Water and Carbon: The Chemical Basis of Life – Study Notes

Chapter 2: Water and Carbon – The Chemical Basis of Life

Introduction

This chapter explores the fundamental chemical principles underlying biological systems, focusing on the atoms and molecules that make up living organisms, the types of chemical bonds, and the unique properties of water that are essential for life.

Basic Atomic Structure

Atoms and Elements

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

• Element: A substance consisting entirely of one type of atom.

• Atomic Number: Number of protons in the nucleus of an atom. Written as a subscript to the left of the element symbol.

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

Example: Carbon has 6 protons (atomic number 6) and typically 6 neutrons (mass number 12).

Structure of the Atom

• Nucleus: Contains protons (+1 charge) and neutrons (neutral).

• Electrons: Negatively charged particles orbiting the nucleus.

The Periodic Table and Common Atoms in Living Cells

Major Biological Elements

• Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O): These four elements make up about 96% of the matter in living organisms.

• Other important elements include Phosphorus (P) and Sulfur (S).

Electron Shells

• Electrons fill the innermost shells first, then outer shells.

• The arrangement of electrons determines how atoms interact and bond.

Chemical Bonds in Biology

Ionic Bonds – Electron Transfer

• Ionic Bond: Formed when one atom transfers electrons to another, resulting in oppositely charged ions that attract each other.

• Example: Sodium (Na) transfers an electron to Chlorine (Cl), forming Na+ and Cl- ions, which combine to form table salt (NaCl).

Covalent Bonds – Electron Sharing

• Covalent Bond: Formed when two atoms share one or more pairs of electrons.

• Example: Two hydrogen atoms share electrons to form H2.

Nonpolar vs Polar Covalent Bonds

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

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

• Electronegativity: Tendency of an atom to attract electrons. Order: O > N > C ≈ H.

The Electron-Sharing Continuum

• Bonds range from equal sharing (nonpolar covalent) to unequal sharing (polar covalent) to complete transfer (ionic).

Van der Waals Forces

• Weak attractions between molecules due to temporary partial charges.

• Important for stabilizing biological macromolecules.

Hydrogen Bonds

• Formed when a hydrogen atom covalently bonded to a highly electronegative atom (like oxygen) is attracted to another electronegative atom.

• Responsible for many unique properties of water.

Water: The Molecule of Life

Unique Properties of Water

• Versatility as a Solvent: Water dissolves many substances due to its polarity, allowing it to interact with ions and polar molecules.

• Cohesive Properties: Water molecules stick together via hydrogen bonds, leading to surface tension.

• Expansion Upon Freezing: Water forms a crystalline structure when frozen, making ice less dense than liquid water.

• Ability to Stabilize Temperature: Water has a high specific heat, absorbing and releasing heat slowly, which helps regulate temperature in organisms and environments.

Water as a Solvent

• Polar molecules and ions dissolve easily in water.

• Example: Table salt (NaCl) dissolves in water as Na+ and Cl- ions are surrounded by water molecules.

Functional Groups and Carbon Chemistry

Role of Carbon

• Carbon forms the backbone of organic molecules due to its ability to form four covalent bonds.

• Promotes structural diversity in organic molecules.

Functional Groups

• Groups of atoms attached to carbon atoms that determine the chemical properties and reactivity of molecules.

• Example: Hydroxyl group (-OH) is polar and can form hydrogen bonds, making molecules soluble in water.

Key Terms Table

Term	Definition
Atom	Smallest unit of matter retaining element properties
Element	Substance consisting of one type of atom
Atomic Number	Number of protons in nucleus
Mass Number	Sum of protons and neutrons
Ionic Bond	Bond formed by electron transfer between atoms
Covalent Bond	Bond formed by electron sharing between atoms
Hydrogen Bond	Weak bond between hydrogen and electronegative atom
Electronegativity	Atom's tendency to attract electrons
Functional Group	Specific group of atoms affecting molecule's properties

Important Equations

• Atomic Number: $\text{Atomic Number} = \text{Number of Protons}$

• Mass Number: $\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}$

Summary Table: Types of Chemical Bonds

Bond Type	Mechanism	Example	Strength
Ionic	Electron transfer	NaCl	Strong (in dry conditions)
Covalent (Nonpolar)	Equal electron sharing	H2	Strong
Covalent (Polar)	Unequal electron sharing	H2O	Strong
Hydrogen Bond	Attraction between polar molecules	Between H2O molecules	Weak (but significant in large numbers)
Van der Waals	Temporary partial charges	Between nonpolar molecules	Very weak

Applications and Examples

• Surface Tension: Water's cohesive properties allow small objects to float and insects to walk on water.

• Temperature Regulation: Water's high specific heat helps maintain stable temperatures in organisms and environments.

• Solubility: Polar and ionic substances dissolve in water, facilitating biochemical reactions.

Additional info: These notes expand upon the provided slides and questions, offering definitions, examples, and tables for clarity and completeness.