Chapter 8 An Introduction to Metabolism Metabolism the sum total of an organism s chemical reaction There are two types of metabolisms Catabolic and Anabolic o Catabolic pathways involved in degradation Release energy by breaking down complex molecules to simpler compounds o Anabolic pathways involved in synthesis Consume energy to build complicated molecules from simpler ones Sometimes called biosynthetic pathways o Every chemical reaction in the body is either building something or breaking it down and falls in one of these categories Energy Organisms transform and transfer energy Potential Energy energy that an object possesses because of its structure or Energy the capacity to do work position Usually chemical bond energy in biological systems Potential energy is usually based on location or position It often refers to energy that is in chemical bonds o Example a ball at the top of a hill has a lot of potential energy The ball at the bottom of the hill does not Kinetic Energy the relative motion of an object Every time there is motion energy is released Kinetic energy can also be based upon position Energy transfers by organisms are subject to two laws of thermodynamics o First Law of Thermodynamics energy can be transferred and transformed but it cannot be destroyed i e the energy of the universe is constant The amount of energy in the universe is constant we are not losing or making energy it is being transformed The first law of thermodynamics is known as the principle of conservation of energy Example according to this law Einstein would be right to say that reincarnation exists because energy is not lost when we die o Second Law of Thermodynamics every energy transfer or transformation makes the universe more disordered i e every process increases entropy In other words with every transfer of energy some usable energy is lost as heat There isn t a completely perfect transfer of energy because some of the energy that could ve been used to do work is lost as heat For example with working out your energy is not only used for doing weights or cardio because some of it is lost as heat this is why we get hot when working out Entropy S the quantitative measure of disorder or randomness Free Energy and Spontaneous Reactions Free energy G the portion of energy available to do work G H T S Free energy is the difference between the total energy H or enthalpy and the energy not available to do work T S where T is the absolute temperature and S is entropy Chemical Reacti ons In a chemical reaction the energy change G between the reactants and the products is the amount of useable energy that can be harvested to do work G H T S where G G Final Products G Starting Material reactants o If you use this equation and G is negative it means that energy was released by the reaction o If you use the equation and G is positive it means that the energy of the products was higher than the energy of the reactants As a chemical reaction approaches equilibrium the free energy G of the system When a reaction is pushed away from equilibrium the free energy G of the decreases system increases At equilibrium G 0 At equilibrium no work can be done If G is negative the forward reaction will occur spontaneously and energy will be released If G is positive the forward reaction will not occur spontaneously energy will have to be added to the system in order for the forward reaction to occur Types of Chemical Reacti ons Types of reactions are based on their free energy changes Exergonic Reactions release energy when they occur o Products have less free energy than the reactants o Reaction is energetically downhill o Spontaneous reaction o G is negative Endergonic Reactions require input of energy to occur o Products have more free energy than reactants o Reaction is energetically uphill o Non spontaneous reaction requires an energy source o G is positive Figure 8 6a exergonic reaction o If you were to subtract the reactants from the products you would get a negative number Figure 8 6b endergonic reaction positive number Figure 8 9 hydrolysis of ATP o If you were to subtract the reactants from the products you would get a o One of the most exergonic reactions in cells is the hydrolysis of ATP When water is added to ATP the negatively charged oxygen ions from the phosphate groups repel the water and an inorganic phosphate group is released The release of the phosphate group releases a bunch of energy o High energy molecule G o Phosphate tail These phosphate bonds aren t really high energy as they are often referred to The reason that ATP is a high energy molecule is that the products ADP P have substantially lower free energy than the reactants This means that the energy difference between the products and reactant is enormous which will provide a lot of energy to do work The structure of the ATP molecule is what makes it such an efficient energy source Form dictates Function The 3 phosphate groups all located next to each other have negative charges Like charges repel The phosphate tail is like a compressed spring When one of these bonds is broken a huge amount of energy is released How does ATP drive work o If the change in free energy G for an endergonic reaction is less than the amount of energy released by ATP hydrolysis then the 2 reactions can be coupled so that overall the reactions are exergonic o Coupled reactions energy released from exergonic reactions ATP hydrolysis are used to power endergonic reactions Catalysts Enzymes Catalysts are substances that speed up the rates of exergonic chemical reactions but are not themselves used up or altered Enzymes are biological catalysts Most enzymes are proteins Enzymes are highly specific for the chemicals they act on Activation Energy EA energy that is required for a reaction to occur Figure 8 14 o Even exergonic reactions require activation energy to start the reaction o What enzymes do to speed up chemical reactions is lower activation energy o Enzymes have no effect on reactants or products and are not destroyed while catalyzing the reactions Figure 8 15 o The presence of enzymes decreases the amount of activation energy needed for a reaction to occur o The only thing that is affected by enzymes is activation energy An enzyme o Lowers the activation energy of a reaction o Does not alter the change in free energy between reactants and products Enzymes are substrate specific o Substrate a molecule that reacts with the help of the enzyme o Active Site the
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