The Chemical Basis of Life

Elements, Atoms, Isotopes, and Compounds

All matter is composed of elements, which are substances that cannot be broken down into simpler substances by chemical means. Atoms are the smallest units of elements that retain their chemical properties. Isotopes are atoms of the same element with different numbers of neutrons. Compounds are substances formed when two or more elements combine in fixed ratios.

• Element: A pure substance consisting of only one type of atom (e.g., Oxygen, Carbon).

• Atom: The basic unit of matter, composed of protons, neutrons, and electrons.

• Isotope: Atoms of the same element with different numbers of neutrons (e.g., Carbon-12 and Carbon-14).

• Compound: A substance formed from two or more elements in a fixed ratio (e.g., H2O).

• Example: Water (H2O) is a compound made from hydrogen and oxygen.

Chemical Bonds

Chemical bonds are forces that hold atoms together in compounds. The main types are covalent, ionic, and hydrogen bonds.

• Covalent Bond: Atoms share one or more pairs of electrons.

• Ionic Bond: One atom donates an electron to another, resulting in oppositely charged ions that attract each other.

• Hydrogen Bond: A weak attraction between a hydrogen atom (covalently bonded to an electronegative atom) and another electronegative atom.

• Example: Table salt (NaCl) forms via ionic bonds; water molecules are held together by hydrogen bonds.

Covalent vs. Ionic Bonds

• Covalent Bonds: Involve the sharing of electrons between atoms (e.g., H2O).

• Ionic Bonds: Involve the transfer of electrons from one atom to another, forming ions (e.g., NaCl).

Polar vs. Non-Polar Covalent Bonds

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges (e.g., water).

• Non-Polar Covalent Bond: Electrons are shared equally (e.g., O2).

• Hydrophilic: "Water-loving"; substances that dissolve in water (usually polar).

• Hydrophobic: "Water-fearing"; substances that do not dissolve in water (usually non-polar).

Hydrogen Bonds

Hydrogen bonds are weak attractions between the slightly positive hydrogen atom of one molecule and the slightly negative atom of another molecule. They are crucial in stabilizing the structures of water, proteins, and DNA.

• Example: The double helix structure of DNA is stabilized by hydrogen bonds between base pairs.

Water

Water is essential for life due to its unique properties, which arise from its polarity and ability to form hydrogen bonds.

• Cohesion: Water molecules stick together.

• Adhesion: Water molecules stick to other substances.

• High Specific Heat: Water resists temperature changes.

• Solvent Properties: Water dissolves many substances, making it the "universal solvent."

Organic Molecules and Biological Macromolecules

Isomers

Isomers are compounds with the same molecular formula but different structural arrangements, leading to different properties.

• Example: Glucose and fructose are isomers (both C6H12O6).

Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and chemical reactivity.

• Hydroxyl (-OH): Found in alcohols.

• Carboxyl (-COOH): Found in acids like amino acids and fatty acids.

• Amino (-NH2): Found in amino acids.

• Phosphate (-PO4): Found in nucleic acids.

Biological Molecules

Biological macromolecules are large molecules essential for life, including carbohydrates, nucleic acids, proteins, and lipids.

• Polymers: Large molecules made by joining many smaller units (monomers).

• Monomers: The building blocks of polymers.

Dehydration and Hydrolysis Reactions

• Dehydration Reaction: Joins two monomers by removing a water molecule.

• Hydrolysis Reaction: Breaks a polymer into monomers by adding a water molecule.

• Example: Formation of a disaccharide from two monosaccharides via dehydration; digestion of starch via hydrolysis.

Carbohydrates

Carbohydrates are sugars and polymers of sugars. They serve as energy sources and structural materials.

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

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

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

Nucleic Acids

Nucleic acids store and transmit genetic information. The two main types are DNA and RNA.

• Monomer: Nucleotide (composed of a sugar, phosphate group, and nitrogenous base).

• Polymer: DNA or RNA.

• Example: DNA encodes instructions for protein synthesis.

Proteins

Proteins are polymers of amino acids and perform a wide variety of functions, including catalysis, structure, and transport.

• Monomer: Amino acid.

• Polymer: Polypeptide (protein).

• Levels of Structure: Primary, secondary, tertiary, and quaternary.

• Example: Enzymes are proteins that speed up chemical reactions.

Lipids

Lipids are hydrophobic molecules that include fats, oils, phospholipids, and steroids. They are important for energy storage, membrane structure, and signaling.

• Fats: Composed of glycerol and fatty acids; used for long-term energy storage.

• Phospholipids: Major components of cell membranes.

• Steroids: Include hormones like cholesterol.

Summary Table: Major Classes of Biological Molecules