Chapter 8: An Introduction to Metabolism – Study Notes

Chapter 8: An Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all the chemical reactions that occur within a living organism, enabling it to maintain life, grow, and reproduce. These reactions are organized into metabolic pathways, each catalyzed by specific enzymes.

• Metabolism: The sum total of all chemical reactions in an organism.

• Metabolic pathway: A series of chemical reactions that either build a complex molecule (anabolic) or break down a complex molecule (catabolic), each step catalyzed by a specific enzyme.

• Example: The breakdown of glucose in cellular respiration is a catabolic pathway; the synthesis of proteins from amino acids is an anabolic pathway.

Types of Metabolic Pathways

• Catabolic pathways: "Downhill" reactions that break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways: "Uphill" reactions that build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

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

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

Forms of Energy in Biology

Energy is the capacity to cause change or do work. Living cells transform energy from one form to another to perform the work of life.

• Kinetic energy: Energy of motion (e.g., muscle movement).

• Thermal energy: Kinetic energy associated with the random movement of atoms or molecules; often released as heat.

• Light energy: Used by plants in photosynthesis.

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

• Chemical energy: Potential energy available for release in a chemical reaction (e.g., energy stored in glucose).

Thermodynamics and Biological Systems

Thermodynamics is the study of energy transformations. Biological systems are open systems, exchanging energy and matter with their surroundings.

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

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

• Living organisms increase the disorder of their surroundings through metabolism.

Table: Application of Thermodynamics in Biology

Law	Description	Biological Example
First Law	Energy is transferred/transformed, not created or destroyed	Light energy from the sun is converted to chemical energy in plants
Second Law	Entropy increases with every energy transfer	Heat is released during metabolic processes, increasing disorder

Free Energy and Spontaneity of Reactions

Free energy (Gibbs free energy, G) is the portion of a system's energy that can perform work at constant temperature and pressure. The change in free energy ($\Delta G$) determines whether a reaction is spontaneous.

• Equation: $\Delta G = \Delta H - T\Delta S$ Where: $\Delta G$ = change in free energy $\Delta H$ = change in enthalpy (total energy) $T$ = temperature in Kelvin $\Delta S$ = change in entropy

• Spontaneous reactions: $\Delta G < 0$ (negative); these reactions can perform work.

• Nonspontaneous reactions: $\Delta G \geq 0$ (zero or positive); require input of energy.

• At equilibrium, $\Delta G = 0$; no work can be done.

Exergonic and Endergonic Reactions

• Exergonic reaction: Proceeds with a net release of free energy ($\Delta G < 0$); spontaneous.

• Endergonic reaction: Absorbs free energy from surroundings ($\Delta G > 0$); nonspontaneous.

• Cells couple exergonic and endergonic reactions to drive necessary processes.

ATP and Energy Coupling

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

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

• Phosphorylation: Transfer of a phosphate group from ATP to another molecule, making it more reactive.

• ATP is regenerated from ADP and inorganic phosphate using energy from catabolic reactions.

Table: Types of Cellular Work Powered by ATP

Type of Work	Description	Example
Chemical	Pushing endergonic reactions	Synthesis of macromolecules
Transport	Pumping substances across membranes	Active transport of ions
Mechanical	Movement	Muscle contraction, cilia movement

Enzymes and Activation Energy

Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy barrier, without being consumed in the reaction.

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

• Enzymes do not change $\Delta G$; they only speed up reactions that would occur eventually.

• Substrate: The reactant an enzyme acts on.

• Active site: The region on the enzyme where the substrate binds.

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

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature; higher temperatures may denature the enzyme.

• pH: Each enzyme has an optimal pH; deviations can reduce activity or denature the enzyme.

• Cofactors: Nonprotein helpers (inorganic ions or organic coenzymes) required for enzyme activity.

Table: Examples of Enzyme Optima

Enzyme	Optimal Temperature (°C)	Optimal pH
Human enzyme	37	Varies (e.g., pepsin: 2, trypsin: 8)
Thermophilic bacteria enzyme	75	Varies

Enzyme Inhibition and Regulation

• Competitive inhibitors: Resemble the substrate and bind to the active site, blocking substrate binding; can be overcome by increasing substrate concentration.

• Noncompetitive inhibitors: Bind to another part of the enzyme, causing a shape change that reduces enzyme activity; cannot be overcome by increasing substrate concentration.

• Allosteric regulation: Regulatory molecules bind to a site other than the active site, affecting enzyme activity (can inhibit or stimulate).

• Cooperativity: Substrate binding to one active site affects binding at other sites (often in multimeric enzymes).

• Feedback inhibition: The end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction.

Organization of Enzymes in the Cell

• Enzymes may be organized into complexes, embedded in membranes, or compartmentalized within organelles to increase efficiency and regulation of metabolic pathways.

• Example: Enzymes for cellular respiration are located in the mitochondria.