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 o Contained within sugar glycogen and fat o Cells use ATP to accept and transfer energy from one chemical reaction to the next 2 types of energy 1 Potential energy is stored energy o Ex Chemical energy in bonds electrical charge in a battery rock at top of a hill 2 Kinetic energy is the energy of movement o Ex Light heat electricity and the movement of objects The laws of thermodynamics describe the quantity total amount and the quality usefulness of energy o The first law of thermodynamics AKA Law of conservation of energy energy can neither be created nor destroyed but can change form o Total amount of energy within a closed system remains constant unless energy is added or removed o Second law of thermodynamics the amount of useful energy decreases when energy is converted from one form to another o Entropy is a measure of disorder or more precisely unpredictability o Systems move towards maximum entropy o Our ecosystem is becoming more random more entropy Energy Conversions Result in a Loss of Useful Energy o When gasoline is burned the orderly arrangement of 8 carbons bound together in a gasoline molecule are converted to 8 randomly moving molecules of carbon dioxide 6 2 How Does Energy Flow in Chemical Reactions A chemical reaction is a process that forms or breaks chemical bonds holding atoms together o Chemical reactions convert reactants into products o All chemical reactions require a small input of energy o You have to give a little to get a little bit of energy o Exergonic reactions release energy ATP ADP P o Occur spontaneously o Reactants contain more energy than products o Ex The burning of glucose C6H12O6 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 o All chemical reactions require an initial energy input activation energy to get started The speed of a reaction is determined mostly by its activation energy o The negatively charged electron shells of atoms repel one another and inhibit o Molecules need to be moving fast to overcome electronic repulsion and react o Increasing the temperature increases kinetic energy and thus the rate of bond formation reaction o Endergonic reactions require a net input of energy ADP P ATP o The reactants in endergonic reactions contain less energy than the products o Ex 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 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 1st transferred to an energy carrier molecule o High energy unstable molecules o Present at the site of an exergonic reaction o Capture some of the released energy from a reaction o Transfer energy to an endergonic reaction elsewhere in a cell ATP is the principal energy carrier in cells o ATP synthesis Energy is stored in ATP o ATP breakdown Energy is released o Energy is stored in the high energy phosphate bonds of ATP o ATP is well suited to its role as an energy carrier molecule in cells because the covalent bond between that last 2 phosphates can be broken to release substantial amounts of energy o The formation of ATP is an endergonic reaction o At sites in the cell where energy is needed ATP is broken down into ADP P and its stored energy is released o Unlike glycogen and fat ATP stores energy very briefly before being broken down Electron carriers also transport energy within cells o ATP is not the only energy carrier molecule in cells o Energy can be transferred to electrons in glucose metabolism and photosynthesis o Electron carrier molecules such as NAD and FAD transport high energy electrons o Electron carriers donate their high energy electrons to other molecules often leading to ATP synthesis Coupled reactions link exergonic with endergonic reactions o In a coupled reaction an exergonic reaction provides the energy needed to drive an endergonic reaction o The two reactions may occur in different parts of the cell so energy carrier molecules carry the energy from one to the other 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 o Reaction speed is determined by the activation energy required to start process o Reactions with low activation energies proceed rapidly at body temperature o 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 o 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 conversion of carbon monoxide CO to carbon dioxide CO2 2 CO O2 2 CO2 heat energy Enzymes are biological catalysts o Composed of protein synthesized by living organisms o May require small non protein helper molecules called coenzymes in order to function o Many water soluble vitamins some B vitamins are essential to humans because they are used by the body to synthesize coenzymes o Enzymes orient distort and reconfigure molecules in the process of lowering activation energy Enzyme structures allow them to catalyze specific reactions o Each enzyme has a pocket called an active site into which one or more reactant molecules called substrates can enter lock and key o The distinctive shape of the active site is both complementary and specific to the substrate o Three steps of enzyme catalysis 1 Shape charge of the active site
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