chapter 8

Chapter 8: An Introduction to Metabolism

Overview

  • Discusses metabolic pathways, energy transformations, and enzyme functionality.

Thermodynamics and Biological Processes

Laws of Thermodynamics

  • First Law: Energy can be transformed or transferred but not created or destroyed.

  • Second Law: Every energy transfer increases the universe's entropy.

  • Light energy from the sun is transformed into chemical energy in plants, termites, and click beetle larvae, demonstrating energy transformation.

Metabolism

  • Defined as the totality of an organism's chemical reactions.

  • Metabolic Pathway: Sequence of chemical reactions where specific enzymes alter specific molecules to produce products.

Types of Pathways

Catabolic Pathways

  • Described as "downhill" reactions, releasing energy.

Anabolic Pathways

  • Described as "uphill" reactions, requiring energy.

  • Energy from catabolic reactions powers anabolic pathways.

Bioenergetics

  • The study of how energy flows through living organisms.

Forms of Energy

  • Types:

    • Kinetic (motion)

    • Thermal (heat)

    • Potential (stored)

    • Chemical (available for release in reactions)

  • Energy transformations (e.g., kinetic energy of muscle movement changing to potential energy while climbing).

Energy Transformation Principles

First Law of Thermodynamics

  • Energy of the universe is constant; it can be transformed but not created or destroyed.

Second Law of Thermodynamics

  • Energy transfers increase entropy; living organisms create disorder in their surroundings via metabolism.

Free Energy and Metabolism

Free Energy (

G)

  • Free energy is the portion of a system's energy that can do work (constant temperature and pressure).

  • Related to temperature, enthalpy ( H), and entropy ( S): ΔG = ΔH - TΔS.

  • Negative ΔG indicates a spontaneous process; positive ΔG indicates nonspontaneous.

Equilibrium and Cell Function

  • Equilibrium is achieved when forward and reverse reactions occur at the same rate, indicating maximum stability.

  • Living cells do not reach equilibrium; maintain metabolic flux by allowing material flow, similar to open systems.

ATP and Cellular Work

ATP's Role

  • ATP hydrolysis releases energy used to drive cellular work (chemical, transport, mechanical).

  • Energy coupling occurs when an exergonic reaction drives an endergonic reaction, typically mediated by ATP.

ATP Energy Release

  • When the terminal phosphate bond is broken, energy is released; ATP is often compared to a compressed spring due to repulsive forces in triphosphate tail.

Enzyme Functionality

Catalysis and Activation Energy

  • Enzymes speed up reactions by lowering the activation energy (EA) needed for reactions to proceed.

  • The transition state occurs when reactants are unstable; absorbing sufficient energy begins the reaction.

Enzyme Specificity

  • Enzymes bind to substrates (reactants) at their active site, forming an enzyme-substrate complex and catalyzing reaction rapidly.

Factors Affecting Enzymatic Activity

Temperature and pH

  • Each enzyme has an optimal temperature; excessive heat can denature enzymes.

  • Optimal pH varies per enzyme; examples: pepsin (pH=2) vs. trypsin (pH=8).

Cofactors

  • Nonprotein helpers needed for enzyme functionality; may be inorganic (zinc, iron) or organic (coenzymes from vitamins).

Enzyme Regulation

Inhibitors

  • Inhibitors can be competitive or noncompetitive:

    • Competitive Inhibitors: Resemble substrates and block active sites.

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

Feedback Inhibition

  • The end product of a metabolic pathway inhibits its own production, preventing waste of resources.

Localization of Enzymes

  • Enzymes may be localized within cellular compartments, forming multi-enzyme complexes to improve metabolic efficiency.