Enzymes, Water, Acids & Bases

Learning Outcomes

• Understand the notation and basic energetics of chemical bond formation

• Describe the factors that influence reaction rates

• Explain the properties, actions, and importance of enzymes

• Discuss the physiologically important properties of water

• Define and describe the properties of acids and bases with respect to hydrogen ions

• Explain what the pH scale measures

• Explain how a pH buffer works

Chemical Bond Formation & Energetics

Types of Chemical Energy

Chemical reactions in biological systems involve the making and breaking of chemical bonds, which are associated with energy changes. Understanding these energetics is essential for grasping how cells function.

• Chemical Energy: Potential energy stored in chemical bonds.

• Electrical Energy: Energy from the flow of ions (e.g., nerve impulses).

• Mechanical Energy: Energy directly transferred from one object to another (e.g., muscle contraction).

Endergonic vs. Exergonic Reactions

• Endergonic Reactions: Require energy input; products have more energy than reactants. Example: Photosynthesis Energy is absorbed from sunlight to build glucose molecules.

• Exergonic Reactions: Release energy; products have less energy than reactants. Example: Cellular Respiration Energy is released and used to produce ATP.

Chemical Reactions in Cells

Components of a Chemical Reaction

• Reactants: Substances that enter into a reaction.

• Intermediates: Substances formed in the middle of a reaction.

• Products: End results of a reaction.

Chemical reactions may be reversible or irreversible. For example:

• Reversible Reaction:

• Exchange Reaction:

Factors Influencing Reaction Rates

Key Factors

The rate at which chemical reactions occur in the body is influenced by several factors:

• Concentration: Higher concentration of reactants increases the likelihood of collisions and reaction rate.

• Temperature: Increasing temperature generally speeds up reactions, but excessive heat can denature proteins (including enzymes).

• Catalysts: Substances that increase reaction rate without being consumed (e.g., enzymes).

• Reactant Properties: Physical and chemical characteristics of reactants affect how easily they interact.

Enzymes: Biological Catalysts

Properties and Actions of Enzymes

Enzymes are proteins that act as biological catalysts, speeding up chemical reactions by lowering the activation energy required.

• Highly Specific: Each enzyme has a unique active site that binds specific substrates.

• Reusable: Enzymes are not consumed in the reaction and can be used repeatedly.

• Lower Activation Energy: Enzymes facilitate the making or breaking of covalent bonds.

Example: Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Factors Affecting Enzyme Activity

• Temperature: Moderate increases speed up reactions; high temperatures denature enzymes.

• Substrate Concentration: Increasing substrate increases reaction rate up to a saturation point.

• Product Concentration: High product concentration can slow reaction (negative feedback).

• Enzyme Concentration: More enzyme increases activity up to a point.

• pH: Enzymes have optimal pH ranges; extreme pH can denature enzymes. Example: Pepsin (stomach enzyme) works best at acidic pH.

Enzyme Deficiencies

Physiologically Important Properties of Water

Key Properties

Water is essential for life and has several important physiological properties:

• High Heat Capacity: Absorbs heat without significant temperature change.

• Heat of Vaporization: Carries heat away when it evaporates (e.g., sweating).

• Cushioning: Protects body structures (e.g., cerebrospinal fluid).

• Lubrication: Reduces friction between adjacent surfaces (e.g., joints).

Water as a Solvent

• Hydrophilic: Substances with charged or polar regions dissolve easily in water.

• Hydrophobic: Nonpolar substances do not dissolve well in water.

• Hydration Sphere: Water molecules surround ions and polar molecules, keeping them dissolved and allowing movement and interaction.

Acids, Bases, and pH

Properties of Acids

• Electrolytes: Acids ionize and dissociate in water.

• Proton Donors: Release hydrogen ions () into solution.

• Acidity: Determined by concentration of ions. Example:

Properties of Bases

• Electrolytes: Bases ionize and dissociate in water.

• Proton Acceptors: Accept ions; release hydroxyl ions () into solution.

• Alkalinity: Determined by concentration of ions. Example:

pH Scale

The pH scale measures the concentration of hydrogen ions in a solution, indicating its acidity or alkalinity.

• Neutral pH: Pure water has a pH of 7.

• Acidic Solutions: pH < 7; higher

• Basic (Alkaline) Solutions: pH > 7; higher

Formula:

• Higher = Lower pH (more acidic)

• Lower = Higher pH (more alkaline)

pH Buffers

Function and Importance

Buffers are molecules that prevent rapid shifts in pH, maintaining homeostasis in biological systems.

• Weak Acids and Bases: Buffers consist of weak acids and their conjugate bases.

• Mechanism: Release when pH rises; bind when pH falls.

• Example: Carbonic acid-bicarbonate buffer in blood:

Additional info:

• Mass action principle: The rate of a chemical reaction is directly proportional to the concentration of reacting substances.

• Reversible chemical reactions are common in physiology, allowing dynamic equilibrium.