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 resulting from the orderly interactions between molecules.

• Metabolic Pathways: A series of chemical reactions where a specific molecule is altered stepwise to produce a final 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.

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.

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).

• 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: Where: = change in free energy = change in enthalpy (total energy) = change in entropy = temperature in Kelvin

• Spontaneous Reactions: Occur when ΔG is negative; energetically favorable.

• Nonspontaneous Reactions: Occur when ΔG is zero or positive; require energy input.

Exergonic vs. Endergonic Reactions

Chemical reactions are classified by their free-energy changes:

• Exergonic Reactions: Net release of free energy; ΔG is negative; spontaneous.

• Endergonic Reactions: Absorb free energy; ΔG is positive; nonspontaneous.

ATP and Energy Coupling

Structure and Function of ATP

ATP (adenosine triphosphate) is the primary energy currency of the cell, composed of ribose, adenine, and three phosphate groups.

• ATP Hydrolysis: 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 Powers Cellular Work

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 within the cell (e.g., cilia, muscle contraction).

The ATP Cycle

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

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 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 upon.

• Active Site: The region of the enzyme where substrate binding and catalysis occur.

• Induced Fit: Enzyme changes shape for optimal substrate binding.

Factors Affecting Enzyme Activity

Enzyme activity is influenced by environmental conditions (temperature, pH) and by cofactors and inhibitors.

• Optimal Conditions: Each enzyme has an optimal temperature and pH for activity.

• Cofactors: Nonprotein helpers (inorganic or organic) required for enzyme function.

• Enzyme Inhibitors: Competitive inhibitors bind to the active site; noncompetitive inhibitors bind elsewhere, altering enzyme shape.

Summary Table: Exergonic vs. Endergonic Reactions

Key Terms and Definitions

• Metabolism: The sum of all chemical reactions in an organism.

• Catabolic Pathway: Pathway that breaks down molecules and releases energy.

• Anabolic Pathway: Pathway that builds molecules and consumes energy.

• ATP: Adenosine triphosphate, the main energy carrier in cells.

• Enzyme: Protein catalyst that speeds up biochemical reactions.

• Activation Energy (EA): The energy required to initiate a reaction.

• Free Energy (G): Energy available to do work in a system.

• ΔG: Change in free energy during a reaction.

• Entropy (S): Measure of disorder or randomness.

• Exergonic Reaction: Reaction that releases energy.

• Endergonic Reaction: Reaction that absorbs energy.

• Cofactor: Nonprotein molecule required for enzyme activity.

• Competitive Inhibitor: Molecule that competes with substrate for active site.

• Noncompetitive Inhibitor: Molecule that binds elsewhere on enzyme, altering its function.