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: 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., synthesis of proteins from amino acids).

• 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, which dictate 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.

• Biological Order: Cells create complex structures from simpler materials, increasing order locally while increasing disorder globally.

Forms of Energy

Types of Energy Relevant to Biology

Energy is the capacity to cause change and exists in various forms, each playing a role in biological processes.

• 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 molecules).

• 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) during a reaction determines whether the reaction occurs spontaneously.

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

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

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

• Stability: Systems with higher free energy are less stable and tend to become more stable (lower free energy).

Exergonic vs. Endergonic Reactions

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

• Exergonic Reaction: Net release of free energy; is negative; occurs spontaneously.

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

• Example: Cellular respiration is exergonic; photosynthesis is endergonic.

ATP and Energy Coupling

Structure and Function of ATP

ATP (adenosine triphosphate) is the primary energy currency of the cell, mediating energy coupling between exergonic and endergonic reactions.

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

• 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 in Cellular Work

Cells use ATP to perform three main types of work: chemical, transport, and mechanical.

• Chemical Work: Driving endergonic reactions (e.g., synthesis of macromolecules).

• Transport Work: Pumping substances across membranes against their concentration gradient.

• Mechanical Work: Movement, such as muscle contraction or cilia beating.

The ATP Cycle

ATP is regenerated from ADP and inorganic phosphate through energy from catabolic reactions. This cycle couples energy-yielding and energy-consuming processes.

• ATP Cycle: Shuttling of inorganic phosphate and energy between catabolism and cellular work.

Enzymes and Metabolic Regulation

Activation Energy and Catalysis

Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy barrier.

• Activation Energy (EA): The initial energy required to break bonds in reactants.

• Catalyst: A chemical agent that speeds up a reaction without being consumed.

• Enzyme: A protein that acts as a catalyst for specific reactions.

Substrate Specificity and Enzyme Action

Enzymes are highly specific for their substrates, binding 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 the substrate binds.

• Induced Fit: Enzyme changes shape to fit the substrate, enhancing catalysis.

Factors Affecting Enzyme Activity

Enzyme activity is influenced by environmental factors such as temperature, pH, and the presence of cofactors or inhibitors.

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

• Metabolism

• Catabolic Pathway

• Anabolic Pathway

• ATP (Adenosine Triphosphate)

• Activation Energy (EA)

• Enzyme

• Substrate

• Active Site

• Induced Fit

• Cofactor

• Competitive Inhibitor

• Noncompetitive Inhibitor