Energy and Chemical Reactions Ch. 5 Pre-AP Biology Ms. Haut

Laws of Thermodynamics The study of energy First Law of Thermodynamics (Conservation of Energy) –Energy cannot be created or destroyed However, Energy can be changed from one form to another Figure 5.2A Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

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

Energy Energy—the capacity to do work –Kinetic energy— energy of motion (heat) –Potential energy— stored capacity to perform work (chemical energy stored in chemical bonds) Figure 5.1x3 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

Chemical Reactions Cells carry out thousands of chemical reactions –The sum of these reactions constitutes cellular metabolism Chemical reactions either store or release energy –Endergonic reactions—absorb and store energy in a reaction –Exergonic reactions—energy is released from reaction –Metabolism = endergonic rxns + exergonic rxns

Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings

Chemical Reactions Reactants  Products Chemical bonds between atoms of reactants are broken and new bonds are formed. –6 CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2 But, you always have the same number of atoms of each element on both sides of the equation

Energy in Cells ATP is used to shuttle chemical energy within the cell ATP—energy-rich covalent bonds b/w outer 2 phosphate bonds

ATP, molecular model Figure 5.4Ax Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

When ATP gives up its energy, it forms ADP and an energy shuttle, the phosphate group. Energy coupling—using energy released from exergonic reactions to drive endergonic reactions Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings

For a chemical reaction to begin, reactants must absorb some energy –This energy is called the energy of activation (E A ) –This represents the energy barrier that prevents molecules from breaking down spontaneously E A barrier Reactants 1Products2 Enzyme Figure 5.5A A protein catalyst called an enzyme can decrease the energy barrier Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

Enzyme—catalytic protein that speeds up the chemical reactions by lowering the activation energy (end with –ase) Activation energy (E A )—amount of energy needed to start a chemical reaction before the reaction will proceed

Energy Profile Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings

Enzymes are substrate-specific—has unique 3-D shape which determines what the enzyme works on (substrate) The 3-D shape of the enzyme creates a “pocket” called the Active Site in which the substrate binds Active site has particular amino acid side-chains that match up with side-chains of substrate 20Site.gif

”Induced Fit Model” How an enzyme works The enzyme is unchanged and can repeat the process Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings

Affecting Enzyme Function Pre-AP Biology Ms. Haut

Enzymes speed up the rate of chemical reactions because they lower the energy of activation. Enzymes lower the energy of activation by forming an enzyme-substrate complex.

Enzyme Activity Enzyme activity is influenced by –temperature –salt concentration –pH –Presence of other molecules

Enzyme Activity A cell’s physical and chemical environment affects enzyme activity Each enzyme has optimal environmental conditions that favor the most active enzyme conformation (shape)

Effects of Temperature Optimal temp. allows greatest number of molecular collisions without denaturing the enzyme  Reaction rate  when temperature   Kinetic energy increases and collisions increase  Beyond optimal temperature, reaction rate slows  Too low, collisions b/w substrate and active site don’t occur fast enough  Too high, agitation disrupts weak bonds of the tertiary structure of enzyme (enzyme unfolds)

Effects of pH Optimal pH range for most enzymes is pH  Beyond optimal pH, reaction rate slows  Too low (acidic) H + ions interact with amino acid side-chains and disrupt weak bonds of the tertiary structure of enzyme  Too high (basic) base interacts with H + ions on amino acid side-chains and disrupt weak bonds of the tertiary structure of enzyme

Enzyme Inhibitors Certain chemicals selectively inhibit enzyme activity –Irreversible—inhibitor attaches by covalent bonds –Reversible—inhibitor attaches by weak bonds

Effects of Enzyme Inhibitors Competitive inhibitors— chemicals that resemble an enzyme’s normal substrate and compete with it for the active site Blocks active site from substrate If reversible, can be overcome by increasing substrate concentration

Competitive Inhibitor

Effects of Enzyme Inhibitors Noncompetitive inhibitors— chemicals that bind to another part of the enzyme molecule (allosteric site) Causes enzyme to change shape so active site can’t bind substrate May act as metabolic poison (DDT, many antibiotics) Essential mechanism in cell’s regulating metabolic reactions

Negative Feedback

Metabolic Control often Depends on Allosteric Regulation Allosteric enzymes have 2 shapes, catalytically active and inactive Binding of an activator to the allosteric site stabilizes active shape Binding of an inhibitor (noncompetitive) to the allosteric site stabilizes inactive shape

Connection: Some pesticides and antibiotics inhibit enzymes Certain pesticides are toxic to insects because they inhibit key enzymes in the nervous system Many antibiotics inhibit enzymes that are essential to the survival of disease-causing bacteria –Penicillin inhibits an enzyme that bacteria use in making cell walls

Control of Metabolism Feedback inhibition—regulation of a metabolic pathway by its end product, which inhibits an enzyme within the pathway  Prevents cell from wasting chemical resources synthesizing more product than is needed Negative Feedback inhibition Threonine C B A isoleucine D Enzyme 1 Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5

Negative Feedback

Acknowledgements Unless otherwise noted, illustrations are credited to Pearson Education which have been borrowed from BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2003. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher. BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2001. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.