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Energy and Metabolism Chapter 6

Flow of Energy Thermodynamics Branch of chemistry concerned with energy changes Cells are governed by the laws of physics and chemistry

Energy – capacity to do work 2 states Kinetic – energy of motion Potential – stored energy Many forms – mechanical, heat, sound, electric current, light, or radioactivity Heat the most convenient way of measuring energy 1 calorie = heat required to raise 1 gram of water 1ºC calorie or Calorie?

a. Potential energy b. Kinetic energy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. Potential energy b. Kinetic energy

Energy flows into the biological world from the sun Photosynthetic organisms capture this energy Stored as potential energy in chemical bonds

Redox reactions Oxidation Reduction Atom or molecule loses an electron Reduction Atom or molecule gains an electron Higher level of energy than oxidized form Oxidation-reduction reactions (redox) Reactions always paired

Loss of electron (oxidation) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Loss of electron (oxidation) e – A B A + B A + + B – Gain of electron (reduction) Lower energy Higher energy

Laws of thermodynamics First law of thermodynamics Energy cannot be created or destroyed Energy can only change from one form to another Total amount of energy in the universe remains constant During each conversion, some energy is lost as heat

Second law of thermodynamics Entropy (disorder) is continuously increasing Energy transformations proceed spontaneously to convert matter from a more ordered/less stable form to a less ordered/ more stable form

Disorder happens spontaneously Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Disorder happens spontaneously Organization requires energy © Jill Braaten

Free energy G = Energy available to do work G = H – TS H = enthalpy, energy in a molecule’s chemical bonds T = absolute temperature S = entropy, unavailable energy “molecule’ s” – close up # after apostrophe

ΔG = ΔH – TS ΔG = change in free energy Positive ΔG Negative ΔG Products have more free energy than reactants H is higher or S is lower Not spontaneous, requires input of energy Endergonic Negative ΔG Products have less free energy than reactants H is lower or S is higher or both Spontaneous (may not be instantaneous) Exergonic

Energy Released Energy Supplied Energy Released Energy Supplied Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Endergonic Products Energy must be supplied Energy Released Energy Supplied Free Energy (G)  G > 0 Reactants Course of Reaction a. Exergonic Reactants Energy Released Energy Supplied Free Energy (G) Energy is released Products  G < 0 Course of Reaction b.

Activation energy Extra energy required to destabilize existing bonds and initiate a chemical reaction Exergonic reaction’s rate depends on the activation energy required Larger activation energy proceeds more slowly Rate can be increased 2 ways Increasing energy of reacting molecules (heating) Lowering activation energy “Exergonic reaction’ s …” Close up # after apostrophe. Fix # in No. list from No. to entry?

Energy Released Energy Supplied Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. uncatalyzed catalyzed Activation energy Activation energy Energy Released Energy Supplied Free Energy (G) Reactant ΔG Product Course of Reaction

Catalysts Substances that influence chemical bonds in a way that lowers activation energy Cannot violate laws of thermodynamics Cannot make an endergonic reaction spontaneous Do not alter the proportion of reactant turned into product

Energy Released Energy Supplied Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. uncatalyzed catalyzed Activation energy Activation energy Energy Released Energy Supplied Free Energy (G) Reactant ΔG Product Course of Reaction

ATP Adenosine triphosphate Chief “currency” all cells use Composed of Ribose – 5 carbon sugar Adenine Chain of 3 phosphates Key to energy storage Bonds are unstable ADP – 2 phosphates AMP – 1 phosphate – lowest energy form

a. b. ATP Triphosphate group O– O P O– O ADP High-energy bonds O P O– Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ATP Triphosphate group O– a. O P O– O ADP High-energy bonds O P O– Adenine NH2 N C O C N H C C C H N N AMP CORE O P O O CH2 O H H H H OH OH Ribose b.

ATP cycle ATP hydrolysis drives endergonic reactions Coupled reaction results in net –ΔG (exergonic and spontaneous) ATP not suitable for long-term energy storage Fats and carbohydrates better Cells store only a few seconds worth of ATP

+ H2O ATP Energy from exergonic cellular reactions Energy for Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. + H2O ATP Energy from exergonic cellular reactions Energy for endergonic cellular processes + ADP Pi

Enzymes: Biological Catalysts Most enzymes are protein Some are RNA Shape of enzyme stabilizes a temporary association between substrates Enzyme not changed or consumed in reaction Carbonic anhydrase 200 molecules of carbonic acid per hour made without enzyme 600,000 molecules formed per second with enzyme

a. b. Active site Substrate Enzyme Enzyme–substrate complex Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Active site Substrate Enzyme Enzyme–substrate complex a. b.

Active site Pockets or clefts for substrate binding Forms enzyme–substrate complex Precise fit of substrate into active site Applies stress to distort particular bond to lower activation energy Induced fit

consists of glucose and fructose bonded together. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1. The substrate, sucrose, consists of glucose and fructose bonded together. 3. The binding of the substrate and enzyme places stress on the glucose– fructose bond, and the bond breaks. 2. The substrate binds to the active site of the enzyme, forming an enzyme– substrate complex. Glucose Bond Fructose H2O 4. Products are released, and the enzyme is free to bind other substrates. Active site Enzyme sucrase

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Enzymes may be suspended in the cytoplasm or attached to cell membranes and organelles Multienzyme complexes – subunits work together to form molecular machine Product can be delivered easily to next enzyme Unwanted side reactions prevented All reactions can be controlled as a unit

Nonprotein enzymes Ribozymes 1981 discovery that certain reactions catalyzed in cells by RNA molecule itself 2 kinds Intramolecular catalysis – catalyze reaction on RNA molecule itself Intermolecular catalysis – RNA acts on another molecule

Enzyme function Rate of enzyme-catalyzed reaction depends on concentrations of substrate and enzyme Any chemical or physical condition that affects the enzyme’s three-dimensional shape can change rate Optimum temperature Optimum pH

Optimum temperature for enzyme from hotsprings prokaryote Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Optimum temperature for human enzyme Optimum temperature for enzyme from hotsprings prokaryote Rate of Reaction 30 40 50 60 70 80 Temperature of Reaction (˚C) a. Optimum pH for pepsin Optimum pH for trypsin Rate of Reaction 1 2 3 4 5 6 7 8 9 pH of Reaction b.

Inhibitors Inhibitor – substance that binds to enzyme and decreases its activity Competitive inhibitor Competes with substrate for active site Noncompetitive inhibitor Binds to enzyme at a site other than active site Causes shape change that makes enzyme unable to bind substrate

Competitive inhibitor interferes with active site of enzyme so Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Substrate Substrate Inhibitor Inhibitor Active site Active site Enzyme Enzyme Allosteric site Competitive inhibitor interferes with active site of enzyme so substrate cannot bind Allosteric inhibitor changes shape of enzyme so it cannot bind to substrate a. Competitive inhibition b. Noncompetitive inhibition

Allosteric Enzymes Allosteric enzymes – enzymes exist in active and inactive forms Most noncompetitive inhibitors bind to allosteric site – chemical on/off switch Allosteric inhibitor – binds to allosteric site and reduces enzyme activity Allosteric activator – binds to allosteric site and increases enzyme activity

Metabolism Total of all chemical reactions carried out by an organism Anabolic reactions/anabolism Expend energy to build up molecules Catabolic reactions/catabolism Harvest energy by breaking down molecules

Biochemical pathways Reactions occur in a sequence Product of one reaction is the substrate for the next Many steps take place in organelles

Initial substrate Enzyme1 Intermediate substrate A Enzyme2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Initial substrate Enzyme1 Intermediate substrate A Enzyme2 Intermediate substrate B Enzyme3 Intermediate substrate C Enzyme4 End product

Feedback inhibition End-product of pathway binds to an allosteric site on enzyme that catalyzes first reaction in pathway Shuts down pathway so raw materials and energy are not wasted

a. b. Initial substrate Initial substrate Enzyme 1 Enzyme 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Initial substrate Initial substrate Enzyme 1 Enzyme 1 Intermediate substrate A Enzyme 2 Enzyme 2 Intermediate substrate B Enzyme 3 End product Enzyme 3 End product a. b.

Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.