🧬 AP Biology: Cellular Energetics — Free Energy Changes and Enzymes

🔹 1–2. Catabolic vs. Anabolic Pathways

• Catabolic pathways: Break down complex molecules → simpler ones; release energy (exergonic).

  • Example: Cellular respiration (glucose → CO₂ + H₂O).

• Anabolic pathways: Build complex molecules from simpler ones; require energy (endergonic).

  • Example: Photosynthesis (CO₂ + H₂O → glucose).

Cells use energy coupling — using energy from catabolism (ATP) to power anabolism.

🔹 3. Kinetic vs. Potential Energy

• Kinetic energy: Energy of motion (e.g., diffusion, heat, moving electrons).

• Potential energy: Stored energy due to position or structure (e.g., chemical bonds in glucose, ATP).

🔹 4. Open vs. Closed Systems

• Open system: Exchanges energy and matter with surroundings — living organisms are open systems.

• Closed system: No exchange; reactions eventually reach equilibrium (death for cells).

🔹 5. Spontaneous vs. Nonspontaneous Reactions

• Spontaneous: Occurs without energy input (∆G < 0); releases free energy (exergonic).

• Nonspontaneous: Requires energy input (∆G > 0); consumes free energy (endergonic).

🔹 6. Exergonic vs. Endergonic Reactions

• Exergonic: Energy released, ∆G negative; e.g., cellular respiration.

• Endergonic: Energy absorbed, ∆G positive; e.g., photosynthesis.

• Coupling: ATP hydrolysis (exergonic) drives endergonic processes.

🔹 7. Free Energy and Entropy

• Free energy (G): Portion of energy that can do work.

  • ∆G = ∆H – T∆S

  • Negative ∆G = spontaneous.

• Entropy (S): Measure of disorder; increases in spontaneous reactions.

  • Organisms maintain order by increasing entropy in surroundings.

🔹 8. Coupled Reactions

• Energy from an exergonic reaction (like ATP hydrolysis) drives an endergonic reaction.

• Example: ATP → ADP + Pi provides energy for muscle contraction or active transport.

🔹 9. Second Law of Thermodynamics in Biology

• Every energy transfer increases entropy (disorder) in the universe.

• Living systems remain ordered by using energy (sunlight or food) and releasing heat to surroundings.

🔹 10–11. Cellular Energy and ATP

• ATP (adenosine triphosphate) stores energy in its high-energy phosphate bonds.

• Hydrolysis (ATP → ADP + Pi) releases ~7.3 kcal/mol of energy.

• ATP drives work by transferring a phosphate group (phosphorylation) to another molecule.

🔹 12. Activation Energy

• The energy barrier that must be overcome to start a reaction.

• Enzymes lower this barrier, allowing spontaneous reactions to occur faster.

⚙ ENZYMES

🔹 13. Monomer of an Enzyme

• Enzymes are proteins, made up of amino acids (linked by peptide bonds).

• Enzyme structure = specific shape → specific function.

🔹 14. How a Substrate Binds

• The substrate binds to the enzyme’s active site, forming an enzyme–substrate complex.

• Induced fit model: Active site changes shape slightly to fit substrate snugly.

🔹 15. What Happens After Binding

• The enzyme stabilizes the transition state → lowers activation energy.

• Reaction occurs → products released → enzyme unchanged.

🔹 16–17. Enzyme Function and Mechanism

• Function: Catalyze reactions by lowering activation energy.

• Mechanisms:

  • Orienting substrates correctly.

  • Straining bonds in substrates.

  • Providing optimal microenvironments (e.g., pH pocket).

🔹 18. True or False: Enzymes Affect ∆G

❌ False — Enzymes speed up reactions but do not change ∆G (free energy of reaction).

🔹 19. Effect on Reaction Rate

• Enzymes increase the rate of biological reactions by lowering activation energy.

🔹 20–23. Enzyme-Catalyzed vs. Uncatalyzed Reactions

• Activation energy: Lower in catalyzed reactions.

• ∆G: Same for both.

• Rate: Much faster when catalyzed.

• Overall energy output: Unchanged.

🔹 24. Mechanisms of Lowering Activation Energy

• Align reactants properly.

• Distort substrate bonds.

• Stabilize transition states.

• Provide correct pH or ionic environment.

🔹 25. Substrate Specificity

• Due to unique 3D shape of active site — only specific substrates fit.

• Example: Lactase acts only on lactose.

🔹 26–27. Environmental Effects

Factors affecting enzyme structure:

• Temperature:

  • Too low = slow.

  • Too high = denaturation (enzyme unfolds).

• pH:

  • Each enzyme has an optimum pH; extremes alter ionic bonds → denaturation.

• Concentration:

  • ↑ Substrate = ↑ rate until saturation (all enzymes occupied).

Changes in hydrogen ion concentration

• ↑ H⁺ → ↓ pH → enzyme denatures.

• ↓ H⁺ → ↑ pH → enzyme denatures.

Temperature effects

• Moderate heat increases collisions → faster rate.

• Excess heat breaks hydrogen bonds → denaturation.

🔹 28. Enzyme Activity Graphs

• Rate vs. Substrate: Increases, then plateaus (enzyme saturation).

• Rate vs. Temperature: Bell curve; optimum temperature yields max rate.

• Rate vs. pH: Bell curve centered at enzyme’s optimum pH.

🔹 29–30. Denaturation

• Denaturation: Unfolding of a protein → loss of structure and function.

• Reversibility: Sometimes reversible if conditions return to normal.

🔹 31–33. Regulation of Enzyme Activity

Regulatory Factors

• Activators/inhibitors control enzyme function.

Allosteric Regulation

• Regulatory molecules bind to an allosteric site (not active site), changing enzyme shape.

• Can activate or inhibit the enzyme.

• Often seen in multi-subunit enzymes (e.g., hemoglobin).

Feedback Inhibition

• End product of a pathway inhibits an early-step enzyme.

• Prevents overproduction and conserves energy.

  • Example: Isoleucine inhibits threonine deaminase.

🧫 Additional Concepts to Review

• Biomolecules: Understand protein structure (primary → quaternary) and how shape affects function.

• Protein Structure Affected by: pH, temperature, salt concentration, mutations.

• Virtual Labs: Be able to interpret enzyme activity data (e.g., catalase reaction, substrate graphs).

⚡ Summary Table