Study Notes: Introduction to Metabolism

Chapter 8: An Introduction to Metabolism

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

Metabolism encompasses all chemical reactions that occur within a living organism, enabling it to maintain life, grow, and reproduce. These reactions are organized into metabolic pathways, which can be classified as either catabolic or anabolic.

• 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).

• Kinetic Energy: The energy of motion; associated with moving objects.

• Potential Energy: Stored energy due to an object's position or structure (e.g., chemical bonds).

• 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.

Example: During cellular respiration, glucose is broken down, releasing energy that is used to form ATP.

Concept 8.2: The Free-Energy Change of a Reaction Tells Us Whether or Not the Reaction Occurs Spontaneously

The direction and spontaneity of chemical reactions are determined by changes in free energy, entropy, and enthalpy.

• Entropy (S): A measure of disorder or randomness in a system.

• Enthalpy (H): The total heat content of a system.

• Gibbs Free Energy (G): The energy available to do work in a system at constant temperature and pressure.

• Gibbs Free Energy Equation:

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

• Exergonic Reactions: Release free energy; $\Delta G$ is negative; occur spontaneously.

• Endergonic Reactions: Absorb free energy; $\Delta G$ is positive; non-spontaneous.

Example: Hydrolysis of ATP is an exergonic reaction that releases energy for cellular work.

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

ATP (adenosine triphosphate) acts as the main energy currency in cells, coupling energy-releasing (exergonic) processes to energy-consuming (endergonic) ones.

• Role of ATP: Provides energy for cellular processes by transferring a phosphate group to other molecules (phosphorylation).

• Chemical Coupling: The energy from exergonic reactions (like ATP hydrolysis) is used to drive endergonic reactions.

• Phosphorylation: The addition of a phosphate group to a molecule, often changing its shape and function.

• Energy Level Changes: A large change in free energy occurs when ATP is hydrolyzed, making it an effective energy carrier.

Example: The sodium-potassium pump uses ATP to transport ions against their concentration gradients.

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 required for the reaction to proceed.

• Enzyme Function: Enzymes bind to substrates, facilitating the conversion to products without being consumed in the reaction.

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

• Enzyme-Substrate Complex: The temporary association between an enzyme and its substrate(s).

• Transition State: The high-energy state during a reaction when bonds are breaking and forming.

• Enzyme Specificity: Determined by the shape of the enzyme's active site and the substrate.

• Induced Fit: The enzyme changes shape slightly to better fit the substrate upon binding.

• Enzyme Inhibitors: Molecules that decrease enzyme activity. They can be competitive (bind to the active site) or noncompetitive (bind elsewhere, changing enzyme shape).

Example: The enzyme 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 control metabolic pathways and respond to changing conditions.

• Metabolic Pathway Regulation: Ensures that resources are used efficiently and prevents the buildup of unnecessary products.

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

• Cooperativity: A form of allosteric regulation where the binding of a substrate to one active site affects binding at other sites (common in multimeric enzymes).

Example: Feedback inhibition, where the end product of a pathway inhibits an early enzyme in the pathway, is a common regulatory mechanism.