Metabolism and Energy Transformations

Concept 8.1: An Organism’s Metabolism Transforms Matter and Energy

Metabolism encompasses all chemical reactions within an organism, arising from the coordinated interactions of molecules. These reactions are organized into metabolic pathways, each step catalyzed by a specific enzyme, allowing cells to efficiently manage energy and matter.

• Metabolic Pathways: Series of chemical reactions that convert a starting molecule to a product through defined steps, each catalyzed by a specific enzyme.

• Enzymes: Biological macromolecules (usually proteins) that accelerate chemical reactions and regulate metabolic pathways.

• Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy (exergonic, spontaneous, ΔG < 0). Example: Cellular respiration (glucose + O2 → CO2 + H2O).

• Anabolic Pathways: Build complex molecules from simpler ones, consuming energy. Examples: Synthesis of amino acids, proteins.

• Energy Flow: Energy released from catabolic pathways is used to drive anabolic pathways.

• Bioenergetics: The study of how energy flows through living organisms.

Forms of Energy

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

• Kinetic Energy: Energy of motion (e.g., moving objects, thermal energy).

• Thermal Energy: Kinetic energy due to random movement of atoms/molecules; transferred as heat.

• Light Energy: Powers processes like photosynthesis.

• Potential Energy: Stored energy due to position or structure (e.g., water behind a dam, energy in chemical bonds).

• Chemical Energy: Potential energy available for release in chemical reactions (e.g., glucose breakdown).

The Laws of Energy Transformation (Thermodynamics)

• Thermodynamics: Study of energy transformations.

• System: Matter under study; Surroundings: Everything else.

• Isolated System: No exchange of energy/matter with surroundings (e.g., liquid in a thermos).

• Open System: Energy/matter can be exchanged (e.g., living organisms).

• First Law of Thermodynamics: Energy can be transferred and transformed, but not created or destroyed (conservation of energy).

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

• Entropy (S): Measure of disorder/randomness. Spontaneous processes increase entropy.

• Spontaneous Process: Occurs without energy input, increases entropy (e.g., diffusion, water flowing downhill).

• Nonspontaneous Process: Requires energy input, decreases entropy (e.g., pumping water uphill).

• Biological Order and Disorder: Living systems increase the entropy of their surroundings by converting organized matter/energy into less ordered forms (e.g., CO2 and H2O from food).

Example: Diffusion across a membrane increases entropy as molecules spread out evenly.

Concept 8.2: Free-Energy Change, ΔG

Gibbs free energy (G) quantifies the portion of a system's energy available to do work at constant temperature and pressure. The change in free energy (ΔG) determines whether a process is spontaneous.

• Gibbs Free Energy Equation:

• ΔH: Change in enthalpy (total energy).

• ΔS: Change in entropy.

• T: Absolute temperature (Kelvin).

• Spontaneous Process: ΔG < 0 (negative); system loses free energy, becomes more stable.

• Nonspontaneous Process: ΔG ≥ 0; requires energy input.

• Equilibrium: Lowest possible free energy; no net change; cannot do work.

• Exergonic Reaction: Releases free energy (ΔG negative); spontaneous. Example: Cellular respiration.

• Endergonic Reaction: Absorbs free energy (ΔG positive); nonspontaneous.

• Metabolic Equilibrium: Cells avoid equilibrium by being open systems; continuous flow of materials prevents equilibrium and allows work.

Example: Cellular respiration is exergonic and spontaneous; energy released is used to make ATP.

Concept 8.3: ATP Powers Cellular Work by Coupling Exergonic and Endergonic Reactions

ATP (adenosine triphosphate) is the main energy currency of the cell, mediating energy coupling between exergonic and endergonic reactions.

• Types of Cellular Work:

  • Chemical Work: Driving endergonic reactions (e.g., polymer synthesis).

  • Transport Work: Pumping substances across membranes against gradients.

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

• ATP Structure: Ribose sugar, adenine base, three phosphate groups.

• ATP Hydrolysis: ATP + H2O → ADP + Pi + energy

• Energy released under cellular conditions is about -13 kcal/mol.

• Phosphorylation: ATP transfers a phosphate group to another molecule, making it more reactive (higher free energy).

• Energy Coupling: Exergonic ATP hydrolysis drives endergonic reactions.

• ATP Cycle: ATP is regenerated from ADP and Pi using energy from catabolic reactions (e.g., cellular respiration).

Example: Synthesis of glutamine from glutamic acid and ammonia is driven by ATP hydrolysis.

Concept 8.4: Enzymes Speed Up Metabolic Reactions by Lowering Energy Barriers

Enzymes are biological catalysts that accelerate chemical reactions by lowering the activation energy (EA), enabling life-sustaining metabolic processes to occur rapidly and efficiently.

• Activation Energy (EA): Initial energy required to start a reaction; brings reactants to the transition state.

• Enzyme Function: Lower EA without changing ΔG; do not get consumed in the reaction.

• Substrate: The reactant an enzyme acts on.

• Active Site: Region on the enzyme where the substrate binds; specificity due to 3D structure.

• Induced Fit: Enzyme changes shape to fit substrate more closely, enhancing catalysis.

• Catalytic Cycle: Substrate binds → enzyme-substrate complex → product formation → product release.

• Factors Affecting Enzyme Activity: Substrate concentration, enzyme concentration, temperature, pH, cofactors, inhibitors.

• Cofactors: Non-protein helpers (inorganic ions or organic coenzymes, often vitamins).

• Enzyme Inhibitors:

  • Competitive Inhibitors: Compete with substrate for active site; effect reduced by increasing substrate concentration.

  • Noncompetitive Inhibitors: Bind elsewhere, change enzyme shape, reduce activity.

Example: Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Concept 8.5: Regulation of Enzyme Activity Helps Control Metabolism

Cells regulate enzyme activity to coordinate metabolic pathways and maintain homeostasis, preventing wasteful or chaotic chemical activity.

• Regulation Methods:

  • Switching genes encoding enzymes on/off (genetic regulation).

  • Regulating enzyme activity after production (allosteric regulation, feedback inhibition).

• Allosteric Regulation: Regulatory molecules bind to sites other than the active site, stabilizing active or inactive enzyme forms.

• 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 early enzyme, preventing overproduction.

• Enzyme Localization: Enzymes may be organized in complexes, attached to membranes, or compartmentalized within organelles for efficiency and regulation.

Example: Isoleucine synthesis from threonine is regulated by feedback inhibition; isoleucine inhibits the first enzyme in its pathway.

Additional Topic: Combustion Reaction (Related to Cellular Respiration)

A combustion reaction is a chemical process where a fuel (often a hydrocarbon) reacts with oxygen, releasing energy as heat and light, and forming oxidized products. This is analogous to cellular respiration, where organic molecules are oxidized to release energy.

General Equation for Hydrocarbon Combustion:

Example: Cellular respiration is a controlled biological combustion of glucose.

Summary Table: Types of Metabolic Pathways

Summary Table: Types of Enzyme Inhibition

Key Equations

• Gibbs Free Energy:

• ATP Hydrolysis:

• Combustion (General):

Additional info: Some explanations and examples were expanded for clarity and completeness, including the analogy between combustion and cellular respiration, and the inclusion of summary tables for metabolic pathways and enzyme inhibition.