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 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

Biological processes are governed by the laws of thermodynamics, which describe energy transformations and the tendency toward disorder.

• 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.

• 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) during a reaction determines whether it 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 without energy input; $\Delta G$ is negative.

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

Exergonic vs. Endergonic Reactions

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

• Exergonic Reaction: Proceeds with a net release of free energy ($\Delta G < 0$); spontaneous.

• Endergonic Reaction: Absorbs free energy ($\Delta G > 0$); 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.

• Mechanical Work: Moving structures within the cell (e.g., muscle contraction).

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.

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

Substrate Specificity and Enzyme Action

Enzymes are highly specific for their substrates, binding them 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 fit substrate, enhancing reaction.

Factors Affecting Enzyme Activity

Environmental Effects

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

• Optimal Temperature: Each enzyme has a temperature at which it works best; too high leads to denaturation.

• Optimal pH: Varies by enzyme; e.g., pepsin (pH 2), trypsin (pH 8).

• Cofactors: Nonprotein helpers; inorganic (metal ions) or organic (coenzymes, often vitamins).

• Inhibitors: Competitive (bind active site) or noncompetitive (bind elsewhere, change enzyme shape).

Summary Table: Exergonic vs. Endergonic Reactions

Key Terms and Definitions

• Metabolism: All chemical reactions in an organism.

• Catabolic Pathway: Breaks down molecules, releases energy.

• Anabolic Pathway: Builds molecules, consumes energy.

• Enzyme: Protein catalyst for specific reactions.

• ATP: Main energy carrier in cells.

• Activation Energy: Energy required to start a reaction.

• Free Energy (G): Energy available to do work.

• ΔG: Change in free energy.

• Entropy (S): Measure of disorder.

• Enthalpy (H): Total energy of a system.