Chapter 8: Introduction to Metabolism

Overview of Metabolism

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

• Metabolic Pathway: A series of chemical reactions that either build a complex molecule (anabolic pathway) or break down a complex molecule into simpler compounds (catabolic pathway).

• Example: Cellular respiration is a catabolic pathway that breaks down glucose to produce ATP.

What is Metabolism? What is Energy?

Definitions and Concepts

• Metabolism: The totality of an organism’s chemical reactions.

• Energy: The capacity to cause change or do work. Exists in various forms, including kinetic and potential energy.

• Metabolic Pathway: Begins with a specific molecule and ends with a product, each step catalyzed by a specific enzyme.

Catabolic and Anabolic Pathways

Types of Metabolic Pathways

• Catabolic Pathways: Release energy by breaking down complex molecules into simpler compounds. Example: Cellular respiration.

• Anabolic Pathways: Consume energy to build complex molecules from simpler ones. Example: Synthesis of proteins from amino acids.

Energy released from catabolic reactions can be stored and used to drive anabolic reactions.

Forms of Energy

Kinetic and Potential Energy

• Kinetic Energy: Energy associated with motion. Example: A cyclist pedaling a bike.

• Potential Energy: Stored energy due to location or structure. Example: Water behind a dam.

• Energy can be converted from one form to another.

Laws of Thermodynamics

Energy Transformations

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

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

Gibbs Free Energy (G)

Free Energy and Spontaneity

• Gibbs Free Energy (G): 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 entropy of the universe.

• Equation:

• = change in free energy

• = change in enthalpy (total energy)

• = temperature in Kelvin

• = change in entropy

Exergonic and Endergonic Reactions

Types of Chemical Reactions

• Exergonic Reaction: Proceeds with a net release of free energy; is negative; spontaneous.

• Endergonic Reaction: Absorbs free energy from its surroundings; is positive; nonspontaneous.

Equilibrium and Metabolism

Metabolic Disequilibrium

• Cells are not in equilibrium; they are open systems experiencing a constant flow of materials.

• Metabolic pathways are regulated to prevent equilibrium, allowing cells to do work.

Cellular Work and ATP

Types of Cellular Work

• Mechanical Work: Movement of cilia, muscle contraction.

• Transport Work: Pumping substances across membranes.

• Chemical Work: Synthesis of polymers from monomers.

• Most energy coupling in cells is mediated by ATP (adenosine triphosphate).

Adenosine Triphosphate (ATP)

The Cell’s Energy Shuttle

• ATP: Composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups.

• ATP hydrolysis releases energy that can be used to drive endergonic reactions.

• Energy released: kcal/mol (under standard conditions).

ATP and Energy Coupling

How ATP Powers Cellular Work

• ATP hydrolysis is coupled to endergonic reactions via phosphorylation (transfer of a phosphate group).

• Phosphorylated intermediates are more reactive, allowing cellular work to proceed.

The ATP Cycle

ATP as a Renewable Resource

• ATP is regenerated by the addition of a phosphate group to ADP.

• Energy to phosphorylate ADP comes from catabolic reactions in the cell.

Enzymes and Metabolism

Enzyme Function

• Enzyme: A catalytic protein that speeds up chemical reactions by lowering the activation energy barrier.

• Enzymes are not consumed by the reaction.

• Each enzyme is specific to its substrate.

How Enzymes Speed Up Reactions

Activation Energy and Catalysis

• Enzymes lower the activation energy () required for a reaction to proceed.

• They do not affect the change in free energy () of the reaction.

Enzyme Active Site and Catalysis

Mechanism of Enzyme Action

• The substrate binds to the enzyme’s active site, forming an enzyme-substrate complex.

• The active site can lower activation energy by orienting substrates, straining substrate bonds, providing a favorable microenvironment, or covalently bonding to the substrate.

Enzyme Structure

Protein Conformation and Function

• Enzymes are globular proteins with a unique three-dimensional structure.

• The active site is typically a pocket or groove on the enzyme’s surface.

• Enzyme specificity results from the complementary fit between the shape of the active site and the substrate (induced fit model).