Chapter 8: An Introduction to Metabolism

Concept 8.1: Introduction to Metabolism

Metabolism encompasses all chemical reactions occurring within an organism, transforming matter and energy according to the laws of thermodynamics. These reactions are organized into metabolic pathways, each catalyzed by specific enzymes.

• Metabolism: The sum of all chemical reactions in the body, involving reactants and products.

• Metabolic Pathway: A series of interconnected reactions, each step catalyzed by a unique enzyme, starting with a reactant and ending with a product.

• Anabolic Pathways: Build complex molecules from simpler ones; require energy (e.g., dehydration synthesis).

• Catabolic Pathways: Break down complex molecules into simpler ones; release energy (e.g., hydrolysis).

• Metabolic reactions are often reversible.

Forms of Energy:

• Energy: The capacity to do work or cause change.

• Kinetic Energy: Energy of motion; includes thermal energy (random movement of atoms/molecules).

• Potential Energy: Stored energy based on position or structure; chemical energy is potential energy stored in chemical bonds.

• Energy can be transformed from one form to another, but cannot be created or destroyed.

Thermodynamics: The study of energy transformations.

• First Law: Energy conservation – energy can be transferred and transformed, but not created or destroyed.

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

• Entropy: A measure of disorder or randomness; dispersal of energy increases entropy.

• Release of heat during energy transformations increases entropy.

• Diffusion across a membrane is explained by the second law: substances move from ordered to disordered states.

Biological Order and Disorder:

• Cells create order from disorder, but overall entropy of the universe increases.

• Evolution of complexity does not violate the second law; local decreases in entropy are offset by increases elsewhere.

Concept 8.2: Free Energy Change

The change in free energy (ΔG) determines whether a reaction is spontaneous. Free energy is the energy available to do work in a system.

• Free Energy (ΔG): The energy available to perform work; calculated as:

• Exergonic Reactions: ΔG < 0; release energy, are spontaneous.

• Endergonic Reactions: ΔG > 0; require energy input, are non-spontaneous.

• Cellular Respiration: Glucose and oxygen react to form CO2 and H2O; this is a spontaneous, exergonic process.

• Reactions in closed systems reach equilibrium and do no work; cells are open systems, constantly exchanging materials and never at equilibrium.

Concept 8.3: ATP and Coupled Reactions

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

• ATP Structure: Composed of adenine, ribose, and three phosphate groups.

• High Potential Energy: The phosphate bonds are unstable and release energy when hydrolyzed.

• ATP Hydrolysis: Releases energy for cellular processes.

• Coupled Reactions: Energy from exergonic reactions is used to synthesize ATP; ATP hydrolysis then drives endergonic reactions.

Functions of ATP:

• Chemical Work: Synthesis of macromolecules.

• Transport Work: Pumping substances across membranes (active transport).

• Mechanical Work: Muscle contraction, movement of flagella.

Example: ATP is used to move ions against their concentration gradient in active transport.

Concept 8.4: Enzymes

Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy barrier, without affecting the overall free energy change (ΔG).

• Enzymes: Proteins that catalyze specific reactions.

• Catalyst: Substance that increases reaction rate without being consumed.

• Activation Energy (EA): The initial energy required to start a reaction.

• Enzymes lower EA, allowing reactions to proceed faster.

• Each enzyme is specific to its substrate.

Enzyme-Substrate Complex:

• Substrate: The reactant an enzyme acts upon.

• Active Site: Region on the enzyme where substrate binds.

• Induced Fit: Enzyme changes shape for optimal substrate binding.

Catalysis in the Active Site:

• Enzyme can orient substrates, strain bonds, provide a favorable environment, or form temporary covalent bonds.

Degradation vs. Synthesis:

• Degradation: Enzyme breaks substrate into products.

• Synthesis: Enzyme joins substrates into a single product.

Factors Affecting Enzyme Activity:

• Substrate Concentration: Higher concentration increases activity up to a saturation point.

• Temperature: Activity increases with temperature, but extreme heat denatures enzymes.

• pH: Each enzyme has an optimal pH; deviations reduce activity.

Enzyme Cofactors:

• Cofactors: Non-protein helpers; can be inorganic (metal ions) or organic (coenzymes).

• Coenzymes: Organic cofactors, often derived from vitamins.

• Vitamins: Essential for coenzyme synthesis.

Concept 8.5: Regulation of Enzyme Activity

Cells regulate metabolism by controlling enzyme activity, ensuring proper function and efficiency of metabolic pathways.

• Regulation Methods: Protein synthesis (enzyme presence/amount), activation/deactivation (binding of inhibitors or activators).

• Feedback Inhibition: End product of a pathway inhibits an earlier enzyme, preventing overproduction.

Enzyme Inhibitors:

• Competitive Inhibitors: Bind to the active site, blocking substrate access.

• Noncompetitive Inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

• Examples: Poisons, pesticides, antibiotics (e.g., penicillin).

Localization of Enzymes:

• Cellular structures organize metabolic pathways; some enzymes are membrane-bound or located in specific organelles (e.g., mitochondria for cellular respiration).

Key Terms Table

Additional info: Academic context was added to clarify definitions, provide examples, and expand explanations for completeness. Allosteric regulation was omitted as instructed.