Biology 172 - Lecture 15: Free Energy and Reactions

Outline

• Free Energy Concepts
• Announcements
  • Quiz 7 due Tuesday by midnight
• Energy and Reactions
• Free energy and G
• Exergonic and Endergonic Reactions
• Activation Energy
• Catalysts and Enzymes

Energy Concepts

• Energy: The potential to do work
  • Potential Energy: Energy stored in an object
  • Kinetic Energy: Energy of motion, including molecular motion (related to temperature)

Free Energy

• Free Energy (G): Energy available to do work
  • Chemical reactions in cells often involve breakdown of sugar into simpler molecules
  • Diffusion of molecules (e.g., dye dispersing in water)
• Gravitational Motion: Objects move from higher to lower altitudes spontaneously
  • More free energy (higher G): Less stable, greater work capacity
  • Less free energy (lower G): More stable, less work capacity
• Systems move from high free energy to low free energy
  • In spontaneous changes, free energy decreases (G < 0), system becomes stable, and released energy can do work

Reaction Types

• Exergonic Reactions: Energy-releasing, spontaneous (G < 0)
  • Reactants have more energy than products
• Endergonic Reactions: Energy-consuming, non-spontaneous (G > 0)
  • Reactants have less energy than products
• Change in Free Energy (G): Determines if a reaction is spontaneous
• Equilibrium Constant (Keq): Ratio of products to reactants at equilibrium
  • If [B] > [A], then K > 1
  • If [B] < [A], then K < 1

How Reactions Occur

• Synthesis (e.g., Protein synthesis): Requires energy input (endergonic)
  • Proteins are more ordered, less stable than amino acids
• Coupling Reactions: Endergonic reactions coupled to exergonic reactions (e.g., ATP hydrolysis) to proceed
  • ATP provides energy for cellular reactions

Activation Energy

• Activation Energy (Ea): Energy required to reach transition state
  • High activation energy can impede reactions
  • Catalysts lower activation energy, increasing reaction rate

Catalysts and Enzymes

• Catalysts: Speed up reactions without being consumed
  • Do not change G or equilibrium constant
• Enzymes: Protein catalysts, specific to reactions
  • Lower activation energy by stabilizing transition states
  • Have substrate specificity, binding specific molecules
  • Active site: Where substrate binds and catalysis occurs

Enzyme Action

1. Active Site: Where substrates bind, causing a shape change (induced fit)
2. Substrate Specificity: Depends on fit with enzyme’s active site
3. Enzyme-Substrate Complex: Lowers activation energy, speeding up reactions

Enzyme Kinetics

• Factors Affecting Reaction Rate:
  • Substrate concentration
  • Enzyme concentration
  • Temperature
  • pH
  • Ions presence/concentration

Key Terms & Concepts

• Free energy, G, Exergonic/Endergonic, Exothermic/Endothermic, Non/Spontaneous, Keq, Entropy, Activation Energy, Catalysis, Enzyme, Substrate specificity, Active site, Reaction coupling
• Identify higher/lower free energy states, reaction types, and how catalysis occurs
• Understanding how endergonic reactions occur in biological systems