Chapter 2: The Chemistry of Life

Introduction

The study of chemistry is fundamental to understanding human anatomy and physiology. All structures and functions in the body are based on chemical principles, from the composition of bones to the processes that sustain life. This chapter explores the chemical building blocks of the human body, the types of chemical bonds, and the major classes of biological molecules.

Atoms and Elements

Basic Definitions

• Matter: Anything that has mass and occupies space.

• Chemistry: The study of matter and its interactions.

• Atom: The smallest unit of matter that retains the properties of an element.

• Element: A substance composed of one type of atom; cannot be broken down by chemical means.

Atomic Structure

• Protons: Positively charged particles in the atomic nucleus.

• Neutrons: Uncharged particles, slightly larger than protons, also in the nucleus.

• Electrons: Negatively charged particles that orbit the nucleus in electron shells.

Elements in the Human Body

• Elements are defined by their atomic number (number of protons).

• The Periodic Table organizes elements by atomic number and recurring properties.

• Major elements in the body: Oxygen (65%), Carbon (18%), Hydrogen (10%), Nitrogen (3%).

• Mineral elements (less than 4%): Sodium, Potassium, Calcium, Chlorine, Magnesium, Phosphorus, Sulfur.

• Trace elements (less than 1%): Iron, Copper, Iodine, Zinc, and others.

Isotopes and Radioactivity

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

• Radioisotopes: Unstable isotopes that emit radiation; used in medical imaging and cancer therapy.

• Example: Hydrogen has three isotopes—protium (no neutrons), deuterium (1 neutron), tritium (2 neutrons).

Matter Combined: Mixtures and Chemical Bonds

Mixtures

• Mixture: Physical combination of two or more substances; components retain their properties and can be separated physically.

• Molecule: Chemical combination of two or more atoms; properties differ from original atoms; separation requires chemical means.

Types of Mixtures

• Suspensions: Large particles, visible, settle out (e.g., blood).

• Colloids: Small particles, not visible, do not settle (e.g., milk).

• Solutions: Solute dissolves in solvent, appears translucent (e.g., glucose in water).

Chemical Bonds

• Chemical Bond: An energy relationship between atoms involving valence electrons.

• Valence Electrons: Electrons in the outermost shell; involved in bond formation.

• Molecule: Two or more atoms of the same element bonded.

• Compound: Two or more atoms of different elements bonded.

• Macromolecule: Very large compound made of many atoms.

Ions and Ionic Bonds

• Ionic Bond: Formed when electrons are transferred from a metal to a nonmetal, creating charged ions.

• Cation: Positively charged ion (lost electrons).

• Anion: Negatively charged ion (gained electrons).

• Attraction between cations and anions forms salts.

Covalent Bonds

• Covalent Bond: Electrons are shared between two or more nonmetal atoms; strongest type of bond.

• Single, double, or triple bonds can form depending on the number of shared electron pairs.

Types of Covalent Bonds

• Nonpolar Covalent Bond: Electrons shared equally (e.g., O2, H2).

• Polar Covalent Bond: Electrons shared unequally, creating partial charges (e.g., H2O).

• Dipole: Molecule with partially positive and negative ends due to polar bonds.

Hydrogen Bonds

• Hydrogen Bond: Weak attraction between partially positive hydrogen and partially negative atoms in polar molecules.

• Responsible for properties like surface tension in water.

Chemical Notation and Reactions

Chemical Reactions

• Chemical Reaction: Occurs when bonds are formed, broken, or rearranged, or electrons are transferred.

• Chemical Equation: Shows reactants (left) and products (right).

• Reversible Reactions: Can proceed in both directions (⇌).

• Irreversible Reactions: Proceed in one direction (→).

Energy in Chemical Reactions

• Energy: Capacity to do work; can be potential (stored) or kinetic (in motion).

• Chemical Energy: Stored in bonds; drives cellular processes.

• Endergonic Reactions: Require energy input; products have more energy than reactants.

• Exergonic Reactions: Release energy; products have less energy than reactants.

Types of Chemical Reactions in the Body

• Catabolic Reactions: Break down larger substances into smaller ones; generally exergonic.

• Exchange Reactions: Atoms or electrons are transferred between reactants; includes oxidation-reduction (redox) reactions.

• Anabolic Reactions: Build new chemical bonds; generally endergonic.

Reaction Rates and Enzymes

• Activation Energy: Minimum energy required for a reaction to occur.

• Catalyst: Substance that lowers activation energy; in biology, these are enzymes.

• Enzymes: Biological catalysts, usually proteins, that speed up reactions without being consumed.

• Enzymes are highly specific, have an active site, and use an "induced fit" mechanism.

Inorganic Compounds: Water, Acids, Bases, and Salts

Water

• Makes up 50-65% of body mass; vital for life.

• Absorbs heat, carries heat, cushions, protects, and lubricates body structures.

• Solvent: Dissolves hydrophilic (water-loving) substances; does not dissolve hydrophobic (water-hating) substances.

Acids and Bases

• Acid: Proton (hydrogen ion) donor; increases H+ in solution.

• Base: Proton acceptor; decreases H+ in solution.

• pH Scale: Measures hydrogen ion concentration; 7 is neutral, below 7 is acidic, above 7 is basic.

• Buffers: Resist changes in pH; major buffer in blood is the carbonic acid–bicarbonate system.

Salts and Electrolytes

• Salt: Compound formed from a metal cation and a nonmetal anion held by ionic bonds.

• Electrolytes: Ions in solution that conduct electricity; essential for nerve and muscle function.

Organic Compounds

Hydrocarbons

• Organic compounds containing only carbon and hydrogen; form chains and rings that are the backbone of all organic molecules.

Monomers and Polymers

• Four major organic compounds: Carbohydrates, Lipids, Proteins, Nucleic Acids.

• Monomer: Single subunit; Polymer: Many monomers linked together.

• Dehydration Synthesis: Links monomers by removing water.

• Hydrolysis: Breaks polymers by adding water.

Carbohydrates

• Composed of carbon, hydrogen, and oxygen (1:2:1 ratio); polar and hydrophilic.

• Monosaccharides: Simple sugars (glucose, fructose, galactose, ribose, deoxyribose).

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

• Polysaccharides: Long chains (starch in plants, glycogen in animals).

• Glycoproteins/Glycolipids: Carbohydrates covalently bonded to proteins/lipids; important for cell recognition.

Lipids

• Composed of carbon, hydrogen, and some oxygen; nonpolar and hydrophobic.

• Fatty Acids: Hydrocarbon chains; can be saturated (no double bonds), monounsaturated (one double bond), or polyunsaturated (multiple double bonds).

• Triglycerides: Three fatty acids linked to glycerol; energy storage.

• Phospholipids: Glycerol, two fatty acids, and a phosphate group; amphiphilic; main component of cell membranes.

• Steroids: Four-ring structure; includes cholesterol, bile acids, and hormones.

Proteins

• Composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur; may be polar or nonpolar.

• Amino Acids: 21 types; central carbon, amino group, carboxyl group, hydrogen, and R group.

• Peptide Bonds: Link amino acids via dehydration synthesis.

• Protein Structure:

  • Primary: Amino acid sequence.

  • Secondary: Alpha helix or beta-pleated sheet (hydrogen bonds).

  • Tertiary: 3D folding (R group interactions).

  • Quaternary: Multiple polypeptide chains.

• Denaturation: Loss of structure and function due to heat, pH, or chemicals.

Nucleotides and Nucleic Acids

• Composed of carbon, hydrogen, oxygen, nitrogen, and phosphorus.

• Nucleotide: Nitrogenous base (purine or pyrimidine), five-carbon sugar, phosphate group.

• ATP (Adenosine Triphosphate): Main energy currency of the cell; produced from ADP and phosphate.

• DNA: Double helix, deoxyribose sugar, bases A, T, G, C; stores genetic code.

• RNA: Single strand, ribose sugar, bases A, U, G, C; involved in protein synthesis.

Summary Table: Major Organic Compounds