Chapter 8: An Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions that occur within living organisms to maintain life. These reactions manage the material and energy resources of the cell and are organized into metabolic pathways.

• Metabolic Pathways: Series of chemical reactions where the product of one reaction serves as the substrate for the next.

• Anabolic Pathways: Build complex molecules from simpler ones; require energy input (e.g., synthesis of proteins from amino acids).

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

Thermodynamics in Biological Systems

Thermodynamics is the study of energy transformations. The laws of thermodynamics apply to biological systems and help explain how organisms manage energy.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed from one form to another.

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

• Energy Types: Kinetic energy (energy of motion) vs. Potential energy (stored energy due to position or structure).

• Example: Water behind a dam has potential energy; glucose has chemical potential energy.

Free Energy and Spontaneous Processes

Free energy (Gibbs free energy, G) is the portion of a system's energy that can perform work when temperature and pressure are uniform.

• Spontaneous Process: Occurs without energy input; increases the stability of a system.

• Equation:

• Exergonic Reaction: Releases energy; is negative; spontaneous (e.g., cellular respiration).

• Endergonic Reaction: Requires energy input; is positive; non-spontaneous (e.g., photosynthesis).

• Example: Cellular respiration is exergonic; photosynthesis is endergonic.

ATP and Energy Coupling

Adenosine triphosphate (ATP) is the cell's energy currency. It powers cellular work by coupling exergonic reactions to endergonic reactions.

• Structure of ATP: Adenine (nitrogenous base), ribose (sugar), and three phosphate groups.

• Hydrolysis of ATP:

• Energy Coupling: The use of exergonic processes (like ATP hydrolysis) to drive endergonic ones.

• Types of Cellular Work:

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

  • Transport work: Pumping substances across membranes.

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

Enzymes and Metabolic Reactions

Enzymes are biological catalysts that speed up metabolic reactions by lowering activation energy barriers.

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

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

• Effect of Enzymes: Lower but do not affect .

• Enzyme Structure: Globular proteins with a specific active site where substrates bind.

• Induced Fit: The enzyme changes shape slightly to fit the substrate more snugly.

• Enzyme Specificity: Determined by the shape and chemical properties of the active site.

Mechanisms of Enzyme Action

Enzymes use several mechanisms to lower activation energy and facilitate reactions.

• Orienting substrates correctly

• Straining substrate bonds

• Providing a favorable microenvironment

• Directly participating in the reaction

Factors Affecting Enzyme Activity

Enzyme activity can be influenced by several factors:

• Substrate concentration: Higher concentration increases reaction rate up to a point.

• pH: Each enzyme has an optimal pH; extremes can denature the enzyme.

• Temperature: Each enzyme has an optimal temperature; high temperatures can denature proteins.

Enzyme Regulation and Inhibition

Cells regulate enzyme activity to control metabolism. Inhibitors can decrease enzyme activity.

• Competitive Inhibitors: Bind to the active site, competing with the substrate.

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

• Allosteric Regulation: Regulatory molecules bind to a site other than the active site, affecting enzyme function.

• Allosteric Activator: Increases enzyme activity.

• Allosteric Inhibitor: Decreases enzyme activity.

• Cooperativity: Substrate binding to one active site affects binding at other sites (e.g., hemoglobin).

Cofactors and Coenzymes

Many enzymes require non-protein helpers for activity.

• Cofactors: Inorganic ions (e.g., Mg2+, Zn2+).

• Coenzymes: Organic molecules (e.g., vitamins, NAD+).

Summary Table: Types of Enzyme Inhibition

Key Terms and Definitions

• Metabolism: All chemical reactions in an organism.

• Enzyme: Protein catalyst that speeds up reactions.

• Substrate: Reactant acted upon by an enzyme.

• Active Site: Region on enzyme where substrate binds.

• Product: Substance formed from substrate after reaction.

• Induced Fit: Enzyme changes shape to fit substrate.

• Activation Energy (): Energy needed to start a reaction.

• Allosteric Regulation: Control of enzyme activity by binding at a site other than the active site.

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