Energy, Metabolism, and Thermodynamics in Biological Systems

An Organism’s Metabolism Transforms Matter and Energy

Overview of Metabolism

Metabolism encompasses all chemical reactions within an organism, enabling the transformation of matter and energy necessary for life. These reactions are organized into metabolic pathways, each catalyzed by specific enzymes.

• Metabolism: The sum total of an organism’s chemical reactions.

• Emergent property resulting from orderly molecular interactions.

• Metabolic pathway: Begins with a specific molecule and ends with a product, with each step catalyzed by a unique enzyme.

Example: The conversion of a starting molecule (A) to a product (D) through intermediates (B, C), each step catalyzed by a different enzyme.

Metabolic Pathways

Catabolic and Anabolic Pathways

Metabolic pathways are classified based on whether they release or consume energy.

• 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 from amino acids).

Bioenergetics

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

Forms of Energy

Types of Energy

Energy is the capacity to cause change and exists in various forms, some of which can perform work.

• Kinetic energy: Energy associated with motion.

• Thermal energy: Kinetic energy due to 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 energy to potential energy).

The Laws of Energy Transformation

Thermodynamics

Thermodynamics is the study of energy transformations. Biological systems are considered open systems (exchange energy and matter with surroundings), while isolated systems do not exchange with their environment.

The First Law of Thermodynamics

The first law states that energy can be transferred and transformed, but it cannot be created or destroyed (principle of conservation of energy).

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

The Second Law of Thermodynamics

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

• Entropy: 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: Require energy input and decrease entropy (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 the disorder of their surroundings. The evolution of complex organisms does not violate the second law because the total entropy of the universe still increases.

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

• 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. The change in free energy (ΔG) determines whether a process is spontaneous.

• 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 Nonspontaneous Reactions

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

• Nonspontaneous (endergonic) reactions: $\Delta G > 0$ (positive); require energy input (e.g., photosynthesis).

Examples

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

$ \Delta G = -686 - (298 \times 0.18) = -739.64 \text{ kcal/mol} $

• Negative $\Delta G$ confirms respiration is exergonic and spontaneous.

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

$ \Delta G = 686 - (298 \times -0.18) = 739.64 \text{ kcal/mol} $

• Positive $\Delta G$ confirms photosynthesis is endergonic and nonspontaneous.

Free Energy, Stability, and Work Capacity

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

• Spontaneous changes decrease free energy, increase stability, and reduce work capacity.

Exergonic and Endergonic Reactions in Metabolism

Definitions and Examples

• Exergonic reaction: Net release of free energy ($\Delta G$ negative); spontaneous (e.g., cellular respiration).

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

ATP and Energy Coupling

ATP Powers Cellular Work

ATP (adenosine triphosphate) mediates energy coupling in cells, linking exergonic and endergonic reactions.

• Chemical work: Pushing endergonic reactions.

• Transport work: Pumping substances across membranes against gradients.

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

Energy Coupling Example: Sodium-Potassium Pump

The sodium-potassium pump uses the exergonic hydrolysis of ATP to drive the endergonic transport of Na+ and K+ ions across the cell membrane, maintaining cellular electrochemical gradients.

The Structure and Hydrolysis of ATP

ATP Structure

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

• Bonds between phosphate groups can be broken by hydrolysis (addition of water).

ATP Hydrolysis

• Releases energy, producing ADP (adenosine diphosphate) and inorganic phosphate ($P_i$).

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

• The triphosphate tail acts like a compressed spring due to repulsion between negatively charged 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.

Example:

• Glutamic acid + NH3 → Glutamine ($\Delta G = +3.4$ kcal/mol; endergonic)

• ATP hydrolysis ($\Delta G = -7.3$ kcal/mol; exergonic)

• Net $\Delta G = -3.9$ kcal/mol (overall reaction is exergonic and spontaneous)

ATP transfers a phosphate group to glutamic acid, forming a phosphorylated intermediate, which then reacts with ammonia to form glutamine.

Type of Reaction	$\Delta G$	Spontaneity	Example
Exergonic	Negative	Spontaneous	Cellular respiration
Endergonic	Positive	Nonspontaneous	Photosynthesis