Define electron transport chain Biochemistry Lecture

The electron transport chain (ETC) is a series of protein complexes and electron carriers embedded in the inner mitochondrial membrane in eukaryotic cells (or the plasma membrane in prokaryotic cells). It is a critical component of oxidative phosphorylation, a key process in cellular respiration.

Key Components and Functions:

1. Protein Complexes:
   • The electron transport chain consists of a series of protein complexes, usually labeled as Complex I, Complex II, Complex III, and Complex IV.
2. Electron Carriers:
   • Electron carriers, such as coenzyme Q (ubiquinone) and cytochrome c, shuttle electrons between the protein complexes.
3. Location:
   • In eukaryotic cells, the electron transport chain is located in the inner mitochondrial membrane.
   • In prokaryotic cells, it may be located in the plasma membrane.

Function:

The primary function of the electron transport chain is to transfer electrons derived from the breakdown of nutrients (such as glucose) through a series of protein complexes and carriers. This transfer of electrons is coupled with the pumping of protons (H⁺ ions) across the inner mitochondrial membrane, creating a proton gradient.

The overall process can be summarized as follows:

1. Complex I (NADH Dehydrogenase):
   • Accepts electrons from NADH, transferring them to coenzyme Q.
   • As electrons move through Complex I, protons are pumped across the inner mitochondrial membrane.
2. Coenzyme Q (Ubiquinone):
   • Acts as a mobile electron carrier, transferring electrons to Complex III.
3. Complex II (Succinate Dehydrogenase):
   • Accepts electrons from FADH₂ (produced in the citric acid cycle), transferring them to coenzyme Q.
   • Does not pump protons into the intermembrane space.
4. Complex III (Cytochrome bc₁ Complex):
   • Accepts electrons from coenzyme Q and transfers them to cytochrome c.
   • Protons are pumped across the inner mitochondrial membrane.
5. Cytochrome c:
   • Acts as a mobile carrier, transferring electrons to Complex IV.
6. Complex IV (Cytochrome c Oxidase):
   • Accepts electrons from cytochrome c and transfers them to oxygen (O₂), forming water.
   • Protons are pumped across the inner mitochondrial membrane.

Proton Gradient and ATP Synthesis:

The pumping of protons across the inner mitochondrial membrane creates a proton gradient, with a higher concentration of protons in the intermembrane space. This proton gradient generates a proton motive force, and the flow of protons back into the mitochondrial matrix through ATP synthase is utilized to produce ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi) in a process known as chemiosmosis.

In summary, the electron transport chain is a crucial part of oxidative phosphorylation, playing a central role in the production of ATP, the cell’s primary energy currency. The flow of electrons through the chain is coupled with the pumping of protons, creating a gradient that drives ATP synthesis.