METABOLISM

• Metabolism


  The totality of an organism's chemical reactions
• Metabolism


  • It is an emergent property of life that arises from interactions between molecules within the cell
• Metabolic pathway


  1. Begins with a specific molecule and ends with a product

  2. Each step is catalyzed by a specific enzyme
• Catabolic pathways


  Release energy by breaking down complex molecules into simpler compounds
• Anabolic pathways


  Consume energy to build complex molecules from simpler ones
• Bioenergetics


  The study of how organisms manage their energy resources
• Energy


  The capacity to cause change
• Forms of energy

  • Kinetic energy
  • Heat (thermal energy)
  • Potential energy
  • Chemical energy
• Energy can be converted from one form to another
• Thermodynamics


  The study of energy transformations
• Closed system


  Isolated from its surroundings
• Open system


  Energy and matter can be transferred between the system and its surroundings
• Organisms are open systems
• First law of thermodynamics


  The energy of the universe is constant: Energy can be transferred and transformed, but it cannot be created or destroyed
• Second law of thermodynamics


  • Living cells unavoidably convert organized forms of energy to heat
  • Spontaneous processes occur without energy input and increase the entropy of the universe
• The evolution of more complex organisms does not violate the second law of thermodynamics
• Free energy


  Energy that can do work when temperature and pressure are uniform, as in a living cell
• Free-energy change (∆G)


  Related to the change in enthalpy (∆H), change in entropy (∆S), and temperature in Kelvin (T): ∆G = ∆H - T∆S
• Only processes with a negative ∆G are spontaneous
• Equilibrium


  A state of maximum stability where a process is spontaneous and can perform work only when it is moving toward equilibrium
• Exergonic reaction


  Proceeds with a net release of free energy and is spontaneous
• Endergonic reaction


  Absorbs free energy from its surroundings and is nonspontaneous
• Cells are not in equilibrium; they are open systems experiencing a constant flow of materials
• A defining feature of life is that metabolism is never at equilibrium
• Energy coupling


  The use of an exergonic process to drive an endergonic one
• Most energy coupling in cells is mediated by ATP
• ATP (adenosine triphosphate)


  The cell's energy shuttle, composed of ribose, adenine, and three phosphate groups
• ATP hydrolysis


  Energy is released when the terminal phosphate bond is broken, coming from the chemical change to a state of lower free energy, not from the phosphate bonds themselves
• How ATP performs work

  1. The energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction

  2. Overall, the coupled reactions are exergonic
• ATP phosphorylation


  ATP drives endergonic reactions by transferring a phosphate group to some other molecule, such as a reactant
• Regeneration of ATP


  • ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP)
  • The energy to phosphorylate ADP comes from catabolic reactions in the cell
• Phosphorylation


  Transferring a phosphate group to some other molecule, such as a reactant
• Recipient molecule


  Now phosphorylated
• Mechanical work


  ATP binds noncovalently to motor proteins, then is hydrolyzed
• Transport work


  ATP phosphorylates transport proteins
• ATP


  A renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP)
• Energy to phosphorylate ADP

  Comes from catabolic reactions in the cell
• Chemical potential energy stored in ATP


  Drives most cellular work
• Catalyst


  A chemical agent that speeds up a reaction without being consumed by the reaction
• Enzyme


  A catalytic protein