Chapter 8: An Introduction to Metabolism – Study Notes

Metabolism and Thermodynamics in Biology

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 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).

• Enzymes: Biological catalysts that speed up reactions without being consumed.

The Laws of Thermodynamics and Biological Processes

Biological systems obey the laws of thermodynamics, which govern energy transformations and the direction of metabolic processes.

• 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 in Biological Systems

Types of Energy

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

• Kinetic Energy: Energy associated with motion (e.g., muscle movement).

• Thermal Energy: Kinetic energy from random movement of atoms/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 of Reactions

Free Energy Change (ΔG)

The change in free energy (ΔG) determines whether a reaction occurs spontaneously. Free energy is the portion of a system’s energy that can do work under constant temperature and pressure.

• Equation: $\Delta G = \Delta H - T \Delta S$ Where: $\Delta G$ = change in free energy $\Delta H$ = change in enthalpy (total energy) $\Delta S$ = change in entropy $T$ = temperature in Kelvin

• Spontaneous Processes: Occur when $\Delta G$ is negative; energetically favorable.

• Nonspontaneous Processes: Require energy input; $\Delta G$ is zero or positive.

Exergonic vs. Endergonic Reactions

Chemical reactions are classified by their free-energy changes:

• Exergonic Reactions: Net release of free energy; $\Delta G$ is negative; spontaneous.

• Endergonic Reactions: Absorb free energy; $\Delta G$ is positive; nonspontaneous.

ATP and Energy Coupling

ATP Structure and Function

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 Work: Driving endergonic reactions.

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

• Mechanical Work: Moving cellular structures (e.g., muscle contraction, cilia movement).

The ATP Cycle

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

Enzymes and Activation Energy

Activation Energy Barrier

Every chemical reaction requires an initial input of energy to break bonds, known as activation energy (EA).

• Enzymes: Lower the activation energy barrier, allowing reactions to occur at moderate temperatures.

• Catalysts: Speed up reactions without being consumed.

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.

• Active Site: Region on the enzyme where substrate binds.

• Induced Fit: Enzyme changes shape to better fit the substrate, facilitating the reaction.

Factors Affecting Enzyme Activity

Environmental Effects

Enzyme activity is influenced by 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.

• Coenzymes: Organic cofactors, often derived from vitamins.

Enzyme Inhibition

Enzyme inhibitors reduce enzyme activity by interfering with substrate binding or enzyme function.

• Competitive Inhibitors: Resemble the substrate and bind to the active site, blocking substrate access.

• Noncompetitive Inhibitors: Bind elsewhere on the enzyme, causing a conformational change that reduces activity.

Summary Table: Exergonic vs. Endergonic Reactions

Reaction Type	ΔG	Spontaneity	Energy Flow	Example
Exergonic	Negative	Spontaneous	Energy released	Cellular respiration
Endergonic	Positive	Nonspontaneous	Energy absorbed	Photosynthesis

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.

• Enzyme: Protein catalyst that speeds up biological reactions.

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

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

• Enzyme-Substrate Complex: Temporary association between enzyme and substrate during catalysis.

• 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.