Metabolism, Thermodynamics, and Enzyme Function in General Biology

Chapter 8: Metabolism

Introduction to Metabolism

Metabolism encompasses all chemical reactions in an organism, transforming matter and energy according to the laws of thermodynamics. Understanding metabolism is essential for grasping how cells acquire, use, and regulate energy.

• Metabolic Pathways: A series of chemical reactions that begin with a specific molecule and end with a product, each step catalyzed by a specific enzyme.

  • Catabolic Pathways: "Downhill" reactions that release energy by breaking down complex molecules into simpler ones. Example: Cellular respiration breaks down glucose to produce ATP.

  • Anabolic Pathways: "Uphill" reactions that consume energy to build complex molecules from simpler ones. Example: Synthesis of proteins from amino acids.

• Forms of Energy:

  • Kinetic Energy: Energy of motion, such as a boulder rolling down a hill.

  • Thermal Energy: Random movement of atoms or molecules (heat).

  • Potential Energy: Stored energy, such as a boulder at the top of a hill before it rolls.

  • Chemical Energy: Potential energy available for release in a chemical reaction, such as food being transformed into energy to climb.

Thermodynamics and Metabolism

Thermodynamics is the study of energy transformations. It explains how energy is transferred and conserved in biological systems.

• First Law of Thermodynamics (Conservation of Energy): Energy can be transferred or transformed but cannot be created or destroyed. The total energy of the universe is constant.

• Second Law of Thermodynamics: Every energy transfer increases the disorder (entropy) of the universe. Energy conversions are never 100% efficient; some energy is lost as heat.

• Open vs. Closed Systems:

  • Open System: Living organisms exchange energy and matter with their surroundings.

  • Closed System: Isolated from surroundings; eventually reaches equilibrium and can no longer do work.

Free Energy and Spontaneous Reactions

Gibbs Free Energy (ΔG)

Gibbs free energy is used to determine if a reaction will occur spontaneously. It is calculated using the following equation:

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

• Spontaneous Reactions: Occur without input of energy and are characterized by a negative $\Delta G$.

• Nonspontaneous Reactions: Require energy input and have a positive $\Delta G$.

• Equilibrium: State where forward and reverse reactions occur at equal rates; no net change in free energy.

Exergonic and Endergonic Reactions

• Exergonic Reactions: "Energy outward"; release energy to the surroundings. $\Delta G$ is negative. Example: Cellular respiration.

• Endergonic Reactions: "Energy inward"; absorb energy from the surroundings. $\Delta G$ is positive. Example: Synthesis of glucose during photosynthesis.

Graphical Representation: Exergonic reactions show a decrease in free energy as reactants are converted to products, while endergonic reactions require an input of energy to proceed.

ATP and Energy Coupling

ATP Structure and Function

ATP (adenosine triphosphate) is the primary energy currency of the cell. It powers cellular work by coupling exergonic and endergonic reactions.

• Structure: ATP consists of adenine, ribose, and three phosphate groups.

• Energy Release: The bonds between phosphate groups are unstable and release energy when broken by hydrolysis. $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$ Example: Hydrolysis of ATP releases energy for cellular processes.

• Energy Coupling: Cells use exergonic reactions (like ATP hydrolysis) to drive endergonic reactions (energy-consuming).

• ATP Cycle: ATP is regenerated from ADP and inorganic phosphate via energy from catabolic pathways.

Enzymes: Catalyzing Metabolic Reactions

Enzyme Function

Enzymes are biological catalysts, usually proteins, that speed up chemical reactions by lowering the activation energy (EA) required.

• Substrate: The reactant an enzyme acts upon.

• Active Site: Specific region on the enzyme where the substrate binds.

• Induced Fit: The enzyme changes shape to better fit the substrate upon binding.

• Enzyme Specificity: Determined by the shape of the active site.

Activation Energy (EA)

Activation energy is the initial energy needed to start a chemical reaction. Enzymes lower EA, allowing reactions to occur more rapidly at cellular temperatures.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature for activity.

• pH: Each enzyme has an optimal pH.

• Cofactors: Non-protein helpers (inorganic ions or organic molecules) required for enzyme function.

  • Inorganic cofactors: Metal ions like zinc, iron, copper.

  • Coenzymes: Organic cofactors, often derived from vitamins.

Enzyme Regulation

Cells regulate enzyme activity through various mechanisms:

• Allosteric Regulation: Enzyme activity is regulated by molecules binding at sites other than the active site.

• Feedback Inhibition: End product of a pathway inhibits an earlier step.

• Enzyme Inhibitors: Chemicals that selectively inhibit enzyme function.

Summary Table: Exergonic vs. Endergonic Reactions

Reaction Type	Energy Change ($\Delta G$)	Spontaneity	Example
Exergonic	Negative	Spontaneous	Cellular respiration
Endergonic	Positive	Non-spontaneous	Photosynthesis

Example Calculation: Gibbs Free Energy

• Given:

  • $\Delta H = -50.0$ kJ/mol

  • $\Delta S = -0.150$ kJ/mol·K

  • $T = 300$ K

• Calculation:

  • $\Delta G = \Delta H - T\Delta S$

  • $\Delta G = -50.0 - (300 \times -0.150)$

  • $\Delta G = -50.0 + 45.0$

  • $\Delta G = -5.0$ kJ/mol

• Interpretation: Since $\Delta G$ is negative, the reaction is spontaneous at 300 K.

Key Terms

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

• ATP: Adenosine triphosphate, the main energy currency of the cell.

• Enzyme: Biological catalyst that speeds up chemical reactions.

• Gibbs Free Energy ($\Delta G$): Energy available to do work in a system.

• Activation Energy (EA): Energy required to start a reaction.

• Catabolic Pathway: Breaks down molecules, releases energy.

• Anabolic Pathway: Builds molecules, consumes energy.