biochem lecture 2

Gibbs Free Energy (G)

Definition:

Gibbs Free Energy (G) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. It is a crucial factor in determining the favorability of chemical reactions and processes.

Spontaneous Process:

A spontaneous process is characterized by a negative change in Gibbs free energy (ΔG < 0). This is classified as an exergonic reaction, meaning it can proceed without the input of external energy. Conversely, if ΔG > 0, it is termed endergonic and signifies that the process is non-spontaneous, proceeding in the opposite direction. At equilibrium, ΔG = 0, indicating that the system is in a state of balance, and no net change occurs in the concentrations of reactants and products.

Equation:

The relationship of Gibbs Free Energy with other thermodynamic parameters is given by the equation:ΔG = ΔH - TΔSwhere:

• ΔH = Change in enthalpy, representing heat content.

• T = Absolute temperature, measured in Kelvin.

• ΔS = Change in entropy, denoting the degree of disorder or randomness in the system.

Role of Enzymes:

Enzymes enhance reaction rates by providing an alternative pathway with a lower energy transition state. They achieve this by lowering the activation energy required for the reaction. Notably, enzymes do not affect the overall change in Gibbs free energy (ΔG) of the reaction and cannot catalyze reactions that inherently have a positive ΔG, thus influencing only the rate, not the direction of the reaction.

Thermodynamics – First and Second Law:

• First Law: Energy is conserved—not created or destroyed. This principle asserts that the total energy of an isolated system remains constant, implying energy can change forms, for example, from kinetic to thermal.

• Second Law: Spontaneous processes lead to an increase in entropy (S), which quantifies the level of disorder in a system. This law suggests that natural processes tend toward a state of greater disorder and that energy transformations are not 100% efficient due to inherent energy loss as heat.

Thermodynamic Equations:

• ΔH ≈ ΔU = H_final - H_initial

  • (-ΔH) indicates an exothermic reaction, which often promotes spontaneity by releasing energy.

  • (+ΔH) indicates an endothermic reaction, which generally opposes spontaneity as it absorbs energy from the surroundings.

• ΔS = S_final - S_initial

  • A positive ΔS signifies an increase in entropy, favoring spontaneous processes; a negative ΔS opposes them, often indicating a more ordered state.

Free Energy Dependence on Concentrations:

The free energy change for a reaction in standard state is represented as ΔG°. It consists of:

• A constant term, which is unique to each reaction, indicating the intrinsic energy properties of the reactants and products involved.

• A variable term influenced by:

  • Concentrations of reactants and products, as these affect the direction and extent of the reaction.

  • Stoichiometry of the reaction, which determines how reactants and products are proportionally related.

  • Temperature, which can alter the kinetic energy of particles and subsequently the rates of reaction.

Le Chatelier’s Principle:

This principle describes how a system at equilibrium responds to changes in concentration, temperature, or pressure. A shift in equilibrium occurs in the direction that counteracts the change, thereby re-establishing equilibrium. This phenomenon is symbolized through the equilibrium constant (K_eq), which provides insight into the ratio of product to reactant concentrations at equilibrium.

Properties of Water:

Physical Properties:

• Structure: Water molecules have a bent structure, with oxygen at the center forming angles with hydrogen atoms, leading to polar characteristics.

• Solvent Role: Water is a universal solvent, capable of dissolving many organic and inorganic substances due to its polarity, making it essential for biological processes.

• Hydrophobic Effect: Water tends to clump nonpolar molecules together to minimize contact, a phenomenon critical in biological systems as it drives the formation of cell membranes and the folding of proteins.

Chemical Properties:

• Water can ionize, contributing to acid-base chemistry by acting as both an acid and a base in reactions.

• Buffers are crucial in maintaining pH levels in biological systems by resisting changes upon the addition of small amounts of acids or bases, thus ensuring homeostasis.

Hydrogen Bonds and Their Significance:

Definition:

A hydrogen bond is a weak bond formed between a hydrogen atom and an electronegative atom (such as O or N). These bonds are crucial in influencing molecular structure and behavior.

Characteristics:

• Hydrogen bonds exhibit specific geometries characterized by donor and acceptor atoms, providing orientation for molecular interactions.

• The strength of hydrogen bonds is intermediate between covalent and van der Waals forces, ensuring stability without overly rigid structures.

Biological Importance:

Hydrogen bonds are critical in stabilizing the structures of proteins and nucleic acids (DNA and RNA), significantly affecting their functions and interactions. They also influence the solubility of biologically relevant compounds, facilitating biochemical reactions.

Hydration of Molecules:

Water molecules surround polar solutes, forming hydration shells that enhance solubility. This process allows for the efficient transport of nutrients and waste in biological systems.

Hydrophobic Effect and Amphiphiles:

Molecules with both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts are called amphiphiles.

• Examples: Fatty acids and phospholipids can spontaneously form structures like micelles and bilayers, driven by the desire to minimize contact with hydrophobic regions, ultimately maximizing entropy by reducing the highly ordered water structure around them.

Diffusion and Osmosis:

• Diffusion: The movement of particles from an area of high concentration to low concentration, leading to equal concentrations throughout the system, is vital for cellular processes that require equilibrium.

• Osmosis: A specialized type of diffusion specifically referring to the movement of water across a semi-permeable membrane from areas of higher to lower water concentration, crucial for maintaining cell structure and function.

• Osmotic Pressure: This is the pressure required to halt osmosis and is directly proportional to solute concentration—understanding this concept is essential for applications in fields like medicine and biochemistry.

• Dialysis: A method utilizing diffusion and osmosis to separate small molecules from larger ones across a semi-permeable membrane, commonly used in biochemistry, particularly in kidney function and laboratory processes.