The Chemical Basis of Life: Study Notes for General Biology

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The Chemical Basis of Life

Introduction

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

Elements, Atoms, and Compounds

Definitions and Basic Concepts

Matter: Anything that occupies space and has mass.
Element: A substance that cannot be broken down into other substances by chemical means. Each element is defined by its number of protons.
Compound: A substance consisting of two or more different elements combined in a fixed ratio (e.g., H2O).
Trace Element: An element required by an organism in minute quantities (e.g., iodine, iron, fluoride).

Importance of Trace Elements in the Human Diet

Iodine (I): Essential for thyroid hormone production; deficiency can cause goiter.
Fluoride (F): Added to water and dental products to prevent tooth decay.
Iron (Fe): Vital for oxygen transport in blood (hemoglobin); deficiency leads to anemia.

Structure of Atoms

Proton: Positively charged particle located in the nucleus; defines the atomic number.
Neutron: Electrically neutral particle in the nucleus; contributes to atomic mass.
Electron: Negatively charged particle orbiting the nucleus; involved in chemical bonding.

Atomic Number: Number of protons in an atom; determines the element.
Mass Number: Sum of protons and neutrons in the nucleus.

Isotopes and Radioactivity

Isotope: Atoms of the same element with different numbers of neutrons.
Radioactive Isotope: An isotope whose nucleus decays spontaneously, emitting radiation.

Uses: Medical imaging, cancer treatment, dating fossils.
Dangers: Radiation exposure can damage living tissue and DNA.

Chemical Bonds

Electron Configuration and Chemical Behavior

The arrangement of electrons in an atom's electron shells determines its chemical reactivity. Atoms tend to fill their outermost shell (valence shell) by gaining, losing, or sharing electrons.

Types of Chemical Bonds

Covalent Bond: Atoms share one or more pairs of electrons. Strongest type of bond in biological molecules.
Nonpolar Covalent Bond: Electrons are shared equally (e.g., O2).
Polar Covalent Bond: Electrons are shared unequally, creating partial charges (e.g., H2O).
Ionic Bond: One atom donates an electron to another, forming oppositely charged ions that attract (e.g., NaCl).
Hydrogen Bond: Weak attraction between a hydrogen atom (partially positive) and an electronegative atom (often oxygen or nitrogen) in another molecule.

Relative Strengths: Covalent > Ionic > Hydrogen

Chemical Reactions

Chemical Reaction: The making and breaking of chemical bonds, leading to changes in the composition of matter.
Reactants: Starting materials in a reaction.
Products: Substances formed as a result of the reaction.

Example: Photosynthesis

Reactants: Carbon dioxide (CO2) and water (H2O)
Products: Glucose (C6H12O6) and oxygen (O2)

Water’s Life-Supporting Properties

Special Properties of Water

Water's unique properties are essential for life and are largely due to its ability to form hydrogen bonds.

Cohesion: Tendency of water molecules to stick together due to hydrogen bonding.
Adhesion: Tendency of water molecules to stick to other substances.
Surface Tension: Measure of how difficult it is to stretch or break the surface of a liquid; water has high surface tension.

Heat and Temperature

Heat: Total amount of energy associated with the movement of atoms and molecules in a body of matter.
Temperature: Measure of the average kinetic energy of molecules.
Evaporative Cooling: As water evaporates (e.g., sweating), it removes heat, cooling the surface.

Why Ice Floats

As water freezes, its molecules form a crystalline structure maintained by hydrogen bonds, making ice less dense than liquid water.
This property allows ice to float, insulating aquatic life in winter.

Solutions, Solvents, and Solutes

Solution: A liquid that is a homogeneous mixture of two or more substances.
Solvent: The dissolving agent (e.g., water).
Solute: The substance that is dissolved (e.g., salt).

Acids, Bases, and the pH Scale

Acid: A substance that increases the hydrogen ion (H+) concentration of a solution.
Base: A substance that reduces the hydrogen ion concentration, often by accepting H+ or releasing OH-.
pH Scale: Measures the concentration of H+ in a solution; ranges from 0 (most acidic) to 14 (most basic).

Buffers

Buffer: A substance that minimizes changes in pH by accepting or donating H+ ions.
Buffers are crucial in maintaining stable pH in biological systems.

Ocean Acidification

Cause: Increased atmospheric CO2 dissolves in seawater, forming carbonic acid, which lowers ocean pH.
Consequence: Reduced carbonate ion concentration affects marine organisms, especially those with calcium carbonate shells or skeletons (e.g., corals).

Water and the Search for Extraterrestrial Life

Water is essential for life as we know it due to its solvent properties and role in biochemical reactions.
The search for extraterrestrial life often focuses on finding water or evidence of past water on other planets.

Table: Comparison of Chemical Bonds

Bond Type	How It Forms	Relative Strength	Example
Covalent	Sharing of electron pairs	Strongest	H2O, O2
Ionic	Transfer of electrons; attraction between ions	Strong (but weaker than covalent in water)	NaCl
Hydrogen	Attraction between H (partially positive) and electronegative atom	Weakest	Between water molecules

Additional info: Some explanations and examples have been expanded for clarity and completeness.