Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 5 The Working Cell

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Energy is defined as the capacity to do work –Movement against an opposing force –The ability to bring about change All organisms require energy to stay alive Energy

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Kinetic energy is energy that is actually doing work –Energy in motion Potential energy is stored energy –Potential for motion Figure 5.1A Figure 5.1B

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Unstable systems:

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Types of potential energy found in metabolism Chemical energy – energy stored in the bonds of molecules. Energy stored in the position of electron around the nucleus of an atom.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings First law of thermodynamics Energy cannot be created or destroyed –However, energy can be changed from one form to another Two laws govern energy conversion Figure 5.2A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Second law of thermodynamics Energy changes are not 100% efficient –Energy conversions increase disorder, or entropy Figure 5.2B Some energy is always lost as heat

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings All living things need energy Cells use energy for chemical work, mechanical work, and electrochemical work Living cells process energy by means of enzyme-controlled chemical reactions Membranes are one of the primary sites where chemical reactions occur Membranes compartmentalize the cell ENERGY AND THE CELL

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Cells use energy to carry out thousands of chemical reactions –The sum of these reactions constitutes cellular metabolism –Reactions use energy coupling – run in a sequence of two reactions: one requires energy, one releases energy What Can Cells Do with Energy? endergonic - store energy exergonic - release energy

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Endergonic Reactions Energy input required Product has more energy than starting substances product with more energy (plus by-products 60 2 and 6H 2 O) ENERGY IN 612

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –Endergonic reactions absorb energy and yield products rich in potential energy Figure 5.3A Reactants Potential energy of molecules Products Amount of energy INPUT

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Exergonic Reactions Energy is released Products have less energy than starting substance ENERGY OUT energy-rich starting substance products with less energy 66

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –Exergonic reactions release energy and yield products that contain less potential energy than their reactants Figure 5.3B Reactants Potential energy of molecules Products Amount of energy OUTPUT

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Cells survive by coupling energy inputs to energy outputs Combine endergonic reactions with exergonic reactions Help make these endergonic reactions possible ATP is the main coupling agent

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings ATP molecules are the key to energy coupling ATP molecules store energy during the process of cellular respiration ATP power nearly all forms of cellular work ATP the cell’s chemical energy

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The Role of ATP Cellular money Cells “earn” ATP in exergonic reactions Cells “spend” ATP in endergonic reactions P PP ribose adenine

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Hydrolysis breaks a phosphate group bond from the ATP molecules releasing energy The exergonic reaction supplies energy for cellular work Figure 5.4A Phosphate groups Adenine Ribose Adenosine triphosphate Hydrolysis Adenosine diphosphate (ADP) Energy Energy Coupling

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Phosphorylation –How ATP powers cellular work Figure 5.4B Reactants Potential energy of molecules Products Protein Work

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The ATP cycle Figure 5.4C Energy from exergonic reactions Dehydration synthesis Hydrolysis Energy for endergonic reactions

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings All molecules have energy barriers to prevent spontaneous breakdown Enzymes speed up the cell’s chemical reactions by lowering energy barriers The amount of energy needed to overcome the energy barrier is the energy of activation (EA) –Enzymes lower the EA for chemical reactions to begin; decreases the amount of energy the reactants must absorb ENZYMES AND CELLULAR REACTIONS

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Energy of Activation (EA) For a reaction to occur, an energy barrier must be overcome Enzymes make the energy barrier smaller activation energy without enzyme activation energy with enzyme energy released by the reaction products starting substance

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Four Features of Enzymes 1)Enzymes do not make anything happen that could not happen on its own. They just make it happen much faster. Enzymes cannot make a nonspontaneous reaction spontaneous 2) Reactions do not alter or use up enzyme molecules

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Four Features of Enzymes, cont. 3) The same enzyme usually works for both the forward and reverse reactions 4) Each type of enzyme recognizes and binds to only a certain molecule called the substrate

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Each cellular reaction is catalyzed by a specific enzyme Active site - a pocket with specific shape for a certain substrate The enzyme fits around the substrate when iit enters the active site - induced fit Enzymes are selective

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Enzyme (sucrase) Active site Substrate (sucrose) Enzyme available with empty active site Substrate binds to enzyme with induced fit Substrate is converted to products 4 Products are released GlucoseFructose How an enzyme works The enzyme is unchanged and can repeat the process Figure 5.6

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Some enzymes require nonprotein cofactors –Some are inorganic metal ions of zinc, iron, and other trace elements –Some are organic molecules called coenzymes Includes vitamins or altered vitamin components Enzyme Helpers

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Inhibitors block enzyme action –A competitive inhibitor takes the place of a substrate in the active site –A noncompetitive inhibitor alters an enzyme’s function by changing its shape Enzyme Inhibitors Substrate Enzyme Active site NORMAL BINDING OF SUBSTRATE Competitive inhibitor Noncompetitive inhibitor ENZYME INHIBITION Figure 5.8

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Inhibition usually caused by the product of the enzyme + substrate reaction Product binds to site other than active site and alters enzyme shape Essential process to regulate cellular metabolism Often the last products in an enzymatic pathway inhibits one of the first enzymes in the pathway Negative Feedback

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Enzyme activity is influenced by the cellular environment –temperature –salt concentration –pH Environmental Effects

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Certain pesticides inhibit key enzymes in the nervous system –Noncompetitive inhibitors may permanently damage the enzyme Many antibiotics inhibit enzymes that are essential to disease-causing bacteria –Penicillin inhibits an enzyme that bacteria use in making cell walls Connection: Some pesticides and antibiotics inhibit enzymes

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings