Chapter 5 The Working Cell

Energy 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

Kinetic energy is energy that is actually doing work Energy in motion Figure 5.1A Potential energy is stored energy Potential for motion Figure 5.1B

Unstable systems:

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.

Two laws govern energy conversion First law of thermodynamics Energy cannot be created or destroyed However, energy can be changed from one form to another Figure 5.2A

Second law of thermodynamics Energy changes are not 100% efficient Energy conversions increase disorder, or entropy Some energy is always lost as heat Figure 5.2B

ENERGY AND THE CELL 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

What Can Cells Do with Energy? 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 endergonic - store energy exergonic - release energy

Endergonic Reactions Energy input required Product has more energy than starting substances product with more energy (plus by-products 602 and 6H2O) ENERGY IN 6 12

Potential energy of molecules Endergonic reactions absorb energy and yield products rich in potential energy Products Amount of energy INPUT Potential energy of molecules Reactants Figure 5.3A

Exergonic Reactions Energy is released Products have less energy than starting substance energy-rich starting substance ENERGY OUT + 602 6 6 products with less energy

Amount of energy OUTPUT Potential energy of molecules Exergonic reactions release energy and yield products that contain less potential energy than their reactants Reactants Amount of energy OUTPUT Potential energy of molecules Products Figure 5.3B

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

ATP the cell’s chemical energy 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

The Role of ATP Cellular money Cells “earn” ATP in exergonic reactions Cells “spend” ATP in endergonic reactions adenine P P P ribose

Adenosine triphosphate Adenosine diphosphate (ADP) Energy Coupling Hydrolysis breaks a phosphate group bond from the ATP molecules releasing energy The exergonic reaction supplies energy for cellular work Adenine Phosphate groups Hydrolysis Energy Ribose Adenosine triphosphate Adenosine diphosphate (ADP) Figure 5.4A

Potential energy of molecules Phosphorylation How ATP powers cellular work Reactants Products Potential energy of molecules Protein Work Figure 5.4B

Dehydration synthesis The ATP cycle Hydrolysis Dehydration synthesis Energy from exergonic reactions Energy for endergonic reactions Figure 5.4C

ENZYMES AND CELLULAR REACTIONS 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

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 starting substance activation energy with enzyme energy released by the reaction products

Four Features of Enzymes 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

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

Enzymes are selective 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

The enzyme is unchanged and can repeat the process How an enzyme works Active site Enzyme (sucrase) Substrate (sucrose) Glucose Fructose 1 4 Enzyme available with empty active site Products are released 3 2 Substrate is converted to products Substrate binds to enzyme with induced fit Figure 5.6 The enzyme is unchanged and can repeat the process

Enzyme Helpers 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

NORMAL BINDING OF SUBSTRATE Enzyme Inhibitors 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 Substrate Active site Enzyme NORMAL BINDING OF SUBSTRATE Competitive inhibitor Noncompetitive inhibitor ENZYME INHIBITION Figure 5.8

Negative Feedback 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

Environmental Effects Enzyme activity is influenced by the cellular environment temperature salt concentration pH

Connection: Some pesticides and antibiotics inhibit enzymes 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