 Metabolisms in organisms are chemical reactions and a lot of them are reactions among organic compounds, but when we have primary productivity and respiration and fermentation and other types of energy being extracted from reactions, those influence the chemistry of environments. And so in this video I want to talk about some of those chemical reactions. And we've talked about our primary productivity as in I'm going to write it as Oxygenic Photosynthesis since it is the most abundant type of primary productivity we have on Earth. And that takes, of course, carbon dioxide and water with light and it creates organic compounds plus molecular oxygen. And this reaction takes energy, but the compounds that are made in the cell store that light energy as chemical energy and that the oxygen provides a lot of energy to other organisms. We, of course, have the other reaction where it take organic molecules, just write the same one here, and add oxygen and react that with enzymes and it produces the CO2 plus water and it gets some energy. Some of that energy from the photon goes into the next cell. And this is called aerobic respiration. And that's, of course, how we get our energy. We eat organics, breathe oxygen, our cells react those to form carbon dioxide and water, and we exhale the carbon dioxide. There are a variety of other metabolisms. So one that we use quite a bit as people for things like making bread is fermentation. And in that case, what we do, the organisms start with an organic molecule. We'll draw this one. And it basically breaks the bonds, the carbon-carbon bonds within that and it produces carbon dioxide, which causes the bread to rise. And it creates typically smaller organic molecules. So I'll draw it this way. And it gets energy. So it gets energy from this reaction. So this would be a fermentation. Different molecules in fermentation produce different results. So we could say have one, I'll draw it out as the molecule where you have a carbon with hydrogen surrounding it, bonded to another carbon, maybe with an oxygen, an oxygen, and that oxygen needs to bond. So we have a carbon here. And the fermentation process is breaking this bond right here. So if this bond breaks, we end up with a methane, a hydrogen comes over here, and a carbon dioxide, plus some energy. So this is also fermentation. These are just fermentation of different molecules. And depending on the bond, different amounts of energy come out of the fermentation process. So there are a number of other metabolisms that bacteria and archaea use. One of the others that produces some primary productivity, again, is a reaction between carbon dioxide and hydrogen gas. So we can have CO2. And then the hydrogen is really reduced. You need four hydrogen gas molecules. And the organisms can react this to form a methane. And then it also produces some water. So both this reaction here, the fermentation and this one create methane. And these would be methanogenesis. One is through fermentation and the other is through the reaction of two inorganic compounds. In some cases, some of the methane might go, could also go into organic molecules, which would make this reaction part of the primary productivity, as I stated at the beginning. There are also other types of respiration where you can take the organisms take an organic molecule. Just use generic organics this time and react them with different things. So for example, sulfate, which has the oxidized sulfur in it. And so I'll just write this organics here as sort of being the equivalent of four hydrogen gas. There are organisms that will react the hydrogen gas with sulfate as well. And that will produce a sulfide. So the sulfur here is a plus six. And the sulfur here is a minus two. So it's moving a lot of electrons from the sulfate, excuse me, moving a lot of electrons from the hydrogen onto the sulfur. And this can produce, this ends up producing water with the remaining hydrogens and the oxygen that the sulfate was bound from. So one form of sulfate reduction is what we call this. One form of sulfate reduction uses the hydrogen gas, but another can use various organic molecules. There are also organisms that can eat the methane that's created by methanogens. And so we could start with methane. And that reacts, of course, with oxygen if you have oxygenic photosynthesis. And this creates carbon dioxide plus water plus energy. And this reaction is called methanotrophy. Methano for the methane. Troph is eat. So methanotrophy is a process of eating methane. We can look at the amount of energy that each one of these metabolic reactions we've been talking about can provide to an organism by looking at the half reactions in terms of the electron tower. And this has each electron tower basically has two different types of reactions, one that are oxidizing this side and the equivalents that are reducing. And all the biological metabolisms balance the flow of electrons when we're writing them in this way. And so we need to pair the organisms pair an oxidizing half reaction with a reducing half reaction. And so we can look, for example, at the organic carbon here. So we have CH2O going to CO2. And the H2O is neutral. So the electronic state of carbon in this case is zero. Oxygen always steals the electron. So in this particular case, the carbon is plus four. So this half reaction has an exchange of four electrons and obviously it needs an additional oxygen and a couple of hydrogens. So we can pair that with, for example, the half reaction down here with oxygen and water on the reducing side. And so then we have O2 goes to H2O. And we can check that we have our water here in here. We have our carbon, our carbon. And we have three oxygens on this side and three oxygens on this side. So the overall reaction is CH2O plus O2 goes to CO2 carbon dioxide and water. Now the energy is shown on this side and it's the electronic potential. And for this reaction going from the organic matter to carbon dioxide to oxygen and water creates a lot of energy. So it's minus 0.48 and minus 0.8. So that's over 1.2 units of energy available. There are other reactions that are visible here and they tend to provide less energy. So for example, we could take the organic matter and one of the reactions I mentioned before was sulfate reduction and basically taking the sulfate and turning it to sulfide. That's the difference on the electron towers smaller. So this is less energy but both of these create what we jokingly call happy bacteria because they're actually getting energy from these types of reactions. The ones that can use oxygen get more energy for the same amount of organic matter but the ones that can use sulfate do just fine as well. There are some reactions that don't go so well from the organism. So we talked about methanotrophs eating methane here and converting it to carbon dioxide and when they react that with oxygen they get energy and they're happy but if they react it with say just for example if they're only protons in the water for example they actually cost energy to make the hydrogen gas to get the hydrogen from the methane. So this is an unhappy bacteria and they wouldn't actually do this to get energy. There might be some reason that reactions need to be done to make components for the cells that consume energy but this would not be this would be a reaction that consumes energy as opposed to one that creates energy. And just to remind you of how this whole thing gets started is with the photosynthesis taking water and carbon dioxide to make the organic matter right. So this is actually our primary productivity and this huge jump in energy is provided by the photons that photosynthesis uses and the organisms get back some of that energy when they react the oxygen with the organic carbon. So we can use any of these reactions in the electron tower to evaluate whether or not a particular metabolism might produce energy for bacteria and there are bacteria that create all sorts of reactions that just have a very very small amount of energy released and what that means is that all of the components in this electron tower are things that can be cycled in the environment by microbial communities. And this includes a lot of nitrogen compounds which are nutrients can include iron and the behind iron oxides or dissolve iron oxides. There's quite a bit of different reactions for the iron and the nitrogen and also the sulfur and those are really compounds that are most frequently used in the microbial biogeochemistry.