Lecture 1 Cellular Metabolism.docx

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Biological Sciences
Ted Petit

Cellular Metabolism1 Slide 1 Animals are highly ordered systems: There are three basic statements that animals are highly ordered systems. This is a picture with several levels of structural organizations of animals. These animals are composed of several organ systems, which are composed of several organs and those organs are composed of several tissues which themselves are composed of cells which are composed of molecules and atoms and so forth. These organisms sort of assemble themselves to form populations and ecosystems. Animals are pretty highly organized systems. But animals don't start off their lives as highly ordered systems. Most animals start off as a single cell which is much simpler compared to their highly ordered adult counterpart. They undergo this complex development. They undergo this progressive increase in complexity over their lifetime. Now if you think about animals in their evolutionary history by looking at an evolution tree, the simple animals at the bottom include sponges with very few cell types and very few tiny tissues and then they progressively evolve into complex animals. Therefore there is this evolutionary trend by increasing the complexity of the animal. So animals themselves are highly ordered and throughout their lifetime they become more highly ordered and also throughout their evolutionary time, they are getting more highly ordered. SLIDE 2 Animals are highly ordered systems doesn’t that violate the 2 law of thermodynamics: This bugs scientists and physicists because isn't the fact that animals are highly ordered systems violating the second law of thermodynamics. The second law of thermodynamics states "Isolated systems are spontaneously move towards the way of maximum entropy (disorder)." At first this sounds strange, that animals are highly ordered organisms and become more so over evolutionary time and throughout their life development. At first it may seem like the violation of the second law of thermodynamics. The first two words of the second law are isolated systems. What we mean by a system that is isolated is when it can't exchange energy or mass with the surroundings. Animals are simply not isolated systems. Animals do exchange energy and mass with their environment. An example of an isolated system does include: the universe. Therefore the second law of thermodynamics does apply to the universe, where it doesn't exchange mass or energy with its surroundings, therefore the universe is moving towards a state of maximum entropy. But the things such as organisms within the universe, are not bound by the second law of thermodynamics and thus are not isolated systems. At the same time, animals live in the universe and are still part of the universe. The second law of thermodynamics does not apply to them in a certain way but since they are part of the universe it applies to them indirectly. The way in which animals fit into this second law of thermodynamics is if we think about the types of energy we are exchanging with the environment, typically animals will take in chemical energy (the food we eat); chemical energy means the energy trapped in chemical bonds in molecules and molecules are very structurally ordered things. Animals release energy though mostly through the form of heat. What animals seem to be doing, and how they fit into the second law of thermodynamics, is they are actually taking from their surroundings organized forms of energy like chemical energy and releasing back into the environment highly disorganized forms of energy that being heat energy. So animals themselves are highly organized and can maintain a highly organized system but they do so by increasing disorder and increasing entropy of the universe. This idea of animals not being contradictory to the second law of thermodynamics, this was figured out by a physicist, an author of the book What is Life published in the 1940s. Slide 3 Gibb’s Free Energy (G): The starting point of cellular metabolism is Gibbs Free Energy (G). We got a physical reaction of glucose reacting to oxygen to produce carbon dioxide and water but also produces energy that is liberated in this process mostly through heat. Therefore in this reaction, the oxidation of glucose releases carbon dioxide and water but also releases energy. The idea of free energy though is that only some proportion of energy that is released by this process is actually going to be available to do work. There is a total amount of energy is going to be released but there is only a small amount that is available to do work and that is what we call Gibbs Free Energy (G) If you think about the types of work animals are doing include movement of muscles, transporting ions or protons against their concentration gradients, they are building complex molecules from simple starting material via biosynthesis. This is the type of work animals are doing and most of these types of work are highly ordered actions for instance movement such as walking is linked toward highly organised types of action. So the movements are ordered such as pumping ions or protons across their concentration gradients, we are establishing greater and greater order. Same as building molecules by taking simple molecules and establishing them into greater molecules. We use the free energy, the energy available to do work to do all this work and therefore establish order in the body. The idea that there is energy available to do work would also mean there is energy unavailable to do work. We often see this equation: deltaG (change in free energy) = deltaH (the unavailable energy released) - TdeltaS (Temperature times the change in entropy or measure of disorder). This helps us figure out the energy available to do work is only a subset of the total energy because some of the energy released in the process actually gets used to increase the entropy of the universe. Molecules move around in more random ways. Back to the oxidation of glucose, can we prove that there will be an increase in entropy in this reaction? On the left hand side there are seven molecules (1 glucose molecules=solid and 6 oxygen molecules=gases). We already know that molecules of gases tend to be a lot more disorganized, random with faster speeds than they are in solids. So we got one solid thing with molecules not moving a lot but six gaseous things where molecules are moving quite a bit. On the right hand side, we have twelve molecules and all of them are in gaseous state or liquid state and we are looking at a much greater number of molecules and they are moving around with higher speed because they are now liquid and gases. Therefore, we see an increase in entropy. The energy that has evolved while oxidizing glucose, some of it is not available to do work and it is being tied up in making these molecules go faster. Any free energy that is being released that we don't conserve, where we don't make ATP or we don't build some sort of molecule, that energy gets released as heat and becomes unavailable in terms of in the increase in entropy of the universe. So the free energy that is available to do work and we don't make it do work gets released as heat in the environment. Slide 4 Gibb’s Free Energy and the Laws of Thermodynamics: So let's put Gibbs free energy into perspective. The first law of thermodynamics states that the total energy never changes. That we can't create or store energy. So if we look at total energy over time, the total energy never changes over time. It is a solid flat line. The second line is the unavailable energy where it is constantly increasing and since total energy is not changing that must mean that free energy is decreasing over time. Free energy (out the universe) is decreasing over time. Slide 5 Gibb’s Free Energy and chemical reactions: Gibb's Free energy in context of chemical equilibrium. We have a generalized reaction of A+B --> C+D. If you let this reaction go to equilibrium, what kinds of concentration will all these products and reactants have? Will we have all A and B or all C and D or will there be an equal amount of both. Basically what is the equilibrium balance of this reaction and how can we figure it out. Well the universe is trying to spontaneously minimize free energy so when chemical reactions are allowed to reach equilibrium, they come to a state where the free energy of that system is as low as possible. So whatever concentration of these A, B, C, and D minimizes the free energy of that system is the equilibrium balance. If we have only A and B and there if no C and D, the free energy is very high and similarly if we have C and D and no A and B, the free energy is not quite as high but is relatively high. There is a midpoint with some A and B and some C and D and this is what minimizes the free energy of that system. So by definition, equilibrium is a point at which free energy is minimized. If we take this reaction and we let it go to equilibrium, where is there is a mixture of A and B and C and D, if we want to push that reaction towards A and B, if for some reason we want more A and B then C and D, then we have to push the equilibrium point to the A and B point, we need a source of free energy. In order for us to move from equilibrium point to any point on the curve, we have to input free energy into the system. At the same time, if at some point we let the system go back at equilibrium, free energy is going to be released. So the idea of pushing a system away from equilibrium takes free energy and then allowing a system to go back to equilibrium releases free energy. This idea does not only apply to chemical reactions, it applies equally well to the distribution and transport of ions across membranes. So if there is a mitochondria membrane and we would like a greatly uneven distribution of some substance on one side and not the other side, the same rule will apply. Slide 6 Gibb’s Free Energy and the distribution of chemical species or Electrical Charges: In the equilibrium condition, the distribution across the membrane is equal amounts of each side. So when we let these molecules distribute themselves across the membrane, they will come to a point of equal distribution of each side because that is the distribution of the equilibrium that releases free energy. If we want to push the system away from equilibrium point, (suppose we want A molecules outside the membrane only) we require free energy to stay that way. So we have a relationship between Gibbs free energy and chemical reactions, and we also got a relationship GFE and distribution of the ions or chemical charges and any other species across the membrane. Slide 7 Gibb’s Free Energy and reduction potential: There is also a relationship between Gibb’s Free energy and reduction potential. So this is a quick reminder of the idea of oxidation and reduction here where we got chemical species A, and A has these spherical electrons attached to it. If A passes those electrons on to B, we say A has become oxidized (loss of electrons) and B has become reduced. Leo the lion goes grr. Leo loses electrons is oxidized and gaining electrons, you are being reduced. If we pass electrons from one species to another, the species that loses electrons oxidizes and the species that gains electrons is being reduced. What we find is that different chemical species have different affinities for electrons and this is what happens in redox or reduction potential. The table of reduction potential ranges from -0.6 to +0.8, the most important thing to remember about these values is the more positive they are, the more those species want electrons and the more negative the value is, the less the reaction wants to keep those electrons. Oxygen plus two hydrogen plus two protons equals water and it has a value of +0.186, so what that tells is that oxygen is it really wants electrons. In comparison a little higher up where we have NADH or NAD+ plus two electrons equals NADH has a very negative affinity so NADH really doesn’t want elect
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