now let's move on to stage four of cellular respiration and that is the electron transport chain now the nadh molecule that we generated in the krebs cycle and even in glycolysis as well as pyruvate oxidation it's going to give up a hydrogen and some electrons at complex one complex one is called nadh dehydrogenase as all dehydrogenase enzymes go it's going to catalyze the removal of hydrogen from nadh as a result nadh will lose electrons it's going to give up electrons to complex one those electrons will travel to ubiquinone represented by the symbol q ubiquinone is a mobile electron carrier it actually moves to complex one picks up the electrons and then take it to complex stream keep in mind the electrons have a negative charge and on their own they can't move by themselves along the inner membrane because the membrane is made up of phospholipids and the tails of the phospholipids are nonpolar and so they don't interact well with negatively charged electrons so the electrons has to be carried by a mobile electron carrier in this case ubiquinone now ubiquinone is going to pass on the electrons to complex 3. complex 3 is called cytochrome reductase also known as bc1 complex by some textbooks cytochrome c i mean cytochrome 3 rather is going to give the electrons to cytochrome c whenever cytochrome c receives electrons it becomes reduced reduction as we talked about is the gain of electrons and so this name is fitting for complex 3 because it reduces cytochrome c by giving it electrons cytochrome c is another mobile electron carrier it's free to move but it's a surface protein as opposed to an integral protein complex 1 and 3 these are transmembrane proteins also integral proteins because they're completely embedded within the membrane now c cytochrome c is going to give up the electrons to complex four also known as cytochrome oxidase this particular transmembrane protein it oxidizes cytochrome c because it takes away electrons from it anytime you have a loss of electrons oxidation occurs now the electrons will flow out of complex four where they're going to meet up with oxygen and some hydrogen ions to form the water which is one of the products of cellular respiration now the other electron carrier that we need to talk about is fadh2 now i mentioned this earlier fad and fadh2 they are bound to the inner membrane they're part of complex two so i'm going to write f to represent fad and then f h2 to represent fadh2 let me get rid of this for now now in step 7 of the krebs cycle we saw that sustanate converts into fumarate and as a result it gives up electrons and hydrogen as well converting fad into fadh2 so that's in the krebs cycle succinate dehydrogenase complex ii is the enzyme that removes hydrogens from susana converting that into fumarate and then those hydrogen atoms are transferred to fad now in the electron transport chain fadh2 gives up those hydrogens and the electrons turning back into fad so it's a cyclic process so as fadh2 turns to fad the electrons will be picked up by ubiquinone and then it's going to be carried to complex 3 and then to cytochrome and to complex 4. now as the electrons are transferred through these membrane proteins across the electron transport chain protons are pumped from the mitochondrial matrix into the inter membrane space and so each of these transmembrane proteins are pumping out protons into the intermembrane space and so what happens is we have a buildup of positive charge in the intermembrane space so the ph here is going to be relatively low now as these protons are pumped into this region the intermembrane space is going to develop a positive charge and so the mitochondrial matrix will be less positive or more negative with respect to the intermembrane space and so there's going to be an electric force that these protons will fill because they're going to be attracted to the negatively charged mitochondrial matrix the second thing is a concentration gradient you have a large concentration of protons in the intermembrane space so the electric force combined with this concentration gradient will cause these protons to enter this enzyme atp synthase which you can also say it's a membrane protein it's going to cause the protons to flow through atp synthase and they're going to cause this portion to basically turn like a rotor a good way to think about this is water flowing down into a turbine causing the turbine to spin so as these protons flow down this membrane they will cause this they will create a mechanical force that will smash adp and phosphate to make atp so this process is known as chemi osmosis because we're using diffusion to create atp so the electron transport chain is basically what we see here the electrons traveling from complex 1 all the way to complex 4. chemiosmosis is the production of atp using the diffusion of protons as it flows through atp synthase when you combine the electron transport chain and chemiosmosis you have oxidative phosphorylation now keep in mind phosphorylation occurs when we transfer a phosphate group to something in this case transferring a phosphate group to adp to make atp that is an example of phosphorylation now in the electron transport chain nadh is oxidized to nad plus it's oxidized in a sense that nadh loses electrons to the electron transport chain and you can see its oxidation state increases from zero to plus one so both nadh and fadh2 are oxidized in the electron transport chain and using atp synthase adp is phosphorylated with a phosphate group making atp so combined we have oxidative phosphorylation the first part is oxidation the second part phosphorylation now one more thing that i do want to mention is that there are many electron acceptors in glycolysis we saw that nad plus was an electron acceptor it took electrons as glucose split into pyruvate fad served as an electron acceptor when sustanate was converted to fumarate oxygen is the final electron acceptor in this process oxygen is one of the most electronegative elements besides fluorine and so because of that affinity for electrons oxygen basically pulls electrons through the electron transport chain to itself and so that's really the driving force here it's electronegativity because electrons will travel from atoms that are less electronegative to atoms that are very electronegative so in the beginning the electrons were located in glucose so they were attached to carbon atoms and then in the end those electrons go to oxygen to make water and carbon dioxide so as the electrons flow from carbon to oxygen energy is released carbon is not very electronegative it's electronegativity is like 2.5 for oxygen it's 3.5 so as we take electrons from an atom that is less electronegative to an atom that is more electronegative we can extract energy from that process a good way to illustrate this is the use of a battery so let's say i'm going to draw a double a battery here is the positive terminal and here is the negative terminal and let's connect this to a light bulb let's say this battery is strong enough to light up the light bulb electrons will flow from the negative terminal to the positive terminal and in a process from moving from one side to the other side energy can be extracted from it and in this case lighten up the light bulb so the positive terminal would be the electronegative side of the battery because it pulls the electrons toward it the negative terminal will be the part that's less electronegative because that's where the electrons are coming from in this case carbon would be like the negative terminal of the battery oxygen will be like the positive terminal of the battery because oxygen being electronegative pulls the electrons away from carbon and so anytime electrons flow from one position to another position energy can be extracted as those electrons move from one location to the other as we could see in the case of a battery the same is true for let's say we have a ball rolling down the hill at the top the ball has potential energy but as it moves from one position to another position that potential energy is converted to kinetic energy and in that motion energy can be extracted if you think of water flowing down a hill that water can be used to move the turbine which can then be used to create electricity so whenever you have an object or a particle like an electron moving from one location to another energy can be extracted uh during that process which is what we see here in the electron transport chain so the electrons they travel from carbon to nad plus and then from nad plus they go to complex one and then to q and then complex three and then complex four and then finally to oxygen so oxygen has the greatest electronegativity or electron affinity for electrons so here's a question for you which complex has a greater electron affinity for electrons would you say complex 1 or complex 3. so according to the sequence because complex 3 comes after complex 1 we could say that complex 3 has a greater electron affinity or rather a greater affinity for electrons than complex 1. so we can say that complex 3 is more electronegative than complex 1. now in the case of fad carbon gives its electrons to fad and then fad gives its electrons to complex 2 which gives its electrons to q so if we were to compare the mobile carrier ubiquinone and complex 2 we would say that complex q is more electronegative than complex 2 because it comes after complex 2. the electrons will always flow from something that is less electronegative to something that is more electronegative spontaneously the only way to make the electrons go back is by putting energy into the system but as electrons flow this way energy is released so now we need to do some math so let's go back to nadh as nadh is oxidized to nad plus we said that it's going to give up electrons which will pass through complex 1 q complex 3 and complex 4. so notice that nadh activates three complexes and these three complexes will be shooting out protons into the intermembrane space it turns out that nadh one molecule of nadh will yield three atp molecules and this is proportional to the number of complexes that nadh activates now as fadh2 turns into fad which is basically inside this complex even though i drew it outside it only activates two of the three complex proteins it activates these two and it turns out that one fadh2 molecule produces two atp molecules which makes sense the proportions have to be the same so keep that in mind because we're going to talk about the net amount of atp that one molecule of glucose can yield in cellular respiration