metabolism question on the AAMC MCAT Exam.
- [03:10] The Purpose of the Electron Transport Chain
- [07:24] What is NADH Dehydrogenase
- [11:32] Coenzyme Q in the Electron Transport Chain
- [18:31] Why Oxygen is Called the Terminal Electron Acceptor
- [20:33] The Proton Motive Force
- [27:10] The Effects of Breaking the Electron Transport Chain
- [30:14] DNP: The “Miracle Weight-Loss Drug”
- [34:30] Fatty Acid Metabolism
- [34:58] Quiz and Important Takeaways
The Purpose of the Electron Transport Chain
The electron transport chain is a process that begins with moving electrons through a series of electron transporters that undergo redox reactions and causes hydrogen ions to accumulate within the matrix space. After that, the concentration gradient forms where hydrogen ions diffuse out of the matrix space by passing through ATP synthase. This transfer of electrons releases energy, which facilitates the transfer of protons from the matrix across the membrane into that intermembrane space. The current produced from the hydrogen ions powers the catalytic action of ATP synthase to produce phosphorylates ADP, producing ATP.
So, how does this all work? Well, nobody knows the science at this point. However, we understand that the redox reaction provides energy that is converted into work.
Steps in the Electron Transport Chain
The entire electron transport chain involves four major membrane proteins that work together to accomplish ATP synthesis. The steps are complex 1, complex 2, complex 3, and complex 4.
- Complex one is often referred to as NADH dehydrogenase, and the reason you need to know that term is because it’s testable in the MCAT. In this step, the reduced coenzyme NADH binds to this complex and functions to minimize coenzyme Q10. This reaction donates electrons, which then transfers protons across the membrane into the intermembrane space.
- Complex two acts on the succinate produced by the citric acid cycle and converts it to fumarate. This reaction is driven by the reduction and oxidation of FAD (Flavin adenine dinucleotide) along with the help of a series of iron-sulfur clusters. However, complex two is not a transmembrane protein, and so it doesn’t pump protons.
- Complex 3 oxidizes ubiquinol and also reduces two molecules of cytochrome-c. The electron is transported via these reactions onto complex 4 accompanied by the release of protons. The unique thing about cytochrome c is it is the one-electron carrier. Whereas coenzyme Q can drop off only one electron to complex three, it still carries two at a time. But cytochrome c doesn’t have room for extra electrons. It can only carry one.
- In complex four, the electron is received by molecular oxygen to yield a water molecule in a conversion that occurs in the presence of copper ions. This process drives the oxidation of the reduced cytochrome-c, and protons get pumped out during this reaction.
DNP: The “Miracle Weight-Loss Drug”
DNP, sometimes called the “miracle weight-loss drug,” promises miracle results for those looking to lose weight quickly. Although the results can be noticed after a few ingestions of DNP, the drug is dangerous and can be potentially lethal to even young, healthy adults.
