The Chemical Basis of Life: Study Notes

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Chapter 2 – The Chemical Basis of Life

Introduction

This chapter explores the fundamental chemical principles that underlie biological processes. Understanding the nature of matter, atomic structure, chemical bonds, and the properties of water is essential for studying life at the molecular level.

What is Matter, an Atom, an Element, and a Compound?

Matter: Anything that has mass and occupies space.
Atom: The smallest unit of an element that retains its chemical properties.
Element: A substance that cannot be broken down into simpler substances by chemical means. Each element is defined by its number of protons.
Compound: A substance formed when two or more elements are chemically combined in fixed ratios.
Example: Water (H2O) is a compound made of hydrogen and oxygen.

Trace Elements and Essential Elements

Essential Element: An element required for an organism to survive, grow, and reproduce.
Trace Element: An element required by an organism in only minute quantities (e.g., iron, iodine).

Most Abundant Elements in Living Things

The four most abundant elements in living organisms are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).
Other important elements include phosphorus (P), sulfur (S), calcium (Ca), and potassium (K).

Subatomic Particles and Their Properties

Proton: Positive charge (+1), located in the nucleus, mass ≈ 1 atomic mass unit (amu).
Neutron: No charge (neutral), located in the nucleus, mass ≈ 1 amu.
Electron: Negative charge (–1), orbits the nucleus, mass ≈ 1/1836 amu.

Atomic Number and Atomic Mass

Atomic Number (Z): Number of protons in the nucleus; defines the element.
Atomic Mass (A): Sum of protons and neutrons in the nucleus.
Example: Carbon-12 has 6 protons and 6 neutrons; atomic mass = 12.

Isotopes

Isotope: Atoms of the same element with different numbers of neutrons.
Example: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon.
Isotopes have the same chemical properties but different atomic masses.

Octet Rule and Atomic Structure

Atoms tend to gain, lose, or share electrons to achieve a full outer shell (usually 8 electrons).
Electron arrangement determines chemical reactivity.
Example: Sodium (Na) loses one electron to achieve a stable configuration.

Covalent Bonds

Covalent Bond: A chemical bond formed when two atoms share one or more pairs of electrons.
Polar Covalent Bond: Electrons are shared unequally due to differences in electronegativity (e.g., in water).
Nonpolar Covalent Bond: Electrons are shared equally (e.g., in O2 or H2).
Single Bond: One pair of shared electrons.
Double Bond: Two pairs of shared electrons.

Ions and Ionic Bonds

Ion: An atom or molecule with a net electric charge due to the loss or gain of electrons.
Cation: Positively charged ion (loss of electrons).
Anion: Negatively charged ion (gain of electrons).
Ionic Bond: Attraction between oppositely charged ions (e.g., NaCl).

Hydrogen Bonding

Hydrogen Bond: A weak bond between a hydrogen atom covalently bonded to an electronegative atom (like O or N) and another electronegative atom.
Example: Hydrogen bonds between water molecules.

Chemical Reactions

Reactants: Substances that start a chemical reaction.
Products: Substances formed as a result of a chemical reaction.
Example:

Properties of Water Due to Hydrogen Bonding

Cohesion: Attraction between molecules of the same substance (e.g., water molecules stick together).
Surface Tension: The measure of how difficult it is to stretch or break the surface of a liquid.
Adhesion: Attraction between different substances (e.g., water and glass).
Water as a Heat Buffer: Water has a high specific heat capacity, allowing it to absorb and release large amounts of heat with little temperature change.

Solutions: Solvent, Solute, and Aqueous Solutions

Solvent: The substance that dissolves another substance (e.g., water).
Solute: The substance that is dissolved (e.g., salt).
Solution: A homogeneous mixture of two or more substances.
Aqueous Solution: A solution in which water is the solvent.

pH, Acids, Bases, and Buffers

pH: A measure of hydrogen ion concentration;
pH Scale: Ranges from 0 (most acidic) to 14 (most basic); 7 is neutral.
Acid: Substance that increases the hydrogen ion concentration in a solution (pH < 7).
Base: Substance that decreases the hydrogen ion concentration (pH > 7).
Buffer: A substance that minimizes changes in pH by accepting or donating H+ ions.

Summary Table: Key Chemical Concepts

Term	Definition	Example
Element	Pure substance of one type of atom	Oxygen (O)
Compound	Substance of two or more elements	Water (H2O)
Isotope	Atoms with same protons, different neutrons	Carbon-14
Covalent Bond	Shared electrons	H2O
Ionic Bond	Transfer of electrons	NaCl
Hydrogen Bond	Weak attraction involving H	Between water molecules
Acid	Donates H+	HCl
Base	Accepts H+	NaOH
Buffer	Stabilizes pH	Bicarbonate in blood

Additional info: These concepts form the foundation for understanding biological molecules and processes in subsequent chapters.