The Chemical Basis of Life: Elements, Atoms, and Compounds

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The Chemical Basis of Life

Introduction

The study of biology is deeply connected to chemistry, as all living organisms are composed of chemical substances. Understanding how chemical compounds interact in the environment is essential for explaining biological phenomena, such as the impact of carbon dioxide on coral reefs. When carbon dioxide (CO2) dissolves in water, it reacts to form an acid, which can disrupt the formation of coral skeletons and affect marine ecosystems.

Elements, Atoms, and Compounds

Elements and Matter

All living organisms are composed of matter, which is anything that occupies space and has mass. Matter is made up of chemical elements, substances that cannot be broken down into other substances by chemical means.

Element: A pure substance consisting of only one kind of atom.
About 25 elements are essential for human life.
Four elements—oxygen (O), carbon (C), hydrogen (H), and nitrogen (N)—make up about 96% of the weight of most living organisms.

Compounds

A compound is a substance consisting of two or more different elements combined in a fixed ratio. Compounds have unique properties that are different from the elements that compose them.

Example: Sodium (Na) and chlorine (Cl) combine to form sodium chloride (NaCl), commonly known as table salt.

Trace Elements

Trace elements are elements required by an organism in only minute quantities but are essential for health.

Examples include fluoride (added to water to prevent tooth decay) and iodine (added to salt to prevent thyroid disease).
Other chemicals are added to food to preserve it, enhance nutrition, or improve appearance.

Atoms and Their Structure

Subatomic Particles

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of three types of subatomic particles:

Protons (positively charged, located in the nucleus)
Neutrons (neutral, located in the nucleus)
Electrons (negatively charged, orbit the nucleus)

The atomic number of an element is the number of protons in its atoms. The mass number is the sum of protons and neutrons.

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Some isotopes are stable, while others are radioactive.

Isotope	Protons	Neutrons	Mass Number
Carbon-12	6	6	12
Carbon-13	6	7	13
Carbon-14	6	8	14

Example: A nitrogen atom with 7 protons and 9 neutrons has an atomic number of 7 and a mass number of 16.

Chemical Bonds

Electron Shells and Bonding

Electrons are arranged in electron shells around the nucleus. Atoms with incomplete outer shells tend to interact with other atoms, resulting in chemical bonds that hold atoms together in molecules and compounds.

Ionic Bonds

An ionic bond forms when one atom transfers an electron to another, resulting in oppositely charged ions that attract each other.

Ion: An atom or molecule with an electrical charge due to the loss or gain of electrons.
Example: Sodium (Na) loses an electron to become Na+, and chlorine (Cl) gains an electron to become Cl-. The resulting NaCl is held together by ionic bonds.

Covalent Bonds

A covalent bond forms when two atoms share one or more pairs of electrons. Covalent bonds can be:

Nonpolar covalent bonds: Electrons are shared equally (e.g., O2 molecule).
Polar covalent bonds: Electrons are shared unequally, resulting in partial charges (e.g., H2O molecule).

Hydrogen Bonds

Hydrogen bonds are weak attractions between the slightly positive hydrogen atom of one molecule and the slightly negative atom (often oxygen or nitrogen) of another molecule. These are especially important in water and biological molecules like DNA.

Chemical Reactions

Making and Breaking Bonds

Chemical reactions involve the breaking of existing bonds and the formation of new ones, resulting in the transformation of substances.

Reactants: Starting materials in a chemical reaction.
Products: Substances formed as a result of the reaction.

Example: The formation of water from hydrogen and oxygen:

Chemical reactions do not create or destroy matter; they rearrange it.

Water's Life-Supporting Properties

Cohesion and Adhesion

Cohesion is the tendency of molecules of the same kind to stick together, while adhesion is the clinging of one substance to another. These properties contribute to surface tension, the difficulty of breaking the surface of a liquid.

Example: Sweat forms droplets on the skin due to cohesion among water molecules.

Moderation of Temperature

Water can absorb and release large amounts of heat with only a slight change in its own temperature, helping to moderate Earth's climate and organisms' body temperatures.

Thermal energy: Energy associated with the random movement of atoms and molecules.
Heat: Transfer of thermal energy from a warmer to a cooler body.
Evaporative cooling: As water evaporates, the surface cools down.

Ice Floats

Water is less dense as a solid (ice) than as a liquid due to hydrogen bonding. When water freezes, molecules form a stable lattice, making ice less dense and allowing it to float.

Example: Floating ice insulates bodies of water, protecting aquatic life in winter.

Water as the Solvent of Life

A solution is a liquid consisting of a uniform mixture of two or more substances. Water's polarity makes it an excellent solvent, able to dissolve many ionic and polar substances, forming aqueous solutions.

Example: Table salt (NaCl) dissolves in water as water molecules surround and separate the ions.

Acids, Bases, and pH

The pH Scale

In aqueous solutions, a small fraction of water molecules dissociate into ions. The pH scale measures how acidic or basic a solution is, ranging from 0 (most acidic) to 14 (most basic), with 7 being neutral.

Acid: A substance that increases the hydrogen ion (H+) concentration in a solution.
Base: A substance that reduces the hydrogen ion concentration.
Buffer: A substance that minimizes changes in pH.

Table: Examples of pH Values

Substance	pH Value
Battery acid	1
Lemon juice	2
Human urine	6
Pure water	7
Human blood, tears	7.4
Household bleach	13

Ocean Acidification and Coral Reefs

Impact of Rising CO2 Levels

Human activities, such as burning fossil fuels, increase atmospheric CO2. About 25% of this CO2 is absorbed by oceans, where it reacts with water to form carbonic acid, lowering ocean pH in a process called ocean acidification.

Lower pH increases hydrogen ion concentration, which combines with carbonate ions to form bicarbonate, reducing carbonate available for coral and shell-building animals.
Reduced carbonate ion concentration slows the rate of calcium carbonate deposition, affecting coral growth and reef health.

Example: Controlled experiments show that lower carbonate ion concentrations result in slower calcification rates in reef organisms.

Summary of Key Concepts

Elements and compounds are the chemical building blocks of life.
Atoms consist of protons, neutrons, and electrons; isotopes vary in neutron number.
Chemical bonds (ionic, covalent, hydrogen) hold atoms together in molecules and compounds.
Chemical reactions rearrange matter but do not create or destroy it.
Water's unique properties support life on Earth.
The pH scale measures acidity and basicity; buffers help maintain pH balance.
Rising CO2 levels cause ocean acidification, threatening coral reefs and marine ecosystems.