LESSON 4: ELECTRON TRANSPORT CHAIN
PART 1: THE ELECTRON TRANSPORT CHAIN
The 6 NADH and 2 FADH2 formed in the Krebs Cycle move towards the inner membrane of the mitochondria where the Electron Transport Chain is situated.
As shown in the image, there are four main membrane proteins that are fixated in the inner mitochondrial membrane. To simplify the concept, think of these proteins as stops on a train ride. The Electron Transport Chain can be referred to as the train riding along the "border" of the mitochondria's matrix. The first stop is called NADH dehydrogenase. An NADH molecule gets off at this stop and is reduced. This reduction reaction involves losing 2 electrons (e-) and 1 hydrogen ion (H+), otherwise referred to as a proton. NADH dehydrogenase accepts these 2 electrons and this provides it with the energy it needs to pump the protons from the inside of the matrix, across the inner mitochondrial membrane, and into the intermembrane space. The 2 e- are then passed on to an electron carrier named ubiquinone which then passes these electrons to the next stop, cytochrome b-c1. As these electrons are transferred across the chain, energy is generated.
As shown in the image, there are four main membrane proteins that are fixated in the inner mitochondrial membrane. To simplify the concept, think of these proteins as stops on a train ride. The Electron Transport Chain can be referred to as the train riding along the "border" of the mitochondria's matrix. The first stop is called NADH dehydrogenase. An NADH molecule gets off at this stop and is reduced. This reduction reaction involves losing 2 electrons (e-) and 1 hydrogen ion (H+), otherwise referred to as a proton. NADH dehydrogenase accepts these 2 electrons and this provides it with the energy it needs to pump the protons from the inside of the matrix, across the inner mitochondrial membrane, and into the intermembrane space. The 2 e- are then passed on to an electron carrier named ubiquinone which then passes these electrons to the next stop, cytochrome b-c1. As these electrons are transferred across the chain, energy is generated.
At the second stop, FADH2 enters the system and is also reduced, donating 2 e- and 2 H+ through a reduction reaction. The energy generated from the transfer of electrons allows this stop, just like the first stop, to pump the protons donated by FADH2 into the intermembrane space one by one. The electrons are then again passed to another carrier, this time called cytochrome c, which passes the 2 e- to the next stop called cyctochrome oxidase. At this stop, oxygen in the matrix acts as an electron acceptor and accepts the 2 e- that have been travelling along the chain, joining with them and 2 H+ to form H2O (water). You have already learned that oxygen is inhaled during cellular respiration and is one of the most important parts in helping us produce the energy we need. This is the very process where the oxygen we inhale is consumed. Along with the production of the water, H+ ions donated by NADH and FADH2 are still pumped into the intermembrane space one by one by the oxidase stop as well.
PART 2: OXIDATIVE PHOSPHORYLATION & CHEMIOSMOSIS
The next part of the Electron Transport Chain is where the large majority of ATP in our bodies is formed. It is referred to as Oxidative Phosphorylation. With the pumping of all the hydrogen ions into the intermembrane space, an electrochemical gradient called a hydrogen ion gradient is formed. As there is a high concentration of H+ ions in the intermembrane space as compared to the low concentration inside the matrix, the electrochemical gradient needs to balance out this concentration and move the hydrogen ions back into the matrix through the membrane protein called ATP synthase. This process that uses the energy in the H+ ion gradient to drive the phosphorylation of ADP to form ATP is called Chemiosmosis. As the H+ ions move back into the matrix and "cross the border" yet again, it creates the energy required for ATP synthase to combine ADP in the matrix with Pi to form an ATP molecule. As a whole, 32 ATP are generated from this process.
Final ATP Yield of Cellular Respiration: 36 ATP
Final ATP Yield of Cellular Respiration: 36 ATP