chapter 8

• Chemical reaction


  Occurs when atoms have enough energy to combine or change bonding partners
• Chemical reaction


  1. Reactants

  2. Products
• Metabolism


  Sum total of all chemical reactions occurring in a biological system at a given time
• Metabolic reactions involve energy changes
• Energy


  The capacity to do work, or the capacity for change
• Forms of energy


  • Potential energy
  • Kinetic energy
• Energy can be converted from one form to another
• Anabolic reactions


  Complex molecules are made from simple molecules; energy is required
• Catabolic reactions


  Complex molecules are broken down to simpler ones; energy is released
• Catabolic and anabolic reactions are often linked
• The energy released in catabolic reactions is used to drive anabolic reactions—to do biological work
• First law of thermodynamics


  Energy is neither created nor destroyed
• Second law of thermodynamics


  When energy is converted from one form to another, some of that energy becomes unavailable to do work
• No energy transformation is 100% efficient; some energy is lost to molecular-scale disorder (entropy) often as heat
• In the universe as a whole, or in any isolated system, the degree of disorder can only increase
• Systems will change spontaneously toward those arrangements that have the greatest probability
• Enthalpy (H)


  Total energy
• Free energy (G)


  Usable energy that can do work
• Entropy (S)


  Unusable energy associated with molecular-scale disorder
• H = G + TS
• Change in free energy (ΔG)


  Measured in calories or joules
• ΔG = ΔH - TΔS
• ΔG = Gfinal - Ginitial
• If ΔG is -, free energy is released
• If ΔG is +, free energy is required
• If free energy is NOT available, reaction doesn't occur
• Entropy (i.e., "unusable" energy associated with molecular-scale disorder) tends to increase because of energy transformations
• Reversing the natural tendency toward disorder requires an intentional effort and an input of energy
• Living organisms must have a constant supply of free energy (from catabolism of food molecules) to maintain order
• Exergonic reactions


  Release free energy (-ΔG)
• Endergonic reactions


  Consume free energy (+ΔG)
• In principle, chemical reactions are reversible
• At chemical equilibrium, ΔG = 0
• The further towards completion the point of equilibrium is, the more free energy is released
• ΔG values near zero are characteristic of readily reversible reactions
• ATP


  Captures and transfers free energy
• ATP hydrolysis


  ATP → ADP + Pi
• Formation of ATP


  Endergonic: ADP + Pi + free energy → ATP + H2O
• Coupling of endergonic and exergonic reactions is powered by formation & hydrolysis of ATP
• Coupling is very common in metabolism