The Chemistry of Living Things: Foundations for Human Biology

The Chemistry of Living Things

Introduction

This chapter introduces the essential chemical principles underlying human biology. Understanding the composition and behavior of matter, atoms, and molecules is crucial for grasping the structure and function of the human body. The following sections explore the building blocks of matter, the nature of chemical bonds, the properties of water, and the major classes of biological molecules.

Matter, Atoms, and Elements

Definition of Matter and States

• Matter is anything that has mass and occupies space. It exists in three traditional states: solids, liquids, and gases.

• All matter is composed of atoms, the smallest units that retain the properties of an element.

Structure of the Atom

• Atoms consist of a central nucleus containing protons (positively charged) and neutrons (no charge).

• Electrons (negatively charged) orbit the nucleus in defined energy levels or shells.

• The number of protons equals the number of electrons in a neutral atom.

Isotopes

• Isotopes are atoms of the same element with different numbers of neutrons.

• Stable isotopes do not decay, while unstable isotopes emit radiation as they seek stability (e.g., carbon-14).

• Unstable isotopes have applications in medicine (e.g., x-rays), archaeology (carbon dating), and sterilization.

Free Radicals

• Free radicals are atoms or molecules with unpaired electrons in their outer shell, making them highly reactive.

• They can damage cell membranes and DNA, contributing to diseases such as cancer.

• Antioxidants (e.g., vitamins C and E, selenium) neutralize free radicals by donating electrons.

Elements and the Periodic Table

• An element is a pure substance consisting of only one type of atom.

• There are about 118 known elements, organized in the Periodic Table of Elements.

• Six elements make up about 99% of the human body: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus.

Chemical Bonds

Types of Chemical Bonds

• Covalent bonds: Formed by the sharing of electrons between atoms; strong and stable.

• Hydrogen bonds: Weak attractions between a hydrogen atom and an electronegative atom (e.g., oxygen or nitrogen) in another molecule; important in water and biological molecules.

Biochemistry: Organic and Inorganic Molecules

Organic vs. Inorganic Molecules

• Organic molecules contain carbon and are typically large (e.g., carbohydrates, lipids, proteins, nucleic acids).

• Inorganic molecules do not contain carbon (with exceptions like CO2); examples include water, salts, acids, and bases.

Water and Its Importance

Properties of Water

• Water is a polar molecule, meaning it has regions of partial positive and negative charge, allowing it to form hydrogen bonds.

• It is the universal solvent, dissolving more substances than any other liquid, which is essential for biological reactions.

• About 60% of the human body is water.

Water as a Solvent

• Water dissolves ionic compounds (e.g., NaCl) by surrounding and separating their ions.

• This property is crucial for transporting nutrients and waste in the body.

Salts, Acids, and Bases

Salts and Electrolytes

• Salts are ionic compounds that dissociate into electrolytes (ions) in water, conducting electrical currents.

• Examples: sodium, potassium, calcium, and iron ions are vital for physiological functions and homeostasis.

Acids and Bases

• Acids are proton donors, releasing H+ ions in solution (e.g., HCl → H+ + Cl-).

• Bases are proton acceptors, removing H+ from solution (e.g., NaOH → Na+ + OH-).

• Acids and bases help maintain the body's pH balance.

Macromolecules: Carbohydrates, Lipids, Proteins, and Nucleic Acids

Macromolecules and Their Synthesis

• Monomers are small organic molecules; macromolecules are large molecules made of many monomers.

• Dehydration synthesis joins monomers by removing water; hydrolysis breaks macromolecules by adding water.

Carbohydrates

• Composed of carbon, hydrogen, and oxygen (C, H, O).

• Primary energy source for cells; also provide structural support.

• Types:

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

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

  • Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Lipids

• Composed of C, H, and O; insoluble in water.

• Types:

  • Triglycerides: Energy storage molecules; made of glycerol and three fatty acids. Can be saturated (straight tails) or unsaturated (kinked tails).

  • Phospholipids: Main component of cell membranes; have hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

  • Steroids: Four-ring structures; include cholesterol and hormones like estrogen and testosterone.

Proteins

• Composed of C, H, O, and N; made of amino acids (20 types).

• Essential amino acids must be obtained from the diet; non-essential amino acids can be synthesized by the body.

• Amino acids are joined by peptide bonds via dehydration synthesis to form polypeptides and proteins.

• Protein structure determines function and is organized into four levels:

  • Primary: Sequence of amino acids

  • Secondary: Alpha helices and beta sheets (hydrogen bonds)

  • Tertiary: Three-dimensional folding (disulfide and hydrogen bonds)

  • Quaternary: Association of multiple polypeptide chains

• Proteins serve as enzymes, structural components, transporters, and more.

Essential Amino Acids	Non-essential Amino Acids
Histidine	Alanine
Isoleucine	Arginine
Leucine	Asparagine
Lysine	Aspartic acid
Methionine	Cysteine
Phenylalanine	Glutamic acid
Threonine	Glutamine
Tryptophan	Glycine
Valine	Proline
	Serine
	Tyrosine

Nucleic Acids

• Store and transmit genetic information; composed of C, H, O, N, and P.

• Two main types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Basic unit is the nucleotide, consisting of a nitrogenous base, a pentose sugar, and a phosphate group.

• DNA is double-stranded and contains adenine (A), guanine (G), cytosine (C), and thymine (T).

• RNA is single-stranded and contains uracil (U) instead of thymine.

Adenosine Triphosphate (ATP)

• ATP is the universal energy carrier in cells.

• Composed of adenine, ribose, and three phosphate groups.

• Energy is released when ATP is hydrolyzed to ADP and a phosphate group.

Summary Table: Major Classes of Biological Molecules

Class	Elements	Monomer	Function
Carbohydrates	C, H, O	Monosaccharide	Energy, structure
Lipids	C, H, O	Fatty acids, glycerol	Energy storage, membranes, hormones
Proteins	C, H, O, N	Amino acid	Structure, enzymes, transport
Nucleic Acids	C, H, O, N, P	Nucleotide	Genetic information

Additional info: This guide expands on the original notes with definitions, examples, and tables for clarity and exam preparation.