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

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, especially when considering 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 in Living Organisms

All matter is composed of chemical elements, which are substances that cannot be broken down into other substances by chemical means. Living organisms are primarily made up of a small subset of these elements.

• 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.

• Trace elements are required in minute amounts but are vital for health (e.g., fluoride in water to prevent tooth decay).

Compounds are substances consisting of two or more different elements combined in a fixed ratio. For example, sodium chloride (NaCl) is a compound formed from sodium (Na) and chlorine (Cl).

Atoms: Structure and Properties

An atom is the smallest unit of matter that retains the properties of an element. Each element consists of one kind of atom, which is made up of subatomic particles:

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Electrically neutral particles also located in the nucleus.

• Electrons: Negatively charged particles that 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 in the nucleus. The atomic mass is approximately equal to the mass number.

Isotopes

Isotopes are forms of an element that have the same number of protons but different numbers of neutrons. For example, carbon has three naturally occurring isotopes:

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

Some isotopes are radioactive, meaning they decay spontaneously, giving off particles and energy.

Chemical Bonds

Electron Arrangement and Chemical Properties

The distribution of electrons in an atom determines its chemical behavior. Electrons are arranged in electron shells around the nucleus. Atoms with incomplete outer shells tend to interact with other atoms, forming chemical bonds by sharing, gaining, or losing electrons.

Types of Chemical Bonds

• Ionic Bonds: Formed when one atom transfers an electron to another, resulting in oppositely charged ions that attract each other. Example: NaCl (table salt).

• Covalent Bonds: Formed when two atoms share one or more pairs of electrons. Covalent bonds can be:

  • Nonpolar covalent bonds: Electrons are shared equally (e.g., O2).

  • Polar covalent bonds: Electrons are shared unequally, leading to partial charges (e.g., H2O).

• Hydrogen Bonds: Weak bonds important in biology, formed between a hydrogen atom (already covalently bonded to a more electronegative atom) and another electronegative atom. Example: bonds between water molecules.

Formation of Molecules

Covalent bonds join atoms into molecules through electron sharing. The number of covalent bonds an atom can form is determined by the number of electrons needed to fill its outer shell. For example, carbon can form four covalent bonds.

Chemical Reactions

Making and Breaking Bonds

Chemical reactions involve the breaking of existing bonds and the formation of new ones, changing the composition of matter. The substances present at the start are called reactants, and those produced are products.

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

• Example: Formation of water from hydrogen and oxygen: $2H_2 + O_2 \rightarrow 2H_2O$

Water's Life-Supporting Properties

Cohesion, Adhesion, and Surface Tension

Water molecules are held together by hydrogen bonds, giving water unique properties:

• Cohesion: Tendency of water molecules to stick together.

• Adhesion: Tendency of water molecules to stick to other substances.

• Surface tension: Measure of how difficult it is to break the surface of a liquid.

These properties explain phenomena such as the formation of sweat droplets on skin.

Moderation of Temperature

Water can absorb and release large amounts of heat with only a slight change in its own temperature, due to hydrogen bonding. Evaporative cooling occurs when the surface of a liquid cools as water evaporates, which helps organisms regulate temperature.

Ice Floats

Water is less dense as a solid (ice) than as a liquid because hydrogen bonds hold water molecules in a crystalline structure that is more open than in liquid water. This is why ice floats, insulating 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.

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:

• Acid: Substance that increases the hydrogen ion (H+) concentration.

• Base: Substance that reduces the hydrogen ion concentration.

• Buffer: Substance that minimizes changes in pH.

The pH scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

Impact of Rising CO2 on Coral Reefs

Ocean Acidification

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—a process called ocean acidification.

• Extra hydrogen ions (H+) combine with carbonate ions (CO32−) to form bicarbonate (HCO3−), reducing carbonate available for coral skeleton formation.

• Lower carbonate ion concentration slows the rate of calcification in corals and other shell-building organisms.

Controlled experiments show that as carbonate ion concentration decreases, the rate of calcium carbonate deposition by reef organisms also decreases.

Summary Table: Types of Chemical Bonds

Bond Type	Description	Example
Ionic	Transfer of electrons; attraction between oppositely charged ions	NaCl (table salt)
Covalent	Sharing of electron pairs between atoms	H2O, O2
Hydrogen	Weak attraction between a hydrogen atom and an electronegative atom	Between water molecules

Key Concepts to Remember

• Elements and compounds are the building blocks of matter in living organisms.

• Atoms are composed of protons, neutrons, and electrons; isotopes differ in neutron number.

• Chemical bonds (ionic, covalent, hydrogen) determine molecular structure and function.

• Water's unique properties support life on Earth.

• The pH scale measures acidity and basicity; buffers help maintain stable pH in organisms.

• Rising CO2 levels lead to ocean acidification, threatening coral reefs and marine life.