chapter 8

• Chemical reactions

  Involve a change in energy between products and reactants
• By the Law of Conservation of Energy, the total energy of a system must remain unchanged
• Enthalpy


  The stored chemical energy or heat content of a system
• Exothermic reactions


  Release heat and light into their surroundings
• Exothermic reactions

  Products have less enthalpy than the reactants
• Endothermic reactions


  Absorb heat and/or light from their surroundings
• Endothermic reactions


  Products have more enthalpy than the reactants
• Decomposition of water into hydrogen and oxygen is an endothermic reaction
• ATP


  Adenosine Triphosphate, the cell's primary energy currency
• Even exergonic, energy-releasing reactions require a small amount of activation energy
• Endergonic reactions


  Require much more energy input because their products have more free energy than their reactants
• ATP hydrolysis

  Releases energy that can be harnessed to perform cellular work
• ATP hydrolysis releases -7.3 kcal/mol of energy under standard conditions
• ATP hydrolysis releases -14 kcal/mol of energy under cellular conditions
• Energy coupling


  The process where the energy released from ATP hydrolysis is used to power other cellular processes
• Energy coupling using ATP


  • Sodium-potassium pump
• ATP hydrolysis releases 7.3 kcal/mol of energy


  It takes 2.1 kcal/mol of energy to move one Na+ across the membrane
• ATP hydrolysis is used to phosphorylate molecules, creating unstable intermediates and powering conformational changes
• Catalysts


  Substances that can increase the rate of a reaction without being consumed
• Catalysts


  • Lower the energy of the transition state, thus lowering the activation energy
  • Change the mechanism of the reaction, also changing the nature and energy of the transition state
• Carbonic anhydrase


  An enzyme that catalyzes the reversible reaction of carbon dioxide and water to form carbonic acid
• Carbonic anhydrase is one of the fastest known enzymes, with reaction rates between 10^4 and 10^6 reactions per second
• Carbonic anhydrase catalyzes the reaction 10^5 times faster than the uncatalyzed reaction
• Acid catalysis

  The catalyst is usually H+
• Base catalysis


  The catalyst is usually OH-
• Acid-catalyzed reaction


  • Hydrolysis of sucrose
• Heterogeneous catalysts


  Catalysts in a different phase than the reactants
• Heterogeneous catalysts


  • Catalytic converters in cars
  • Ziegler-Natta catalysts used to make plastics
• Heterogeneous catalysts work through surface catalysis, where the reactants are adsorbed onto the solid catalyst surface