welcome to the second video for chapter 6 in this video we're going to be talking about activation energy the laws of thermodynamics and ATP so earlier when we were talking about exergonic reactions spontaneous or favorable reactions that will happen without an input of energy we saw a little hump which was the activation energy we have to surpass before the reaction can happen so why is this the case so in biology when we're dealing with molecules whether breaking them or building them there's always some kind of transition state we have to get to and it takes a little bit of energy to get to that transition state because the reactants have to be moved around they're usually unstable for a while that allows the bonds to be made or broken so that transition state which is usually the peak of the bump is not quite stable so once we can reach that transition state the energy happens really quickly how do we get through the activation energy or get past that activation energy when we're talking about reactions happening outside of the cell usually this is due to heat so for example breaking down octane to power a race car once you get through that activation energy and you generate a bunch of heat the rest of the heat will power the remaining reactions the breakdown of the remaining octane molecules inside the cell though we can't reach this state without catalysts catalysts are what are going to lower the activation energy inside our cells so inside the cell the method by which catalysts speed up reactions or catalyze reactions is by lowering the activation energy so I can see in the purple line This is the reaction without a catalyst it's uncatalyzed and the reaction in green is with the Catalyst the purpose of the catalyst is to lower the activation energy and thereby speed up the reaction rate so notice that it does not change Gibs free energy so Delta G is left unchanged what's the thing that is changed is the activation energy and the speed of the reaction so activation energy is the reason for example why iron the time of or the length of time it takes to rust is a long time it happens really slowly even though it's favorable or spontaneous it's because of that activation energy and in her textbook it asks if no activation energy were required to break down sucrose which is table sugar would you be able to store it in a Sugar Bowl if there was no activation energy we would not be able to store table sugar in a sugar suable because it would just be broken down uh very very quickly luckily we have that activation energy to keep it stable and going back to that race car example remember we break down octane for energy to fuel or power our cars how do we get past the activation energy outside of the cell in an environment like this is really that heat so we have a spark to get the initial breakdown of the octane to happen and then once that happens the heat that's generated is enough to overcome the activation energy we can't do this in cells because the amount of heat you would need to overcome the activation energy using heat alone would be too much you would be destroying the cell the organel inside the cell the proteins Etc just as you've seen in your chemistry and maybe phys physics classes we have to also follow the laws of thermodynamics for biochemical reactions or chemical reactions in biology so the first law of thermodynamics tells us that energy can never be created or destroyed because the total energy of the universe is always constant although energy can be converted from one form to another and the second law of Thermodynamics tells us that entropy always increases disorder always increases so in any kind of chemical reaction some energy is always lost and this is usually in the form of heat so our book gives us two examples where heat is lost in the chemical reactions here in the first on the left the book tells us that kids eat this ice cream cone so we have glucose which storms chemical energy potential energy in the chemical bonds and as the chemical bonds are broken the energy is captured and used in the form of kinetic energy to power the kids moving on their bikes on the right we have photosynthesis when plants capture light energy to make glucose for us for consumers some of that energy is also lost in the form of heat looking at that second law we know that entropy is the measure of Randomness or disorder in a system and looking at for example water gases have a higher entropy than liquids and liquids have a higher entropy than solids so this has the least entropy and this has more entropy gases would be up here with even greater disorder and our book tells us about how cells and living organisms are very organized they're very ordered so it takes a lot of energy input to maintain that order what provides energy to power endergonic reactions inside of a cell I keep talking about ATP ATP is the form of energy that's most commonly used inside cells what does it stand for what is it so ATP stands for a Denine triphosphate and we're going to look at it more closely in the next slide so the structure of ATP is shown here on the top right and if I look at it kind of closely it kind of reminds me of the nucleotides that I saw that made up a our DNA and RNA macromolecules are nucleic acids if I look closely it is kind of if I had just a single phosphate group and the rest of the structure this would be a nucleotide that makes up my RNA molecules so adenosine adenosine is just this portion so right here what I'm circling is the nitrogenous base adenine and a five carbon sugar ribos so here the ribos and the nitrogenous base together make the nucleoside nucleoside known as adenosine if I add one phosphate group this would be called a nucleotide but a Denine triphosphate has three phosphate groups and they're labeled in order so the one closest to the nucleoside is called the alpha phosphate group then we've got the beta in the middle and then at the end we have our gamma phosphate group because these phosphate groups are negatively charged they really don't want to be together and there's a lot of potential energy in the bonds connecting the three phosphate groups usually what we're going to see is that if you hydrolyze the hydrolysis of one of the phosphates will release a tremendous amount of energy that can be used to power endergonic reactions in the cell again because of the negative charges between the three phosphate groups it's pretty unstable and it will be fairly easy to hydroly and release energy so here is the hydris of ATP the reaction I can see ATP plus some water will break break down that phosphate group the gamma phosphate will be released and this means inorganic phosphate because it'll no longer be attached to a carbon containing molecule and we release energy that can be used to power some kind of endergonic reaction there is some energy that's loss as heat but surprisingly this is a very uh efficient reaction so most of the energy will be captured to drive that reaction this reaction is also reverse ible if you put in energy you can actually take an inorganic phosphate combine it with a Denine diphosphate and form ATP again and sometimes people will ask like why is it this phosphate group that's hydrolized why isn't it the one between the alpha and beta a lot of it is due to steric hindrance where molecules can't get close enough to hydrolize that second one so it's almost always this first one so in chapter five we talked about active transport and one example was the sodium potassium pump a type of primary active transport so remember the sodium pottassium pump pushed three sodium molecules out of the cell while we moved two potassium ions into the cell and we used one ATP so this is an example of how ATP hydrolysis and the release of energy from ATP hydrolysis can power the movement of ions against their concentration gradients what actually happens is as when you break down ATP and you release that inorganic phosphate the inorganic phosphate it's not shown in this picture but it'll bind temporarily to the sodium potassium pump causing it to go through something called a conformational change which is a change in the shape of the pump that allows the sodium to be released and allows the pump to bind to potassium to eventually bring it into the cell so here's one example of how ATP hydrolysis and the energy that's released from ATP hydrolysis can be used to power some kind of endergonic reaction this so this is a pairing between an exonic reaction and an endergonic reaction earlier I mentioned that we have to get past the activation energy of a reaction in order for it to happen and this is true for both endergonic reactions and exergonic reactions in this one picture on the right it looks like this is an endergonic reaction I have to put energy in it's a positive Delta G to get the reaction to happen but I still have this really big activation energy to get over luckily if I have the right enzyme to power or catalyze this reaction I can lower that activation energy and speed up the reaction so how do enzymes work enzymes will bind to the reactants and promote the trans transtion States or promote the formation of that transition state either by breaking bonds or forming Bonds in that process they are very specific so enzymes are specific to what they will bind to and they usually catalyze only a single type of reaction most of the time enzymes are proteins I'd say like 99% of all enzymes are proteins but there are some that are not proteins and in this case they're called ribozymes These are usually made of RNA molecules and ribozymes are RNA molecules that can catalyze reactions as well since most enzymes are comprised or composed of proteins and we learned that proteins really rely on their three-dimensional shape to properly function it makes sense that the three-dimensional shape of enzymes is also very important to their reactions to the types of reactions they will catalyze because the threedimensional shape really depends depends on or will tell you what kind of substrate can bind to that enzyme so let's look at this enzyme here this gray enzyme the location the substrate binds to on the enzyme is called the active site so I can see here I have a green substrate it's going to bind to the active site once it binds it forms something called the enzyme substrate complex usually there's a temporary change in the shape of the enzyme as it catalyzes the reaction in this case it lookss like it's hydrolyzing or breaking down this molecule the substrate into two subunits and then after the products are generated the enzyme will return to its original shape in order to catalyze the next reaction here's a more realistic look at the threedimensional shape of an enzyme and I can see the active sight here where within the active site there's a binding sight in blue as as well as a catalytic site since enzymes are usually proteins there are unique amino acids present at The Binding site or the active site as well as the catalic site and these amino acids could be positively charged they could be negatively charged they could be hydrophobic looking at their side chains or their R groups so since these amino acids are very specific that means they're specific to the substrates they will bind to but it also o means that since enzymes are proteins they're uh they're sensitive to their environment so if you change the temperature too much you can denature the enzyme and it will no longer function similarly if you change the pH of the envir environment you could mess up the side chains the groups the r groups of the amino acids so that they no longer bind to this substrate properly so again since the enzymes are usually proteins they are sensitive to temperature and pH of the environment so scientists used to think that enzymes had to perfectly fit their substrates through something that used to be called the lock and key model but nowadays we know that that's not always the case and sometimes there's a shift in the enzyme shape to accommodate the shape of the substrate and that's called induced fit and I can kind of see that here here's that enzyme's active and I can see the pockets of the active site don't perfectly fit the substrate but when the substrate binds there is a change in the shape of the active site I can see this one looks like it gets a deeper pocket and this one is a broader or wider V so the enzyme changes shape slightly as the substrate binds and that's called induced fit notice again that the end of the reaction at the end of the catalyzed reaction enzymes always returned to their original state again in order to continue catalyzing the same reaction over and over again here's another more realistic picture of induced fit I can see here's my enzyme before it binds to the substrate and then after the substrate binds you can see the enzyme has changed shape in this case it looks like it's closed up the active side a little bit in order to bind the substrate molecules properly again it will return to the original state once the reaction has finished finished that takes us to the end of the second video for chapter 6 in our third and final video we'll talk more about enzymes how they lower activation energy and how we can regulate enzymes