Energy and Chemical ReactionsCh. 2

BiologyMs. Haut

Chemical Reactions

• Reactants—the starting materials

• Chemical bonds are broken, atoms are reorganized and new bonds formed

• Products—the resulting material(s)– 2H2 + O2 2H2O

Reactants → Products

http://www.chem.ufl.edu/~itl/2045/matter/FG03_003.GIF

Chemical Reactions

• Occur when bonds between the outermost parts of atoms are formed or broken

• Involve changes in matter, the making of new materials with new properties, and energy changes

http://upload.wikimedia.org/wikipedia/commons/8/8c/Wohler_synthesis.gif

http://www.personal.kent.edu/~cearley/ChemWrld/balance/H2_O2.gif

Chemical Equations

• Show the kind of reactants and products and their relative amounts in the reaction

4 Al + 3 O2 → 2 Al2O3

Chemical Equations

• Because of the principle of the conservation of matter, an equation must be balanced.

• It must have the same number of atoms of the same kind on both sides

http://www.chem.ufl.edu/~itl/2045/matter/FG03_003.GIF

Energy in Reactions

• Energy is released when chemical bonds are broken

• Energy is absorbed when chemical bonds are formed.

• Because chemical reactions involve breaking and forming bonds, they involve changes in energy.

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)

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

Energy in Reactions

• In order to stay alive, organisms need to carry out reactions that require energy.

•Because matter and energy are conserved in chemical reactions, every organism must have a source of energy to carry out chemical reactions.

•Plants get their energy from the sun.

•Animals get their energy from eating plants or other animals.

Energy in Reactions

• Chemical reactions that release energy do not always occur spontaneously.

• Chemists call the energy that is needed to get a reaction started the activation energy.

Energy in Reactions

• Activation energy is a factor in whether the overall chemical reaction releases energy or absorbs energy.

Energy in Cells• ATP is used to shuttle chemical energy

within the cell• ATP—energy-rich covalent bonds b/w

outer 2 phosphate bonds

•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

• Some chemical reactions that make life possible are too slow or have activation energies that are too high to make them practical for living tissue

• Enzyme—catalytic protein that speeds up the chemical reactions by lowering the activation energy (end with –ase)

EA

barrier

Reactants

1 Products 2E

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me

Figure 5.5A

EA

barrier

Reactants

1 Products 2E

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Figure 5.5A

Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

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

http://www.pickens.k12.sc.us/phsteachers/edmunds/Enzyme%20Active%20Site.gif

”Induced Fit Model”

• How an enzyme works

• The enzyme is unchanged and can repeat the process

Affecting Enzyme FunctionBiology

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 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 high, agitation disrupts weak bonds of the tertiary structure of enzyme (enzyme unfolds)

Too low, collisions b/w substrate and active site don’t occur fast enough

Effects of pH

• Optimal pH range for most enzymes is pH 6 - 8

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 enzymeToo high (basic) base interacts with H+ ions on amino acid side-chains and disrupt weak bonds of the tertiary structure of enzyme

Effect of Amount of Substrate or Enzyme• Increase Concentration of Substrate

– More substrate available to make contact with enzyme

– Speeds up reaction rate up to a point– Reaction rate decreases as substrate is

converted into product• Increase Concentration of Enzyme

– More enzyme available to make contact with more substrate molecules

– Speeds up reaction rate up to a point– Reaction rate decreases as substrate is

converted into product

Chemical Reactions and Enzymes

Chemical Reactions and EnzymesThe elements or compounds produced by a chemical reaction are known a

a)reactants.

b)enzymes.

c)products.

d)waste.

Chemical Reactions and EnzymesChemical reactions always involve

a) changes in energy.

b) enzymes.

c) catalysts.

d) changes in the atomic number of the reactants.

Chemical Reactions and EnzymesThe factor that prevents many energy-releasing reactions from occurring at relatively low temperatures is called

a) catalytic energy.

b) chemical bond energy.

c) enzyme energy.

d) activation energy.

Chemical Reactions and EnzymesWhich of the following statements is true?

a) All proteins are enzymes.

b) All catalysts are enzymes.

c) All enzymes are catalysts.

d) All catalysts are proteins.

Chemical Reactions and Enzymes

What happens to an enzyme after the reaction it catalyzes has taken place?

a) The enzyme is destroyed, and the cell must make another.

b) The enzyme holds on to the product until another enzyme removes it.

c) The enzyme is unchanged and ready to accept substrate molecules.

d) The enzyme changes shape so it can accept a different kind of substrate.

Acknowledgements• Unless otherwise noted, illustrations are credited to Prentice Hall

and have been borrowed from Biology by Miller and Levine, © 2007. 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, ©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.