so along the electron transport chain we have many complexes so as we discussed previously in complex one of the electron transport chain complex one essentially picks up the high energy electrons from NADH molecules and transfers those electrons onto a special carrier molecule known as ubiquinone and when ubiquinone grabs those electrons it is reduced into a ubiquinol form that is given by q h2 so this molecule is known as ubiquinol likewise on complex 2 the high-energy electrons from fadh2 molecules are also transferred onto ubiquinone to reduce ubiquinone into ubiquinol and the q h to the ubiquinone then travels on to complex 3 and so in this lecture I'd like to discuss the details of what happens when you BIC we know the reduced form of ubiquinone actually attaches onto complex 3 now complex 3 goes by many names so complex 3 is also known as Q cytochrome C ox the reductive where the Q stands for ubiquinol or we also have cytochrome reductase both of these names refer to complex 3 and complex 3 actually consists of many polypeptide chains in fact we have 11 polypeptide chains that make up complex 3 now the entire function the entire purpose of complex 3 is to actually transfer those high-energy electrons from qh2 from ubiquinol onto another carrier molecule another electron carrier used by the electron transport chain known as cytochrome c and so in this lecture we're going to discuss the details of how this transfer actually takes place so let's begin by focusing on the three major components that you should be familiar with that we find on complex 3 the first one that you should know is cytochrome C 1 and this is not the same thing as cytochrome C even though they're both cytochrome molecules cytochrome molecules are proteins that contain heme groups that can bind and transfer electrons cytochrome C and cytochrome C 1 are not the same types of molecules now cytochrome C 1 actually contains a single heme group but in the structure and complex 3 we also have another cytochrome molecule known as cytochrome B and this molecule actually contains two different heme groups that are capable of actually attaching electrons and finally we also have a structure known as the risky group and this contains the two fe 2 sulfur group that can also bind electrons and transfer electrons onto different groups so the entire process by which electrons are transferred from the ubiquinol on to the cytochrome c molecule is known as the q cycle and the q cycle is actually composed of two major mini cycles so we have the first mini cycle shown here we also call the half cycle and the second half cycle the second mini cycle is known as shown here and together these two half cycles these two many cycles make up a single Q cycle so the process by which electrons travel from the Q H to the ubiquinol that is produced on either complex 1 or complex to the electron transport chain on to another carrier molecule known as cytochrome C is known as the Q cycle and by the way cytochrome C is actually a water soluble protein and it is attached on to the intermembrane space side of this complex 3 so this is the matrix side this is the inner membrane of the mitochondria we have complex 3 and this is the inter membrane space and this protein carrier molecule cytochrome C when it actually attaches a single electron when the oxidized version is reduced the cytochrome C will actually detach itself and because it's de fusable in water it basically travels through the fluid and eventually attaches onto complex 4 found on the intermembrane side of complex 4 as we'll see in next lecture so let's begin by summarizing what takes place on the in the first mini cycle the first half cycle of the Q cycle so we have ubiquinol attaches onto complex 3 and when it attaches the 2 protons to H+ ions are basically pumped they're transported to the minor to the intermembrane space of the mitochondria and those 2 electrons follow two different pathways remember we not only have two H+ ions attached to our ubiquinone we also have two electrons these two h+ ions are pumped to this side by the two electrons follow two different pathways one of those electrons follows this pathway the other electron follows this pathway so we have one electron being transferred onto the 2 fe 2 sulfur group found on the risky Center and then that that same electron moves onto the heme group of cytochrome C 1 now by the way inside the heme group so in the oxidized version of cytochrome C 1 that's all that F II atom basically exists in this form but when it gains a single electron so we have an electron coming in when it gains a single electron it is reduced into the fe2 plus form so remember in the heme group we have the iron atom that can actually gain that electron and when gains that electron it is reduced and so as the electron travels from the risky Center to the cytochrome C 1 it actually attaches onto the iron atom of the heme group and so it is reduced now the electron travels from the heme group and ultimately ends up on the heme group of cytochrome C and notice only as single electron can actually bind on to cytochrome C so this is the major difference between cytochrome C electron carrier and the ubiquinone electron carrier ubiquinone is able to actually bind to electrons but cytochrome C can only bind a single electron and that's exactly why the second electron cannot follow this same pathway it has to follow a different pathway and buff and by following this different pathway what we ultimately accomplished by the second electron pathway is we're able to actually recycle that electron and use that electron in a future process as we'll see in more detail in just a moment so once this electron binds on said the oxidized cytochrome C it reduces that cytochrome C the cytochrome C then detaches and diffuses within the fluid of the intermembrane space and travels and binds on to complex 4 while the other electron because it cannot go via this same pathway it follows a different pathway and that's where cytochrome B comes into play cytochrome B actually contains two different heme groups and the electron first moves on to one heme group than a second heme group and then it ultimately ends up on to on ubiquinone so this is basically the fully oxidized version of coenzyme q also known as ubiquinone and when it gains that electron it is basically partially reduced into a molecule we call the semi quinol radical ion which contains a negative charge and we'll see why we form that in just a moment so let's summarize the first half cycle of the Q cycle so this Q cycle begins when the ubiquinol qh2 attaches onto complex 3 as shown here and upon binding the two electrons follow two different pathways and the two protons are pumped into the intermembrane space of the mitochondria now one of these electrons moves on to the risky santé the 2fe to self a group of the risky sensor and then it is transferred onto the heme group of cytochrome C one reducing the three plus form into the two plus form from there it is then picked up by the final accept the cytochrome C and when cytochrome C accepts that electron in itself is reduced and that stimulates the detachment of the cytochrome C and then is able to move along the fluid of the intermembrane space and attach on to complex four as we'll see in the next lecture now what about that other electron well the second electron moves on to the heme groups of cytochrome B before actually being picked up by ubiquinone now this ubiquinone is not the same one as this ubiquinone so complex 4 complex 3 actually contains an additional ubiquinone that is attached on to a different site and this ubiquinone is actually used to recycle this electron so that once the ubiquinone picks up that electron it is partially reduced into semi quinone radical ion which is given by this designation so Q a single electron and that negative side now this is the first half cycle let's move on to the second half cycle so next this structure basically detaches and a second consecutive ubiquinol actually binds on to this location and once this ubiquinol a second different ubiquinol binds on to this complex 3 the same type of pathway is basically followed by those two electrons and by the protons so once the binding takes place in the second half cycle the two protons are essentially transported into the intermembrane fluid of the mitochondria and the two electrons follow this same pathway so that this electron follows this pathway binds unto the heme group not the heme group the two Fe two sulfur group and then the electron moves on and binds unto the heme group of cytochrome C the for actually being picked up by a second different cytochrome C molecule so this cytochrome C molecule is different than this cytochrome C molecule so this one basically detaches and an oxidized version another one basically reattaches and so we see that in a single Q cycle we actually generate two reduced cytochrome C molecules because of the fact that a single a single ubiquinol can carry two electrons but a single cytochrome C can only bind and carries seeing electron now the other electron basically moves attaches unto the heme groups of cytochrome C of cytochrome B and then that electron is picked up by the semi quinone radical ion and now gains two electrons and then it takes two protons from the matrix of the mitochondria picks up those two protons to form ubiquinol and the ubiquinol detaches from complex 3 and enters the inner membrane of the mitochondria where now we can use this quaint of this ubiquinol that is formed to attach it onto this complex and use that ubiquinol to basically generate those reduced cytochrome c molecules so the entire point of the second pathway here is to basically use or recycle those electrons so that we can actually use those electrons in a useful way so that we don't lose the electrons and we can recycle and reuse them in this pathway to actually generate that reduced cytochrome C so in a second half cycle of the Q cycle the second QH transfers a second pair of electrons through the same pathways as before except now at the end we form a ubiquinol because that semi quintal radical ion that is formed in the first half cycle picks up a second electron is able to abstract two protons from the matrix of the mitochondria and form regenerate that ubiquinol molecule and that you bit panel can then be used by this pathway here to basically generate the reduced cytochrome C so we conclude the following four important points and I've only listed three so the first important point is the following the ubiquinone can actually carry two electrons while the cytochrome C can only carry a single electron and that's exactly why we need this second pathway here to basically recycle the electron so that we can regenerate those ubiquinol molecules and reuse that ubiquinol molecules in this above pathway so we need this bottom step so we see that in a single Q cycle two ubiquinol molecules are oxidized into ubiquinone releasing four H+ ions to the intermembrane space to our release in this first process and to a release in this second process at the same exact time a single quin known or ubiquinone is reduced into ubiquinol and that allows us to recycle these electrons so this is the recycling step of this process and we can imagine that this cytochrome B structure is actually a device that the complex uses to recycle those electrons so that we don't lose those electrons and we can reuse the electrons to produce even more reduced Sayaka cytochrome C molecules now the two cytochrome C molecule or two cytochrome C molecules are actually reduced in a single Q cycle the first one is reduced as a result of the first half cycle and the second one is produced reduced as a result of that second high cycle and one thing that one thing I didn't mention here is the fact that in the second half cycle because we want to produce that ubiquinol two protons are actually up taken from the matrix of the mitochondria and this additional uptake of protons helps us generate that gradient the pro electrochemical gradient that will be used by ATP synthase to generate those ATP molecules