Chapter 2: Chemistry Comes Alive

Introduction

Chemistry is fundamental to understanding physiological processes in the human body. All body functions, from movement to digestion, are governed by chemical and biochemical reactions. This chapter introduces the essential chemical principles that underpin anatomy and physiology.

2.1 Matter and Energy

Matter

• Matter is anything that has mass and occupies space. It can be seen, smelled, or felt.

• States of Matter:

  • Solid: Definite shape and volume.

  • Liquid: Changeable shape, definite volume.

  • Gas: Changeable shape and volume.

Energy

• Energy is the capacity to do work or put matter into motion.

• Exists in two forms:

  • Kinetic Energy: Energy in action.

  • Potential Energy: Stored energy.

• Forms of energy:

  • Chemical: Stored in bonds of substances.

  • Electrical: Movement of charged particles.

  • Mechanical: Directly moves matter.

  • Radiant/Electromagnetic: Travels in waves (e.g., light, X-rays).

• Energy conversions are inefficient; some energy is always lost as heat.

2.2 Atoms and Elements

Elements

• Elements are substances that cannot be broken down by ordinary chemical means.

• Four elements (C, O, H, N) make up 96% of the human body.

• Each element is represented by a one- or two-letter symbol (e.g., O for oxygen).

Atoms

• Atoms are the smallest units of elements, retaining their properties.

• Composed of three subatomic particles:

  • Protons: Positive charge, 1 amu.

  • Neutrons: No charge, 1 amu.

  • Electrons: Negative charge, virtually no mass.

Atomic Structure

• Protons and neutrons are in the nucleus; electrons orbit the nucleus.

• Atoms are electrically neutral (equal protons and electrons).

• Models:

  • Planetary Model: Electrons in fixed orbits (simplified).

  • Orbital Model: Electrons in regions of probability (electron cloud).

Isotopes and Atomic Weight

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Atomic Weight: Average of mass numbers of all isotopes.

2.3 Combining Matter

Molecules and Compounds

• Molecule: Two or more atoms bonded together.

• Compound: Molecule with two or more different atoms.

Mixtures

• Most matter exists as mixtures (physical combinations of substances).

• Three types:

  • Solutions: Homogeneous, solute particles do not settle out.

  • Colloids: Heterogeneous, larger particles, do not settle out.

  • Suspensions: Heterogeneous, large particles, settle out.

2.4 Chemical Bonds

Role of Electrons in Bonding

• Electrons occupy energy levels (shells) around the nucleus.

• The outermost shell (valence shell) determines chemical reactivity.

• Octet Rule: Atoms tend to gain, lose, or share electrons to achieve 8 in their valence shell (except H and He).

Types of Chemical Bonds

• Ionic Bonds: Transfer of electrons from one atom to another, forming ions (cations and anions).

• Covalent Bonds: Sharing of electrons between atoms. Can be single, double, or triple bonds.

• Hydrogen Bonds: Weak attractions between electropositive hydrogen and electronegative atoms (e.g., between water molecules).

Polar and Nonpolar Covalent Bonds

• Nonpolar: Equal sharing of electrons (e.g., O2, CO2).

• Polar: Unequal sharing, resulting in partial charges (e.g., H2O).

2.5 Chemical Reactions

Chemical Equations

• Represent the formation, rearrangement, or breaking of chemical bonds.

• General format: Reactants → Products

Types of Chemical Reactions

• Synthesis (Combination): A + B → AB (anabolic, building)

• Decomposition: AB → A + B (catabolic, breaking down)

• Exchange (Displacement): AB + C → AC + B (both synthesis and decomposition)

• Redox (Oxidation-Reduction): Involves electron transfer; essential in metabolism.

Energy Flow in Reactions

• Exergonic: Release energy (products have less energy than reactants).

• Endergonic: Absorb energy (products have more energy than reactants).

Factors Affecting Reaction Rate

• Temperature (higher = faster)

• Concentration (higher = faster)

• Particle size (smaller = faster)

• Catalysts (e.g., enzymes) increase rate without being consumed.

2.6 Inorganic Compounds

Water

• Makes up 60–80% of cell volume; most abundant inorganic compound.

• Properties:

  • High heat capacity and vaporization

  • Polar solvent (dissolves ionic substances)

  • Reactivity (in hydrolysis and dehydration reactions)

  • Cushioning (protects organs)

Salts

• Ionic compounds that dissociate in water to form electrolytes (conduct electricity).

• Vital for nerve impulse transmission, muscle contraction, etc.

Acids and Bases

• Acids: Proton donors (release H+).

• Bases: Proton acceptors (release OH−).

• pH Scale: Measures H+ concentration (0–14; 7 is neutral).

• Buffers: Resist changes in pH, crucial for homeostasis.

2.7 Organic Compounds

General Features

• Contain carbon (except CO2 and CO).

• Major classes: carbohydrates, lipids, proteins, nucleic acids.

• Often polymers (chains of monomers).

• Formed by dehydration synthesis; broken by hydrolysis.

2.8 Carbohydrates

• Include sugars and starches; contain C, H, O (2:1 H:O ratio).

• Three classes:

  • Monosaccharides: Simple sugars (e.g., glucose, ribose).

  • Disaccharides: Two monosaccharides joined (e.g., sucrose, lactose).

  • Polysaccharides: Many monosaccharides (e.g., starch, glycogen).

2.9 Lipids

• Contain C, H, O (less O than carbohydrates); insoluble in water.

• Main types:

  • Triglycerides: Energy storage, insulation, protection.

  • Phospholipids: Major component of cell membranes.

  • Steroids: Cholesterol, hormones, vitamin D.

  • Eicosanoids: Signaling molecules (e.g., prostaglandins).

2.10 Proteins

• Composed of amino acids (20 types); contain C, H, O, N, sometimes S and P.

• Functions: structure, enzymes, transport, movement, immunity.

• Levels of structure:

  • Primary: Amino acid sequence.

  • Secondary: Alpha helices, beta sheets.

  • Tertiary: 3D folding.

  • Quaternary: Multiple polypeptides.

• Denaturation: Loss of structure and function due to pH or temperature changes.

• Enzymes: Biological catalysts, lower activation energy, highly specific.

2.11 Nucleic Acids

• Composed of nucleotides (sugar, phosphate, nitrogen base).

• Two types:

  • DNA: Double helix, genetic blueprint.

  • RNA: Single strand, protein synthesis.

2.12 ATP (Adenosine Triphosphate)

• Main energy currency of the cell.

• Energy released by breaking phosphate bonds powers cellular work.

• ATP → ADP + Pi (inorganic phosphate) + energy

Additional info: This chapter provides the chemical foundation necessary for understanding all physiological processes discussed in later chapters of Anatomy & Physiology.