Chapter 2: The Chemical Context of Life

Introduction

Understanding the chemical basis of life is essential for studying biology. All living organisms are composed of matter, which is organized into elements and compounds. The properties and interactions of these chemical substances underlie all biological processes.

Elements and Compounds

Elements: Basic Chemical Substances

• Element: A substance that cannot be broken down into other substances by chemical reactions.

• Each element is defined by its atomic number, which is the number of protons in its nucleus.

• There are 92 naturally occurring elements, but only about 20-25% are essential for life.

Compounds: Combinations of Elements

• Compound: A substance consisting of two or more elements combined in a fixed ratio.

• Compounds have emergent properties that are different from those of their constituent elements.

• Example: Sodium (Na, a metal) and chlorine (Cl, a poisonous gas) combine to form sodium chloride (NaCl, table salt), which is edible and essential for life.

Elements of Life

• Four elements—oxygen (O), carbon (C), hydrogen (H), and nitrogen (N)—make up about 96% of living matter.

• Other essential elements include calcium (Ca), phosphorus (P), potassium (K), and sulfur (S).

• Trace elements are required in minute quantities (e.g., iron, iodine, zinc).

Atomic Structure and Properties

Atoms: The Building Blocks of Matter

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

• Composed of subatomic particles: protons (positive charge), neutrons (no charge), and electrons (negative charge).

• Protons and neutrons form the atomic nucleus; electrons form a "cloud" around the nucleus.

• Proton and neutron mass are nearly identical and measured in daltons.

Atomic Number and Mass

• Atomic number (Z): Number of protons in the nucleus; defines the element.

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

• Atomic mass: Total mass of the atom, approximately equal to the mass number.

Isotopes

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

• Some isotopes are stable; others are radioactive and decay over time.

Electron Arrangement and Chemical Behavior

Energy Levels and Electron Shells

• Energy: The capacity to cause change.

• Potential energy: Energy that matter possesses due to its location or structure.

• Electrons have different potential energies depending on their distance from the nucleus.

• Electrons are arranged in shells with characteristic energy levels.

Valence Electrons and Chemical Properties

• Valence electrons: Electrons in the outermost shell (valence shell).

• The number of valence electrons determines an atom's chemical behavior.

• Atoms with a full valence shell are chemically inert (e.g., noble gases).

Electron Orbitals

• Orbital: A three-dimensional space where an electron is found 90% of the time.

• Each electron shell contains a specific number of orbitals.

• No more than two electrons can occupy a single orbital.

Chemical Bonds and Molecular Formation

Chemical Bonds

• Atoms with incomplete valence shells can share or transfer electrons, forming chemical bonds.

• Major types of chemical bonds: covalent bonds and ionic bonds.

Covalent Bonds

• Covalent bond: Sharing of a pair of valence electrons by two atoms.

• Single covalent bond: Sharing of one pair of electrons (e.g., H—H).

• Double covalent bond: Sharing of two pairs of electrons (e.g., O=O).

• Molecule: Two or more atoms held together by covalent bonds.

• Valence: The bonding capacity of an atom, usually equal to the number of unpaired electrons in the valence shell.

Electronegativity and Polarity

• Electronegativity: The attraction of an atom for the electrons in a covalent bond.

• Nonpolar covalent bond: Electrons are shared equally between atoms.

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

Ionic Bonds

• Formed when one atom strips electrons from another, creating ions.

• Cation: Positively charged ion (lost electrons).

• Anion: Negatively charged ion (gained electrons).

• Oppositely charged ions attract each other, forming an ionic bond.

• Compounds formed by ionic bonds are called ionic compounds or salts (e.g., NaCl).

Chemical Reactions

Making and Breaking Bonds

• Chemical reaction: The making and breaking of chemical bonds, leading to changes in the composition of matter.

• Reactants: Starting materials in a chemical reaction.

• Products: Resulting materials from a chemical reaction.

• Example:

• Chemical reactions are reversible; products can become reactants in the reverse reaction.

Photosynthesis: A Key Biological Reaction

• Photosynthesis converts carbon dioxide and water into glucose and oxygen using sunlight.

• Equation:

Summary Table: Types of Chemical Bonds

Key Terms

• Element

• Compound

• Atom

• Proton

• Neutron

• Electron

• Atomic number

• Mass number

• Isotope

• Covalent bond

• Ionic bond

• Molecule

• Valence

• Electronegativity

• Reactant

• Product

Example Application: The formic acid produced by ants is a compound whose properties depend on the types of atoms present and how they are bonded. The number of protons in each atom determines the element, while the arrangement of electrons determines bonding and reactivity.

Additional info: Hydrogen bonds and van der Waals interactions, though weaker than covalent and ionic bonds, play crucial roles in the structure and function of biological molecules and will be discussed in subsequent chapters.