Overview
This lecture explains Gibbs free energy (ΔG), its connection to spontaneity in chemical reactions, and its importance in biological processes like respiration and photosynthesis.
Understanding Gibbs Free Energy
- Gibbs free energy (ΔG), or available energy, represents energy in a system that can be used to do work.
- The equation involves ΔG, enthalpy (H, total energy), entropy (S, disorder), and temperature (T).
- Spontaneous reactions happen on their own after an initial input and generally release energy to their surroundings.
Key Concepts in Spontaneous Reactions
- Enthalpy (H) decreases in spontaneous reactions, as in a ball rolling down a slide or glucose breaking down.
- Entropy (S) increases in spontaneous reactions, as in diffusion where particles spread out, increasing disorder.
- Increasing temperature can make reactions more likely to be spontaneous.
Gibbs Free Energy and Reaction Types
- ΔG < 0: Spontaneous (exergonic) reactions, release energy.
- ΔG > 0: Non-spontaneous (endergonic) reactions, require energy input.
- ΔG = 0: System is at equilibrium; no net change in free energy.
Biological Examples
- Cellular respiration is an exergonic reaction (ΔG = -686 kcal/mol), breaking down glucose and releasing energy.
- Photosynthesis is an endergonic reaction (ΔG > 0), storing energy in glucose using sunlight.
- Both reactions involve activation energy, the initial energy required to start the process.
ATP: The Energy Currency
- ATP (adenosine triphosphate) stores and transfers usable energy in cells.
- Breaking down ATP to ADP (adenosine diphosphate) and a phosphate releases energy (ΔG < 0).
- Rebuilding ATP from ADP requires energy input (ΔG > 0).
The Flow of Energy in Life
- Energy from the Sun is stored in glucose by plants (photosynthesis: ΔG > 0).
- Organisms release that energy via respiration (ΔG < 0).
- ATP cycles store and supply energy for cellular work, linking these processes.
Key Terms & Definitions
- Gibbs Free Energy (ΔG) — energy available to do work in a system.
- Enthalpy (H) — total energy of a system.
- Entropy (S) — measure of disorder or randomness in a system.
- Spontaneous Reaction — a process that occurs without continuous energy input (ΔG < 0).
- Exergonic Reaction — releases energy (ΔG < 0).
- Endergonic Reaction — requires energy input (ΔG > 0).
- Activation Energy — initial energy needed to start a reaction.
- ATP (Adenosine Triphosphate) — primary energy carrier in cells.
Action Items / Next Steps
- Review examples of exergonic and endergonic reactions in biology.
- Practice interpreting energy diagrams for cellular respiration and photosynthesis.
- Memorize the definitions and relationships between ΔG, H, S, and T.