Energy, Metabolism, and Thermodynamics in Biological Systems

An Organism’s Metabolism Transforms Matter and Energy

Overview of Metabolism

• Metabolism is the totality of an organism's chemical reactions.

• It is an emergent property resulting from orderly molecular interactions.

• A metabolic pathway begins with a specific molecule and ends with a product, with each step catalyzed by a specific enzyme.

Example of a Metabolic Pathway

• Starting molecule (A) is converted to product (D) through intermediates (B, C) by enzymes 1, 2, and 3, respectively.

Metabolic Pathways

Catabolic and Anabolic Pathways

• Catabolic pathways release energy by breaking down complex molecules into simpler compounds.

  • Example: Cellular respiration breaks down glucose into carbon dioxide and water, releasing energy.

• Anabolic pathways (biosynthetic pathways) consume energy to build complex molecules from simpler ones.

  • Example: Protein synthesis from amino acids.

Bioenergetics

• Bioenergetics is the study of how energy flows through living organisms and is fundamental to all metabolic processes.

Forms of Energy

Types of Energy

• Energy is the capacity to cause change.

• Forms of energy include:

  • Kinetic energy: Energy of motion.

  • Thermal energy: Kinetic energy associated with random movement of atoms or molecules; heat is thermal energy in transfer.

  • Light energy: Can be harnessed to perform work (e.g., photosynthesis).

  • Potential energy: Energy that matter possesses due to its position or structure (e.g., a spring, sugar molecule).

  • Chemical energy: Potential energy available for release in a chemical reaction.

• Energy can be converted from one form to another (e.g., chemical to potential energy).

The Laws of Energy Transformation

Thermodynamics

• Thermodynamics is the study of energy transformations.

• In an open system, energy and matter can be exchanged with the surroundings (organisms are open systems).

• In an isolated system, exchange with the surroundings does not occur.

• Two laws of thermodynamics govern energy transformations.

The First Law of Thermodynamics

• The first law of thermodynamics (principle of conservation of energy):

  • Energy can be transferred and transformed, but it cannot be created or destroyed.

• Example: Light energy from the sun is transformed by plants into chemical energy, which is then transferred through food chains and can be converted to light (bioluminescence) or heat.

The Second Law of Thermodynamics

• The second law of thermodynamics:

  • Every energy transfer or transformation increases the entropy (disorder) of the universe.

• Entropy is a measure of molecular disorder or randomness.

• Energy transfers increase entropy because some energy is always lost as heat, increasing disorder.

• Spontaneous processes occur without energy input and increase entropy (e.g., rusting of iron).

• Nonspontaneous processes decrease entropy and require energy input (e.g., synthesis of proteins from amino acids).

Biological Order and Disorder

• Cells and organisms create order from less organized materials (anabolism) but also increase disorder elsewhere.

• Energy flows into ecosystems as light and exits as heat.

• The evolution of complex organisms does not violate the second law because total entropy of the universe still increases.

• Organisms are islands of low entropy in an increasingly random universe.

The Free-Energy Change of a Reaction

Gibbs Free Energy

• Free energy (G) is the portion of a system's energy that can do work at constant temperature and pressure.

• Gibbs free energy equation: $\Delta G = \Delta H - T\Delta S$

  • $\Delta G$ = change in free energy

  • $\Delta H$ = change in enthalpy (total energy)

  • $T$ = temperature (Kelvin)

  • $\Delta S$ = change in entropy (disorder)

Spontaneous and Non-Spontaneous Reactions

• Spontaneous reactions (exergonic): $\Delta G < 0$ (negative), release energy, occur without input.

  • Example: Cellular respiration (breakdown of glucose).

• Non-spontaneous reactions (endergonic): $\Delta G > 0$ (positive), require energy input.

  • Example: Photosynthesis (energy from sunlight drives the reaction).

Examples of Free-Energy Change

• Respiration Example:

  • Given: $\Delta H = -686$ kcal/mol, $\Delta S = +0.18$ kcal/mol·K, $T = 298$ K

  • Calculation: $\Delta G = -686 - (298 \times 0.18) = -739.64$ kcal/mol

  • Interpretation: $\Delta G$ is negative, so the reaction is exergonic and spontaneous.

• Photosynthesis Example:

  • Given: $\Delta H = +686$ kcal/mol, $\Delta S = -0.18$ kcal/mol·K, $T = 298$ K

  • Calculation: $\Delta G = 686 - (298 \times -0.18) = 739.64$ kcal/mol

  • Interpretation: $\Delta G$ is positive, so the reaction is endergonic and non-spontaneous.

Free Energy, Stability, and Work Capacity

• Systems with higher free energy are less stable and have greater work capacity.

• Spontaneous changes decrease free energy ($\Delta G < 0$), making systems more stable and releasing energy for work.

Exergonic and Endergonic Reactions in Metabolism

Definitions and Examples

• Exergonic reaction: Net release of free energy, spontaneous ($\Delta G$ negative).

  • Example: Cellular respiration (glucose + O2 → CO2 + H2O + energy).

• Endergonic reaction: Absorbs free energy, nonspontaneous ($\Delta G$ positive).

  • Example: Photosynthesis (CO2 + H2O + energy → glucose + O2).

Free Energy Changes in Reactions

Type of Reaction	Reactants	Products	Energy Change
Exergonic	High free energy	Low free energy	Energy released ($\Delta G < 0$)
Endergonic	Low free energy	High free energy	Energy required ($\Delta G > 0$)

ATP and Energy Coupling

ATP Powers Cellular Work

• ATP mediates energy coupling in cells, linking exergonic and endergonic reactions.

• Cells use energy coupling to do three main kinds of work:

  • Chemical work: Pushing endergonic reactions.

  • Transport work: Pumping substances across membranes against gradients.

  • Mechanical work: Beating cilia, muscle contraction.

Energy Coupling Example: Sodium-Potassium Pump

• The sodium-potassium (Na+/K+) pump actively transports Na+ out and K+ into the cell, maintaining the electrochemical gradient.

• This endergonic process is coupled with the exergonic hydrolysis of ATP.

The Structure and Hydrolysis of ATP

• ATP is composed of ribose (sugar), adenine (nitrogenous base), and three phosphate groups.

• The bonds between phosphate groups can be broken by hydrolysis, releasing energy and producing ADP and inorganic phosphate.

• The energy released comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves.

• The triphosphate tail of ATP is like a compressed spring due to repulsive forces between phosphate groups.

How ATP Drives Chemical Work

• ATP hydrolysis can be coupled to endergonic reactions, such as the synthesis of glutamine from glutamic acid and ammonia.

Step	Reaction	ΔG (kcal/mol)
1	Glutamic acid + NH3 → Glutamine	+3.4
2	ATP hydrolysis	-7.3
Net	Coupled reaction	-3.9

• Net ΔG is negative, so the coupled reaction is spontaneous.

Summary Table: Key Terms and Concepts

Term	Definition
Metabolism	Totality of an organism's chemical reactions
Catabolic pathway	Breaks down molecules, releases energy
Anabolic pathway	Builds molecules, consumes energy
Bioenergetics	Study of energy flow in living organisms
Entropy	Measure of disorder
Exergonic reaction	Releases energy, spontaneous
Endergonic reaction	Requires energy, nonspontaneous
ATP	Main energy currency of the cell

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