The Electron Transport Chain

 Introduction #

• The final stage of respiration, forming ATP and H2O. The other steps of respiration create some ATP but not as much as this stage. It takes place across the inner mitochondrial membrane.

1. NADH and FADH from the Krebs Cycle break down into NAD and FAD, releasing hydrogen
2. These hydrogen atoms are broken down into H+ ions and electrons
3. The electrons move down the chain of electron carriers in the mitochondrial membrane (the electron transport chain) and release energy as they do so
4. This energy is used to pump the H+ ions across the membrane against their concentration gradient
5. The H+ ions diffuse back across the membrane through the ATP synthase enzyme channel
6. This movement provides energy for the enzyme to combine ADP and Pi to form ATP
7. The H+ ions and the electrons from the electron transport chain combine with O2 to form H2O

• Glucose is not the only respiratory substrate – lipids and amino acids break down to form molecules that enter the Krebs Cycle and are used for respiration.

The Electron Transport Chain #

The electron transport chain is the final stage of cellular respiration, responsible for the production of ATP (adenosine triphosphate) and the formation of water molecules.

Key points of the electron transport chain #

1. Location: The electron transport chain takes place across the inner mitochondrial membrane, specifically in the cristae, which are the folded structures of the membrane. The mitochondrial membrane provides the necessary environment for the efficient functioning of the electron transport chain.
2. NADH and FADH: NADH (nicotinamide adenine dinucleotide) and FADH (flavin adenine dinucleotide) are molecules derived from the previous step of respiration, the Krebs Cycle. NADH and FADH release hydrogen atoms, which are essential for the electron transport chain.
3. Hydrogen Atom Breakdown: The hydrogen atoms released from NADH and FADH are split into hydrogen ions (H+) and electrons. This breakdown is a crucial step in the electron transport chain.
4. Electron Movement: The electrons from the hydrogen atoms move through a series of protein complexes called electron carriers embedded in the mitochondrial membrane. These carriers form the electron transport chain. As the electrons move along the chain, they release energy.
5. Proton Pumping: The released energy is used to pump protons (H+ ions) across the inner mitochondrial membrane, from the matrix to the intermembrane space. This pumping creates an electrochemical gradient.
6. ATP Synthesis: The H+ ions generated by the proton pumping process diffuse back across the membrane through the ATP synthase enzyme channel. This movement of H+ ions provides the necessary energy for ATP synthase to combine ADP (adenosine diphosphate) and Pi (inorganic phosphate) to form ATP. This step is often referred to as oxidative phosphorylation.
7. Formation of Water: At the end of the electron transport chain, the electrons and H+ ions combine with oxygen (O2), which serves as the final electron acceptor. This combination forms water (H2O) as a byproduct.
8. Respiratory Substrates: Although glucose is the main respiratory substrate, other molecules such as lipids (fats) and amino acids can be broken down to form molecules that enter the Krebs Cycle and contribute to the electron transport chain.

Practice question #

1. Describe the process of oxidative phosphorylation in the mitochondrion
2. Explain the roles of roles of :-
   • Oxygen
   • Nicotinamide Adenine Dinucleotide (NAD)