Chapter 8: An Introduction to Metabolism – Study Notes

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Metabolism and Thermodynamics in Biology

Overview of Metabolism

Metabolism refers to the totality of an organism’s chemical reactions, encompassing both the breakdown and synthesis of molecules. It is an emergent property of life, arising from the orderly interactions between molecules within cells.

Metabolic Pathways: A series of chemical reactions in which a specific molecule is altered stepwise to produce a product. Each step is catalyzed by a specific enzyme.
Catabolic Pathways: Release energy by breaking down complex molecules into simpler compounds (e.g., cellular respiration).
Anabolic Pathways: Consume energy to build complex molecules from simpler ones (e.g., protein synthesis).
Energy Coupling: Cells manage energy by coupling exergonic (energy-releasing) and endergonic (energy-consuming) reactions.

Diagram of catabolic and anabolic pathways Linear metabolic pathway with enzymes

The Laws of Thermodynamics

Biological processes are governed by the laws of thermodynamics, which describe how energy is transferred and transformed in living systems.

First Law (Conservation of Energy): Energy can be transferred and transformed, but not created or destroyed.
Second Law (Entropy): Every energy transfer increases the entropy (disorder) of the universe. Some energy is lost as heat and becomes unavailable to do work.

How the laws of thermodynamics relate to biological processes Bear illustrating first and second law of thermodynamics

Forms of Energy in Biological Systems

Types of Energy

Energy is the capacity to cause change and exists in various forms relevant to biological processes.

Kinetic Energy: Energy associated with motion (e.g., moving water, muscle contraction).
Thermal Energy: Kinetic energy due to random movement of atoms or molecules; transfer is called heat.
Potential Energy: Energy due to location or structure (e.g., water behind a dam, arrangement of electrons in bonds).
Chemical Energy: Potential energy available for release in a chemical reaction (e.g., glucose breakdown).

Diver illustrating conversion between kinetic and potential energy

Free Energy and Spontaneity of Reactions

Free Energy Change (ΔG)

The change in free energy (ΔG) during a reaction determines whether the reaction occurs spontaneously.

Formula: $\Delta G = \Delta H - T \Delta S$ Where: $\Delta G$ = change in free energy $\Delta H$ = change in enthalpy (total energy) $\Delta S$ = change in entropy $T$ = temperature in Kelvin
Spontaneous Reactions: Occur when $\Delta G$ is negative; energetically favorable.
Nonspontaneous Reactions: Occur when $\Delta G$ is zero or positive; require energy input.

Relationship of free energy to stability and spontaneous change

Exergonic vs. Endergonic Reactions

Chemical reactions are classified based on their free-energy changes.

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.

Exergonic and endergonic reactions Exergonic reaction energy diagram Endergonic reaction energy diagram

ATP and Energy Coupling

Structure and Function of ATP

ATP (adenosine triphosphate) is the cell’s primary energy currency, composed of ribose, adenine, and three phosphate groups. It powers cellular work by coupling exergonic and endergonic reactions.

Hydrolysis of ATP: Energy is released when the terminal phosphate bond is broken by hydrolysis.
Phosphorylation: Transfer of a phosphate group from ATP to another molecule, making it more reactive.
ATP Cycle: ATP is regenerated by addition of a phosphate group to ADP, using energy from catabolic reactions.

Structure of ATP Hydrolysis of ATP ATP drives chemical work via energy coupling ATP cycle

Cellular Work Powered by ATP

Cells use ATP to perform three main types of work:

Chemical Work: Driving endergonic reactions (e.g., synthesis of macromolecules).
Transport Work: Pumping substances across membranes against concentration gradients.
Mechanical Work: Moving structures within the cell (e.g., muscle contraction, cilia movement).

ATP powers transport and mechanical work

Enzymes and Metabolic Regulation

Activation Energy and Catalysis

Every chemical reaction requires an initial input of energy, called activation energy (EA), to break bonds in reactants. Enzymes act as catalysts, lowering the activation energy barrier and speeding up reactions without being consumed.

Enzyme Function: Enzymes lower EA, allowing reactions to occur at moderate temperatures.
Enzyme Specificity: Each enzyme is specific to its substrate, binding at the active site.
Induced Fit Model: The enzyme changes shape to fit the substrate, enhancing catalysis.

Effect of enzyme on activation energy Sucrase catalyzed hydrolysis of sucrose Active site and catalytic cycle of an enzyme Induced fit between enzyme and substrate

Factors Affecting Enzyme Activity

Enzyme activity is influenced by environmental conditions and specific chemicals.

Temperature: Each enzyme has an optimal temperature; activity decreases beyond this point due to denaturation.
pH: Each enzyme has an optimal pH, often reflecting its typical environment (e.g., pepsin in stomach, trypsin in intestine).
Cofactors: Nonprotein helpers (inorganic or organic) required for enzyme function. Organic cofactors are called coenzymes (e.g., vitamins).
Inhibitors: Chemicals that reduce enzyme activity. Competitive inhibitors bind to the active site, while noncompetitive inhibitors bind elsewhere, altering enzyme shape.

Factor	Effect on Enzyme Activity
Temperature	Optimal range increases activity; extreme heat denatures enzyme
pH	Optimal pH maximizes activity; deviation reduces activity
Cofactors	Required for proper function; absence reduces activity
Inhibitors	Competitive: block active site; Noncompetitive: alter enzyme shape

Environmental factors affecting enzyme activity Competitive inhibition of enzyme activity Noncompetitive inhibition of enzyme activity

Summary Table: Exergonic vs. Endergonic Reactions

Type of Reaction	ΔG	Spontaneity	Energy Flow
Exergonic	Negative	Spontaneous	Energy released
Endergonic	Positive	Nonspontaneous	Energy absorbed

Key Terms and Definitions

Metabolism: All chemical reactions in an organism.
Catabolic Pathway: Breaks down molecules, releases energy.
Anabolic Pathway: Builds molecules, consumes energy.
ATP: Adenosine triphosphate, main energy carrier.
Enzyme: Protein catalyst for biochemical reactions.
Activation Energy (EA): Energy required to start a reaction.
Free Energy (G): Energy available to do work.
ΔG: Change in free energy during a reaction.
Entropy (S): Measure of disorder.
Exergonic Reaction: Releases energy, spontaneous.
Endergonic Reaction: Requires energy, nonspontaneous.
Cofactor: Nonprotein helper for enzyme activity.
Competitive Inhibitor: Blocks active site.
Noncompetitive Inhibitor: Alters enzyme shape.