CC

BIO 201

Overview of Thermodynamics in Chemical Reactions

  • The discussion revolves around classifying chemical reactions as exergonic or endergonic based on energy dynamics.

  • Exergonic Reactions: Reactions that release energy.

  • Endergonic Reactions: Reactions that consume energy.

Case Study: Breakdown of Hydrogen Peroxide (H₂O₂)

  • The reaction: H₂O₂ → H₂O + O₂

    • This reaction involves the breakdown of hydrogen peroxide into water and oxygen.

  • Determining Reaction Type:

    • Show of hands to determine if the reaction is exergonic or endergonic:

    • If exergonic, show a 1.

    • If endergonic, show a 2.

    • Outcome: The reaction is determined to be exergonic.

Explanation of Results

  • The initial reactant is one molecule of hydrogen peroxide, breaking down into two products (water and oxygen).

  • The products are smaller and more disordered, indicating an increase in entropy (disorder) and a release of energy.

Energy Considerations
  • Energy Stored in Molecules:

    • Reactants (H₂O₂) have more energy compared to the products (H₂O + O₂).

    • Energy is released as chemical bonds in the reactants are broken and the products are formed.

Free Energy Change (ΔG)

  • Definition of ΔG:

    • ΔG represents the change in free energy throughout the reaction.

    • Negative ΔG: Indicates a decrease in free energy; indicates an exergonic reaction.

  • As ΔG is negative, more energy is initially stored in the reactants compared to the products, confirming the reaction is spontaneous and does not require energy input.

# Spontaneity of the Reaction

  • The reaction is spontaneous due to it being energetically favorable (releases energy into surroundings).

  • This aligns with the laws of thermodynamics where systems naturally progress toward states of greater disorder.

Energy Input Requirement
  • Energy Requirement:

    • The reaction does not require an external energy input (thumbs up for ‘no energy input’).

Upcoming Events

  • Unit Two Exam Details:

    • Scheduled for Thursday.

    • Items to Bring: Number two pencil or mechanical pencil.

    • Exam format: 40 multiple-choice questions, 50 minutes to complete.

    • Additional review session by a teaching assistant before the exam.

Key Concepts in Reactions

Revisiting Reaction Types

  • Exergonic Reactions:

    • Negative ΔG, products have less energy than reactants, spontaneous reaction, energy release.

  • Endergonic Reactions:

    • Positive ΔG, products have more energy than reactants, non-spontaneous reaction, energy required to be input.

Activation Energy and Transition States

  • Activation Energy:

    • Even exergonic reactions require an initial energy input to start the reaction (nudge necessary to reach the transition state).

  • Activation energy is the energy needed to initiate the reaction.

    • Reaction progress shown along an energy diagram with a peak - transition state - which represents the required energy threshold to move from reactants to products.

  • Analogy for Understanding:

    • Teetering boulder analogy – if a ball is on the edge of a cliff, it will fall alone due to gravity, but must be teetering on the edge to begin its descent, implying initial energy input is needed for reactions.

Catalysts and Enzymes

Definition and Function

  • Catalysts:

    • Substances that speed up reactions without being consumed in the reaction (e.g., enzymes). They work by providing alternative pathways with lower activation energy required for the reaction to occur.

  • Enzymes:

    • A type of protein serving as biological catalysts to facilitate chemical reactions.

Mechanism of Action

  • Enzymes have an active site that specifically binds substrates, increasing the likelihood of appropriate interactions to foster a reaction.

  • Induced Fit Model:

    • When substrates enter the active site, the enzyme changes shape slightly to better fit the substrate, enhancing the likelihood of a successful reaction.

Example of Enzyme Action

  • Enzymes can lower activation energy, making reactions occur faster without changing ΔG.

  • An example provided is when heat increases kinetic energy, leading to more frequent molecular collisions, thereby increasing reaction rates.

Michaelis-Menten Kinetics

Understanding Reaction Rates

  • Michaelis-Menten Graph:

    • Plots substrate concentration (X-axis) against reaction velocity (Y-axis).

    • Vmax: Maximum reaction speed when all enzyme active sites are saturated with substrate.

    • Km (Michaelis constant): Substrate concentration at which the reaction rate is half-maximal.

Competitive Inhibition Analysis

  • Competitive Inhibitors:

    • Molecules that compete with the substrate for the active site.

    • Impact on reaction kinetics:

    • Vmax remains the same; however, a higher Km indicates a higher substrate concentration is needed to reach half of Vmax.

Noncompetitive Inhibition Analysis

  • Noncompetitive Inhibitors:

    • Molecules that bind to another site on the enzyme, changing its shape and preventing the substrate from binding properly.

    • Impact on reaction kinetics:

    • Maximum velocity (Vmax) decreases, but Km remains unchanged as substrate concentration does not affect the inhibitor's ability to bind.

External Factors Affecting Enzyme Activity
  • Enzyme activity can be affected by temperature, pH, and substrate concentration.

  • Denaturation:

    • Condition in which enzymes lose their structure and functionality due to extreme pH or temperature changes.

Cofactors and Coenzymes

  • Cofactors:

    • Non-protein chemical compounds that assist enzymes, often metallic ions like zinc or iron.

  • Coenzymes:

    • Organic molecules, often vitamins, that help enzymes function properly.