The Chemistry of Life

Introduction

Chemistry forms the foundation of human anatomy and physiology. All structures and functions of the body are based on chemical principles, from the composition of bones to the transmission of nerve impulses. Understanding the chemical level of organization is essential for comprehending higher levels of biological structure and function.

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

• Atoms are composed of three subatomic particles:

  • Protons: Positively charged, located in the nucleus.

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

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

• Atoms are electrically neutral when the number of protons equals the number of electrons.

• Electron shells:

  • 1st shell: holds up to 2 electrons

  • 2nd shell: holds up to 8 electrons

  • 3rd shell: holds up to 18 electrons (satisfied with 8 for most biological atoms)

Elements in the Human Body

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

• The periodic table organizes elements by increasing atomic number and recurring properties.

• Four major elements make up 96% of body mass:

  • Oxygen (O) – 65%

  • Carbon (C) – 18%

  • Hydrogen (H) – 10%

  • Nitrogen (N) – 3%

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

• Trace elements (0.01%): Iron, Copper, Iodine, Zinc, and others.

Isotopes and Radioactivity

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

• 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).

Nuclear Medicine Applications

• Cancer radiation therapy uses radioisotopes to destroy cancer cells.

• Radiotracers help visualize organ function and structure.

• Iodine-131 treats thyroid disorders by targeting thyroid cells.

Matter Combined: Mixtures and Chemical Bonds

Mixtures

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

• Three types of mixtures:

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

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

  • Solutions: Solute dissolved in solvent, particles not visible, do not settle (e.g., glucose in water).

Chemical Bonds

• Chemical bond: Attractive force holding atoms together in molecules or compounds.

• Valence electrons: Electrons in the outermost shell; involved in bonding.

• Octet Rule: Atoms are most stable with 8 electrons in their valence shell (except for small atoms, which follow the duet rule—2 electrons).

Ionic Bonds

• Formed when electrons are transferred from a metal to a nonmetal.

• Results in charged particles called ions:

  • Cation: Positively charged (lost electrons).

  • Anion: Negatively charged (gained electrons).

• Ionic compounds are often called salts.

Covalent Bonds

• Formed when two or more nonmetals share electrons.

• Strongest type of chemical bond.

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

Types of Covalent Bonds

• Nonpolar covalent bond: Electrons are shared equally (e.g., O2, H2).

• Polar covalent bond: Electrons are shared unequally, creating partial charges (e.g., H2O).

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

Hydrogen Bonds

• Weak attractions between partially positive hydrogen atoms and partially negative atoms (often oxygen or nitrogen) in polar molecules.

• Responsible for properties like surface tension in water.

Determining Bond Types

• Ionic: Metal + nonmetal

• Nonpolar: Identical nonmetals, or mostly C and H

• Polar: Two nonmetals with different electronegativities

Chemical Notation and Reactions

Chemical Equations

• Chemical reaction: Bonds are formed, broken, or rearranged; electrons may be transferred.

• Reactants: Starting substances (left side).

• Products: Substances formed (right side).

• Reversible reactions:

• Irreversible reactions:

Energy in Chemical Reactions

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

• Forms: Chemical, electrical, mechanical.

• 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 large molecules; generally exergonic.

• Anabolic reactions: Build new molecules; generally endergonic.

• Exchange reactions: Atoms or electrons are exchanged between reactants.

• Redox reactions: Involve electron transfer; oxidation (loss of electrons), reduction (gain of electrons).

Reaction Rates and Enzymes

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

• Enzymes: Biological catalysts (usually proteins) that lower activation energy and increase reaction rates without being consumed.

• Enzymes are highly specific for their substrates and reactions.

• Enzyme action follows the "induced fit" model: substrate binds to active site, enzyme changes shape, reaction occurs, products are released.

Enzyme Deficiencies

• Tay-Sachs Disease: Deficiency of hexosaminidase; leads to fatal accumulation of lipids in brain cells.

• SCIDS: Deficiency of adenosine deaminase; severely compromised immune system.

• Phenylketonuria: Deficiency of phenylalanine hydroxylase; can cause intellectual disability if untreated.

Inorganic Compounds: Water, Acids, Bases, and Salts

Water

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

• Properties:

  • Absorbs heat without significant temperature change.

  • Carries heat when evaporating.

  • Cushions and lubricates body structures.

  • Primary solvent for hydrophilic substances.

• Hydrophilic: Dissolves in water (polar or charged).

• Hydrophobic: Does not dissolve in water (nonpolar).

Acids and Bases

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

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

• Water can dissociate:

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

Buffers

• Resist changes in pH; consist of a weak acid and its conjugate base.

• Major buffer in blood: carbonic acid–bicarbonate system.

• Blood pH must remain between 7.35 and 7.45; deviations cause acidosis or alkalosis.

Salts and Electrolytes

• Salt: Compound formed from a metal cation and a nonmetal anion (ionic bond).

• Electrolytes: Ions released when salts dissolve in water; conduct electricity and are 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 main organic compounds in the body: 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 C, H, O (1:2:1 ratio); polar

• and hydrophilic.

• Functions: Fuel, structural roles, cell recognition (glycoproteins/glycolipids).

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

• Disaccharides: Two monosaccharides (sucrose, lactose).

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

Lipids

• Composed of C, H, and some O; nonpolar and hydrophobic.

• Functions: Energy storage, cell membranes, hormones.

• Fatty acids: Saturated (no double bonds), monounsaturated (one double bond), polyunsaturated (multiple double bonds).

• Triglycerides: Three fatty acids + glycerol; main storage form of fat.

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

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

Proteins

• Composed of C, H, O, N, and sometimes S; may be polar or nonpolar.

• Functions: Structure, enzymes, defense, communication, movement, fuel.

• Amino acids: 21 types; central carbon, amino group, carboxyl group, R group.

• Peptide bonds: Link amino acids via dehydration synthesis.

• Levels of structure:

  • Primary: Sequence of amino acids.

  • 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 C, H, O, N, P; include DNA and RNA.

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

• ATP: Main energy currency of the cell; formed 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.