Chapter 2: The Chemistry of Life

2.1 Atoms and Elements

The study of chemistry is fundamental to understanding anatomy and physiology, as all physiological processes are based on chemical interactions. Matter is anything that has mass and occupies space, and it is composed of atoms, the smallest units that retain the properties of an element.

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

• Element: A substance composed of one or more identical atoms; cannot be broken down by chemical means.

• Subatomic Particles: Atoms are made of protons (positive charge), neutrons (no charge), and electrons (negative charge).

• Atomic Nucleus: The central core containing protons and neutrons; electrons orbit in shells around the nucleus.

Key Fact: Atoms are electrically neutral when the number of protons equals the number of electrons. The number of neutrons can vary, resulting in isotopes.

2.1 Elements in the Human Body

Elements are defined by their atomic number (number of protons). Four major elements make up 96% of the human body: oxygen, carbon, hydrogen, and nitrogen. Seven mineral elements are also essential in smaller amounts.

• Major Elements: Oxygen (O), Carbon (C), Hydrogen (H), Nitrogen (N)

• Mineral Elements: Sodium (Na), Potassium (K), Calcium (Ca), Chlorine (Cl), Magnesium (Mg), Phosphorus (P), Sulfur (S)

2.1 Isotopes and Radioactivity

Isotopes are atoms of the same element with different numbers of neutrons. Some isotopes are unstable (radioisotopes) and emit radiation, which is used in medical imaging and treatments.

• Mass Number: Total number of protons and neutrons in the nucleus.

• Radioisotopes: Unstable isotopes that emit energy as radiation.

Applications: Nuclear medicine uses radioisotopes for cancer therapy and diagnostic imaging (e.g., Iodine-131 for thyroid disorders).

2.2 Chemical Bonds

Chemical Bonds and Molecules

Chemical bonds are energy relationships between atoms. Molecules are formed when two or more atoms are chemically bonded. Macromolecules are large compounds made of many atoms.

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

• Octet Rule: Atoms are most stable with 8 electrons in their valence shell.

Ions and Ionic Bonds

Ionic bonds form when electrons are transferred from one atom (usually a metal) to another (usually a nonmetal), creating charged ions. The attraction between oppositely charged ions forms ionic compounds (salts).

• Cation: Positively charged ion (loss of electron).

• Anion: Negatively charged ion (gain of electron).

Covalent Bonds

Covalent bonds are the strongest type of chemical bond, formed when two or more nonmetal atoms share electrons. Bonds can be single, double, or triple, depending on the number of shared electron pairs.

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

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

Hydrogen Bonds

Hydrogen bonds are weak attractions between the partially positive hydrogen atom of one polar molecule and the partially negative atom of another. They are crucial for the properties of water and the structure of biological molecules.

• Surface Tension: Hydrogen bonds between water molecules create surface tension, important in physiological processes.

2.3 Chemical Notation and Reactions

Chemical Reactions and Notation

Chemical reactions involve the formation, breaking, or rearrangement of chemical bonds. Chemical equations use symbols to represent reactants (starting substances) and products (resulting substances).

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

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

Reaction Rates and Enzymes

The rate of a chemical reaction is influenced by concentration, temperature, properties of reactants, and the presence of catalysts. Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Enzyme Properties: Highly specific, not consumed in the reaction, and increase reaction rates dramatically.

• Induced Fit Mechanism: The enzyme changes shape to fit the substrate, facilitating the reaction.

2.4 Inorganic Compounds: Water, Acids, Bases, and Salts

Water

Water is the primary solvent in the body, dissolving hydrophilic (water-loving) substances but not hydrophobic (water-hating) ones. Its polarity and hydrogen bonding are essential for physiological processes.

Acids and Bases

Acids are proton (hydrogen ion) donors, increasing H+ concentration in water. Bases are proton acceptors, decreasing H+ concentration. The pH scale measures hydrogen ion concentration; values below 7 are acidic, above 7 are basic, and 7 is neutral.

Buffers

Buffers are chemical systems that resist changes in pH, maintaining homeostasis. The major buffer in the body is the carbonic acid–bicarbonate system, which helps regulate blood pH.

Clinical Note: Blood pH must remain between 7.35 and 7.45; deviations can cause acidosis or alkalosis.

2.5 Organic Compounds

Monomers and Polymers

Organic compounds in the body include carbohydrates, lipids, proteins, and nucleic acids. Monomers are single subunits that combine to form polymers, which are larger molecules with diverse functions.

Carbohydrates

Carbohydrates are polar, hydrophilic molecules made of carbon, hydrogen, and oxygen (typically in a 1:2:1 ratio). They serve as fuel and have structural roles. Specialized carbohydrates, such as glycoproteins and glycolipids, are important for cell recognition and communication.

Lipids

Lipids are nonpolar, hydrophobic molecules that include fats, oils, phospholipids, and steroids. They function as energy storage, components of cell membranes, and precursors for hormones.

• Fatty Acids: Monomers of lipids.

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

• Phospholipids: Amphiphilic molecules with a hydrophilic head and hydrophobic tails; main component of cell membranes.

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

Proteins

Proteins are made of amino acids and are essential for structure, function, and regulation of the body’s tissues and organs. They can be polar or nonpolar, hydrophilic or hydrophobic. Protein denaturation (loss of shape) results in loss of function and can be caused by heat, pH changes, or chemicals.

Nucleotides and Nucleic Acids

Nucleic acids (DNA and RNA) store and transmit genetic information. They are polymers of nucleotides, each consisting of a nitrogenous base, a five-carbon sugar, and a phosphate group.

Adenosine Triphosphate (ATP)

ATP is the main energy currency of the cell, synthesized from ADP and a phosphate group. Hydrolysis of ATP releases energy for cellular work. Continuous ATP production is essential for life and requires oxygen.

Summary Table: Organic Compounds