Basic Chemistry: Part II

Introduction

This module focuses on the fundamental chemical principles relevant to Anatomy & Physiology, with an emphasis on inorganic molecules, water, acids, bases, salts, the pH scale, and buffers. Understanding these concepts is essential for comprehending physiological processes at the molecular level.

Inorganic and Organic Molecules

Classification of Chemical Compounds

• Inorganic Compounds: Molecules that generally do not contain both carbon and hydrogen atoms together. Examples include water (H2O), carbon dioxide (CO2), oxygen (O2), and inorganic acids, bases, and salts.

• Organic Compounds: More complex molecules composed primarily of carbon and hydrogen atoms (C-H bonds). Major classes include carbohydrates, proteins, lipids, and nucleic acids (to be discussed in detail in a later module).

Water: Physiological Importance

Role of Water in the Body

• The human body is composed of approximately 70–90% water.

• Even small changes in water content can significantly affect physiological functions.

• Water is a polar molecule, meaning it has a partial positive charge on one side (hydrogen) and a partial negative charge on the other (oxygen), due to the electronegativity of oxygen.

• Hydrogen bonds form between water molecules, giving water unique properties essential for life.

Additional info: The polarity of water allows it to interact with various solutes, making it an excellent medium for biochemical reactions.

Structure of Water Molecule

• Water consists of two hydrogen atoms covalently bonded to one oxygen atom.

• Oxygen is more electronegative, pulling shared electrons closer and creating a partial negative charge on the oxygen and partial positive charges on the hydrogens.

• This unequal sharing leads to the formation of hydrogen bonds between adjacent water molecules.

Properties of Water

Key Properties

• Essential Reactant: Water participates directly in many chemical reactions, such as hydrolysis (breaking molecules using water) and dehydration synthesis (forming molecules by removing water).

• High Heat Capacity: Water can absorb and retain large amounts of heat with only a slight change in its own temperature, helping to stabilize body temperature.

• High Heat of Vaporization: A significant amount of energy is required to convert water from liquid to gas, which is important for cooling mechanisms like sweating.

• Excellent Solvent: Water dissolves a wide variety of substances (solutes), facilitating chemical reactions and transport of materials in the body.

• Dissociation/Ionization: Ionic compounds (e.g., salts) dissociate into ions in water, a process essential for the function of electrolytes.

Role of Water Molecules in Solution

Dissociation of Ionic Compounds

• Water molecules surround and separate the positive and negative ions of a solute (e.g., NaCl), a process called hydration.

• This allows substances like salts to dissolve and form electrolytes, which are crucial for nerve and muscle function.

Example: When NaCl dissolves in water, it dissociates into Na+ and Cl- ions, each surrounded by water molecules.

Acids, Bases, and Salts

Definitions and Examples

• Acids: Substances that release hydrogen ions (H+) when dissolved in water. Example: Hydrochloric acid (HCl) dissociates completely:

• Bases: Substances that release hydroxide ions (OH-) or accept H+ ions. Example: Sodium hydroxide (NaOH) dissociates completely:

• Salts: Ionic compounds formed when an acid reacts with a base. Example:

Additional info: Acids and bases can be classified as strong (complete dissociation) or weak (partial dissociation).

Electrolytes

• Electrolytes are inorganic compounds that dissociate into ions in solution, enabling electrical conductivity.

• Important physiological electrolytes include Na+, K+, Ca2+, Cl-, and HCO3- (bicarbonate).

pH Scale

Definition and Calculation

• pH is a measure of the hydrogen ion concentration [H+] in a solution.

• The pH scale ranges from 0 to 14:

  • pH = 7 is neutral (equal H+ and OH- concentrations)

  • pH < 7 is acidic (more H+ than OH-)

  • pH > 7 is basic (alkaline) (more OH- than H+)

• pH is calculated as:

• The pH scale is logarithmic: a change of 1 pH unit represents a tenfold change in [H+].

Example: A solution with pH 5 has 10 times more H+ ions than a solution with pH 6.

Buffers and pH Regulation

Function of Buffers

• Buffers are substances that minimize changes in pH by absorbing or releasing H+ ions as needed.

• They are essential for maintaining physiological pH, especially in blood and cellular fluids.

• Most buffers consist of a weak acid and its conjugate base, which can reversibly combine with H+.

Bicarbonate Buffer System (Human Blood)

• The main buffer system in human blood is the bicarbonate buffer system:

• Key reaction:

• If [H+] increases (pH drops), the buffer absorbs H+ to form carbonic acid.

• If [H+] decreases (pH rises), the buffer releases H+ from carbonic acid.

Additional info: Proper buffer function is vital for homeostasis and normal organ function.

Summary Table: Properties of Water