Chapter 8: An Introduction to Metabolism – Study Notes

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Metabolism and Thermodynamics

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within an organism, enabling the transformation of matter and energy. It is an emergent property of life, arising from the orderly interactions between molecules.

Metabolic Pathways: A series of chemical reactions where 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 couple exergonic (energy-releasing) and endergonic (energy-consuming) reactions to efficiently manage energy resources.

How do the laws of thermodynamics relate to biological processes? Catabolic and anabolic pathways Metabolic pathway diagram

The Laws of Thermodynamics in Biology

Biological processes are governed by the laws of thermodynamics:

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.

Potential and kinetic energy in a diver Bear demonstrating first and second law of thermodynamics

Forms of Energy

Types of Energy Relevant to Cells

Energy is the capacity to cause change and exists in various forms:

Kinetic Energy: Energy associated with motion.
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).

Free Energy and Spontaneity

Free-Energy Change (ΔG)

The change in free energy (ΔG) determines whether a reaction occurs spontaneously:

ΔG 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.
Nonspontaneous Reactions: Occur when $\Delta G$ is zero or positive.

Free energy, stability, and equilibrium

Exergonic vs. Endergonic Reactions

Chemical reactions are classified by their free-energy changes:

Exergonic Reactions: Net release of free energy; $\Delta G < 0$; spontaneous.
Endergonic Reactions: Absorb free energy; $\Delta G > 0$; 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 mediates most energy coupling in cells.

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.

Structure of ATP Hydrolysis of ATP

ATP Cycle

ATP is regenerated by phosphorylation of ADP, using energy from catabolic reactions. This cycle couples energy-yielding and energy-consuming processes.

ATP cycle diagram

Cellular Work Powered by ATP

Cells use ATP to perform three main types of work:

Chemical Work: Driving endergonic reactions.
Transport Work: Pumping substances across membranes.
Mechanical Work: Moving structures like cilia or muscle cells.

ATP drives chemical work 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. Enzymes act as catalysts, lowering the activation energy barrier and speeding up reactions without being consumed.

Effect of enzyme on activation energy

Enzyme Structure and Function

Enzymes are highly specific, binding to their substrates at the active site to form an enzyme-substrate complex. The induced fit model describes how the enzyme changes shape to enhance catalysis.

Substrate: The reactant an enzyme acts on.
Active Site: The region on the enzyme where substrate binds.
Induced Fit: Enzyme changes shape for optimal substrate binding.

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:

Temperature: Each enzyme has an optimal temperature; activity decreases beyond this point due to denaturation.
pH: Each enzyme has an optimal pH, depending on its environment (e.g., pepsin in stomach, trypsin in intestine).
Cofactors: Nonprotein helpers (inorganic or organic) required for enzyme function.
Inhibitors: Chemicals that reduce enzyme activity; competitive inhibitors bind the active site, noncompetitive inhibitors bind elsewhere.

Factor	Effect on Enzyme Activity
Temperature	Optimal range increases activity; extreme heat denatures enzyme
pH	Optimal pH varies by enzyme; deviation reduces activity
Cofactors	Required for function; can be metal ions or coenzymes
Inhibitors	Competitive or noncompetitive; reduce activity

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