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 in Biological Systems

Energy is the capacity to cause change, including movement or work against opposing forces. It can be converted from one form to another.

• 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 (e.g., movement of molecules).

• Thermal energy: Kinetic energy associated with random movement of atoms or molecules; heat is the transfer of thermal energy.

Thermodynamics in Biology

Thermodynamics is the study of energy transformations in matter. 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 increase entropy, often as heat.

• Spontaneous processes: Occur without energy input; increase entropy (e.g., diffusion).

• Nonspontaneous processes: Require energy input; decrease entropy locally.

• Biological order: Organisms maintain order by increasing entropy in their surroundings, thus not violating the second law.

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:

$\Delta G = \Delta H - T\Delta S$

• Spontaneous reactions: $\Delta G < 0$ (exergonic); release free energy; products are more stable than reactants; can perform work.

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

• Exergonic reactions: Release free energy; spontaneous.

• Endergonic reactions: Absorb free energy; nonspontaneous.

Example: Cellular respiration is exergonic ($\Delta G < 0$); photosynthesis is endergonic ($\Delta G > 0$).

• Hydroelectric system analogy: Open, multistep metabolic pathways prevent equilibrium, allowing cells to extract energy efficiently in steps.

ATP and Cellular Work

ATP (adenosine triphosphate) is the cell's main energy currency, coupling exergonic and endergonic reactions to perform cellular work.

• Three main types of cellular work:

  • Chemical work: Driving endergonic reactions (e.g., synthesis of polymers).

  • Transport work: Pumping substances across membranes against gradients.

  • Mechanical work: Movement (e.g., muscle contraction, cilia beating).

• ATP structure: Adenine, ribose, and three phosphate groups; the bonds between phosphate groups are high-energy bonds.

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

  • Products: ADP (adenosine diphosphate) and inorganic phosphate ($P_i$).

  • Net $\Delta G$ is negative; reaction is spontaneous and exergonic.

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

• ATP regeneration: ATP is regenerated from ADP and $P_i$ by cellular respiration.

Example: ATP hydrolysis drives the synthesis of glutamine from glutamic acid and ammonia by coupling reactions.

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.

• Enzyme function: Lower $E_a$ by stabilizing the transition state; do not affect $\Delta G$.

• Enzyme vs. heat: Enzymes are specific and function under mild conditions; heat is nonspecific and can denature proteins.

• 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; binding induces a fit that facilitates the reaction.

• Enzyme reuse: Enzymes are not consumed and can catalyze multiple reactions.

Factors Affecting Enzyme Activity

• Environmental factors: Temperature, pH, and ionic concentration can affect enzyme activity.

• Cofactors: Non-protein helpers (inorganic ions or organic molecules) required for enzyme function.

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

Enzyme Regulation

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

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

• Reversible vs. irreversible inhibition: Reversible inhibitors bind non-covalently; irreversible inhibitors bind covalently and permanently inactivate the enzyme.

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

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

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

Example: Isoleucine inhibits the first enzyme in its biosynthetic pathway when in excess (feedback inhibition).

Localization and Evolution of Enzymes

• Enzymes are localized to specific cellular structures or organelles, optimizing their function and regulation.

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

Type of Inhibition	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

Additional info: This summary expands on the learning objectives by providing definitions, examples, and explanations for each concept, ensuring a comprehensive understanding suitable for exam preparation.