Chapter 6 Energy Flow in the Life of a Cell 6 1 What Is Energy Energy is the capacity to do work Work is a force acting on an object that causes the object to move Chemical energy is the energy that is contained in molecules and released by chemical reactions Contained within sugar glycogen and fat Cells use ATP to accept and transfer energy from one chemical reaction to the next Two fundamental types of energy Potential energy is stored energy i e chemical energy in bonds electrical charge in a battery rock at top of a hill Kinetic energy is the energy of movement i e light heat electricity and the movement of objects Author Animation Types of Energy The laws of thermodynamics describe the quantity total amount and the quality usefulness of energy 1 Energy can neither be created nor destroyed the first law of thermodynamics but can change form often called the law of conservation of energy total amount of energy within a closed system remains constant unless energy is added or removed The laws of thermodynamics describe the basic properties of energy continued 2 The amount of useful energy decreases when energy is converted from one form to another the second law of thermodynamics Entropy disorder is a measure of disorder or more precisely unpredictability Systems move towards maximum entropy Energy Conversions Result in a Loss of Useful Energy When gasoline is burned the orderly arrangement of eight carbons bound together in a gasoline molecule are converted to eight randomly moving molecules of carbon dioxide Combustion by engine 100 units chemical energy 75 units heat 25 units kinetic energy concentrated energy motion Fig 6 2 6 2 How Does Energy Flow in Chemical Reactions A chemical reaction is a process that forms or breaks chemical bonds holding atoms together Chemical reactions convert reactants into products energy energy All chemical reactions require a small input of you have to give a little to get a little bit of Exergonic reactions release energy Endergonic reactions require an input of energy Author Animation Exergonic and Endergonic Reactions An Exergonic Reaction reactants Energy released products Fig 6 3 An Endergonic Reaction Energy input reactants products Fig 6 4 EXERGONIC reactions release energy Reactants contain more energy than products Example the burning of glucose Overall sugar combines with oxygen to produce carbon dioxide and water releasing energy Why The molecules of sugar contain more energy than the molecules of carbon dioxide and water the reaction releases energy Reactants and End Products of Burning Glucose C6H12O6 glucose 6 O2 oxygen Energy released 6 CO2 carbon dioxide 6 H2O water Fig 6 5 Exergonic reactions release energy continued All chemical reactions require an initial energy input activation energy to get started The negatively charged electron shells of atoms repel one another and inhibit bond formation Molecules need to be moving fast to overcome electronic repulsion and react Increasing the temperature increases kinetic energy and thus the rate of reaction Activation Energy in Exergonic Reactions Activation energy needed to ignite glucose energy level of reactants glucose O2 high energy content of molecules low CO2 H2O progress of reaction Fig 6 6 ENDERGONIC reactions require a net input of energy The reactants in endergonic reactions contain less energy than the products Example process of photosynthesis plants add the energy of sunlight to the lower energy reactants water and carbon dioxide to produce the higher energy product sugar Photosynthesis Energy input 6 CO2 carbon dioxide 6 H2O water C6H12O6 glucose 6 O2 oxygen Fig 6 7 6 3 How Is Energy Transported Within Cells Most organisms powered by breakdown of glucose Energy in glucose cannot be used directly to fuel endergonic reactions Energy released by glucose breakdown is first transferred to an energy carrier molecule high energy unstable molecules present at the site of an exergonic reaction capture some of the released energy from a reaction transfer energy to an endergonic reaction elsewhere in a cell ATP is the principal energy carrier in cells ATP is the principal energy carrier in cells continued Energy is stored in the high energy phosphate bonds of ATP The formation of ATP is an endergonic reaction At sites in the cell where energy is needed ATP is broken down into ADP P and its stored energy is released Unlike glycogen and fat ATP stores energy very briefly before being broken down Electron carriers also transport energy within cells ATP is not the only energy carrier molecule in cells Energy can be transferred to electrons in glucose metabolism and photosynthesis Electron carrier molecules such as NAD and FAD transport high energy electrons Electron carriers donate their high energy electrons to other molecules often leading to ATP synthesis Coupled reactions link exergonic with endergonic reactions In a coupled reaction an exergonic reaction provides the energy needed to drive an endergonic reaction The two reactions may occur in different parts of the cell so energy carrier molecules carry the energy from one to the other Author Animation Coupled Reactions Coupled Reactions Within Living Cells high energy reactants glucose exergonic glucose breakdown low energy products CO2 H2O ATP ADP P high energy products protein endergonic protein synthesis low energy reactants amino acids Fig 6 9 6 4 How Do Enzymes Promote Biochemical Reactions Enzyme structures allow biochemical reactions to catalyze specific reactions Enzymes like all catalysts lower activation energy making reactions more likely to go forward Enzymes control the rate of energy release and capture some energy in ATP At body temperatures spontaneous reactions proceed too slowly to sustain life Reaction speed is determined by the activation energy required to start process Reactions with low activation energies proceed rapidly at body temperature Reactions with high activation energies move very slowly at body temperature Enzymes catalyze speed up chemical reactions in cells by lowering the activation energy Enzymes are biological catalysts and regulate all the reactions in living cells Catalysts speed up the rate of a chemical reaction without themselves being used up All catalysts have three important properties 1 They speed up reactions by lowering the activation energy 2 They speed up only exergonic reactions 3 They are not consumed or changed by the reactions Catalytic converters in cars facilitate the
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