Thermodynamics and Energy 101

Thermodynamics

• The laws of thermodynamics describe energy transformations in the universe.

First Law of Thermodynamics

• The energy of the universe is constant.
• Energy can be transferred or transformed, but not created or destroyed.
• Also known as the principle of conservation of energy.
• Entropy: a thermodynamic quantity representing the unavailability of a system's thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system.
• Enthalpy: a thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume.

Second Law of Thermodynamics

• During energy transfer or transformation, some energy is unusable and often lost as heat.
• 100% efficiency of energy transfer is impossible.
• Many important biological processes (e.g., photosynthesis, oxidation of glucose) have efficiencies around 42%.
• Every energy transfer or transformation increases the entropy (disorder) of the universe.
• Entropy is a measure of disorder and randomness in a system (or its surroundings).

Systems and Entropy

• Systems tend to proceed from ordered (low-entropy) states to disordered (high-entropy) states.
• The entropy of the system plus surroundings is unchanged by reversible processes.
• The entropy of the system plus surroundings increases for irreversible processes.

Spontaneous Reactions

• Spontaneous reactions occur as time passes.
• Organized effort requires energy input (e.g., cleaning a room).
• Cells use energy to resist the natural tendency toward disorder and maintain structure.

Exergonic and Endergonic Reactions

• Exergonic reactions release free energy.
• ΔG is negative.
• Spontaneous reactions.
• Endergonic reactions absorb free energy.
• ΔG is positive.
• Non-spontaneous reactions.
• If ΔG = 0, the reaction is at equilibrium, with no net flow in either direction.

Gibbs Free Energy

• Gibbs free energy is a measure of usable energy in a system.
• ΔG = change in Gibbs free energy = G_final - G_initial
• Activation energy: energy needed to initiate a reaction.
• Catalysts: substances that reduce the activation energy of a reaction.

Reaction Diagrams

• Exergonic reactions: energy released, reactants higher in energy
• Endergonic reactions: energy absorbed, reactants lower in energy

Coupling Reactions and ATP

• Endergonic reactions are often coupled with exergonic reactions to proceed.
• ATP hydrolysis: ATP is converted to ADP + Pi, releasing energy.
• ATP synthesis: ADP + Pi are combined to reform ATP, requiring energy.
• ATP is the energy currency of the cell.

ATP Role

• Transfer of chemical energy.
• Stored energy.
• Energy currency
• ATP structure: Adenosine triphosphate containing 3 phosphate groups.
• Phosphoanhydride bonds are formed by splitting out water between 2 phosphoric acids or between carboxylic & phosphoric acids. They have a large negative ΔG of hydrolysis, releasing energy when broken down
• Hydrolysis of ATP must be coupled with an endergonic process to drive it forward.