Chapter 6: An Introduction to Metabolism – Study Notes

Chapter 6: An Introduction to Metabolism

Overview of Metabolism and Bioenergetics

Metabolism refers to the totality of an organism’s chemical reactions, which are organized into metabolic pathways. Bioenergetics is the study of how organisms manage their energy resources. Metabolic pathways consist of a series of enzymatic reactions that transform molecules through defined steps.

• Catabolic pathways: Break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways: Build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

Forms of Energy

Energy is the capacity to cause change or do work. It exists in various forms relevant to biological systems:

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

• Chemical energy: A form of potential energy stored in chemical bonds (e.g., ATP, glucose).

• Kinetic energy: Energy of motion.

  • Thermal energy: Kinetic energy associated with random movement of atoms or molecules.

  • Heat: Transfer of thermal energy from one object to another.

Thermodynamics in Biological Systems

Thermodynamics is the study of energy transformations. Biological systems obey the laws of thermodynamics:

• Open systems: Exchange energy and matter with their surroundings (e.g., living cells).

• Closed systems: Isolated from their surroundings; no exchange of energy or matter.

• First Law of Thermodynamics (Conservation of Energy): Energy cannot be created or destroyed, only transformed or transferred.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe.

  • Entropy (S): A measure of disorder or randomness.

  • Energy transfers often release heat, increasing entropy.

  • Spontaneous processes: Occur without energy input; increase entropy.

  • Nonspontaneous processes: Require energy input; decrease entropy locally but increase it overall.

  • Biological organisms do not violate the second law because they increase local order at the expense of greater disorder in the environment.

Free Energy and Chemical Reactions

Free energy (G) is the portion of a system’s energy that can perform work. The change in free energy ($\Delta G$) determines whether a reaction is spontaneous.

• Calculation of Free Energy Change:

  • $\Delta G = \Delta H - T\Delta S$

  • Where $\Delta H$ = change in enthalpy (total energy), $T$ = temperature in Kelvin, $\Delta S$ = change in entropy.

• Spontaneous reactions: $\Delta G < 0$ (exergonic, release free energy, can perform work).

• Nonspontaneous reactions: $\Delta G > 0$ (endergonic, require energy input).

• In exergonic reactions, the final state is more stable than the initial state, and free energy is released.

• In endergonic reactions, free energy is absorbed, and the products have higher free energy than the reactants.

Biological reactions differ from physical chemical reactions because they occur in open systems and are often part of multistep pathways, preventing equilibrium and allowing continuous work.

• Hydroelectric system analogy: Open, multistep systems allow cells to release energy gradually and efficiently.

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) couples exergonic and endergonic reactions, enabling cellular work.

• Three main types of cellular work:

  • Chemical work (driving endergonic reactions)

  • Transport work (pumping substances across membranes)

  • Mechanical work (muscle contraction, movement of cilia/flagella)

• Structure of ATP: Adenine, ribose, and three phosphate groups. The bonds between phosphate groups are high-energy bonds.

• ATP hydrolysis: Releases significant free energy by breaking the terminal phosphate bond.

• ATP phosphorylation: Transfer of a phosphate group to another molecule, forming a phosphorylated intermediate, which is more reactive.

• Products of ATP hydrolysis: ADP and inorganic phosphate ($\text{P}_i$).

• The net $\Delta G$ of ATP hydrolysis is negative (spontaneous, exergonic).

• ATP hydrolysis powers mechanical and transport work by changing protein shapes and transporting molecules.

• ATP is regenerated from ADP and $\text{P}_i$ by cellular respiration.

Enzymes and Catalysis

Enzymes are biological catalysts that speed up reactions by lowering activation energy without being consumed.

• Activation energy ($E_a$): The initial energy needed to start a reaction.

• Enzymes lower $E_a$ but do not affect $\Delta G$.

• Enzyme catalysis is more specific and efficient than heat.

• Substrate specificity: Each enzyme acts on a specific substrate, forming an enzyme-substrate complex.

• Active site: The region on the enzyme where the substrate binds, inducing a fit that facilitates the reaction.

• Enzymes can be reused after releasing products.

Factors Affecting Enzyme Activity

• Temperature, pH, and substrate concentration can affect enzyme activity.

• Enzymes have optimal conditions for activity.

Enzyme Regulation

• Cofactors: Non-protein helpers (inorganic, e.g., metal ions).

• Coenzymes: Organic cofactors (e.g., vitamins).

• Enzyme inhibition:

  • Competitive inhibition: Inhibitor binds to active site, blocking substrate.

  • Noncompetitive inhibition: Inhibitor binds elsewhere, changing enzyme shape.

  • Reversible inhibition: Inhibitor can dissociate from enzyme.

  • Irreversible inhibition: Inhibitor binds permanently, inactivating enzyme.

• Allosteric regulation: Regulatory molecules bind to sites other than the active site, affecting enzyme activity.

• Cooperativity: Substrate binding increases enzyme activity at other active sites (seen in multi-subunit enzymes).

• Feedback inhibition: End product of a pathway inhibits an earlier step, preventing overproduction.

Localization and Evolution of Enzymes

• Enzymes are localized to specific cell structures or organelles, optimizing their function.

• Enzyme structure and function are encoded by genes and subject to mutation and natural selection, leading to evolutionary diversity.

Summary Table: Types of Enzyme Inhibition

Type	Binding Site	Effect on Enzyme	Reversibility
Competitive	Active site	Blocks substrate binding	Usually reversible
Noncompetitive	Allosteric site	Changes enzyme shape	Usually reversible
Irreversible	Active or allosteric site	Permanently inactivates enzyme	Irreversible

Key Equations

• Free energy change:

  • $\Delta G = \Delta H - T\Delta S$

• ATP hydrolysis:

  • $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard biology textbooks.