Chapter 2: The Chemical Basis of Life

Introduction

This chapter explores how chemistry underlies biological processes, using the example of how carbon dioxide in the air can harm coral reefs. When carbon dioxide (CO2) dissolves in water, it reacts to form an acid, which can disrupt the formation of coral skeletons. Understanding the chemical basis of life is essential for studying biology at all levels.

Elements, Atoms, and Compounds

Elements and Matter

All living organisms are composed of matter, which consists of chemical elements. Elements are 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), or table salt.

Trace Elements

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

• Example: Fluoride is added to municipal water and dental products to help reduce tooth decay.

• Other trace elements are added to food to preserve it, enhance nutrition, or improve appearance.

Atoms and Subatomic Particles

Structure of Atoms

An atom is the smallest unit of matter that retains the properties of an element. Each element consists of one kind of atom.

• Atoms are composed of three subatomic particles:

  • Protons (positively charged, located in the nucleus)

  • Neutrons (neutral, located in the nucleus)

  • Electrons (negatively charged, orbit the nucleus)

• The atomic number is the number of protons in an atom and defines the element.

• The mass number is the sum of protons and neutrons in the nucleus.

• The atomic mass is approximately equal to the mass number (measured in daltons or atomic mass units).

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

• Example: Carbon has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14.

Chemical Bonds and Molecules

Electron Shells and Chemical Properties

Electrons are arranged in electron shells around the nucleus. The chemical properties of an atom are determined by the distribution of electrons, especially those in the outermost shell (valence shell).

• Atoms with incomplete outer shells tend to interact with other atoms, forming chemical bonds by sharing, gaining, or losing electrons.

Covalent Bonds

A covalent bond forms when two atoms share one or more pairs of electrons. Molecules are formed by 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).

Ionic Bonds

An ionic bond is formed 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.

Hydrogen Bonds

Hydrogen bonds are weak bonds important in the chemistry of life. They form when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom.

• In water, hydrogen bonds form between the slightly positive hydrogen of one molecule and the slightly negative oxygen of another.

• Hydrogen bonds are responsible for many of water's unique properties.

Chemical Reactions

Chemical reactions involve the breaking and forming of chemical bonds, changing the composition of matter. Reactants are transformed into products.

• 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. Surface tension is related to cohesion and measures how difficult it is to break the surface of a liquid.

• Example: Water droplets form on the skin due to cohesion among water molecules.

Moderation of Temperature

Water moderates temperature due to its high specific heat, which is a result of hydrogen bonding. Thermal energy is the energy associated with the random movement of atoms and molecules, and heat is the transfer of thermal energy.

• Evaporative cooling: As water evaporates, the surface cools down, helping 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 property allows ice to float, 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.

• Aqueous solution: A solution in which water is the solvent.

• Positive and negative regions of water molecules surround ions and polar molecules, dissolving them.

Acids, Bases, and pH

In aqueous solutions, a small fraction of water molecules dissociate into ions. The pH scale measures how acidic or basic a solution is.

• Acid: A substance that increases the hydrogen ion (H+) concentration of a solution.

• Base: A substance that reduces the hydrogen ion concentration.

• Buffer: A substance that minimizes changes in pH.

Ocean Acidification and Coral Reefs

Rising atmospheric CO2 levels, mainly from fossil fuel combustion, are absorbed by oceans, leading to ocean acidification. Dissolved CO2 forms carbonic acid, lowering ocean pH and reducing carbonate ion concentration, which is essential for coral and shell-building organisms.

• As seawater acidifies, extra hydrogen ions (H+) combine with carbonate ions (CO32-) to form bicarbonate (HCO3-), reducing carbonate availability.

• Lower carbonate ion concentration slows the rate of calcium carbonate deposition, affecting coral growth.

Example of a controlled experiment: Scientists manipulated carbonate ion concentration and measured the rate of calcification in coral reef organisms. The lower the carbonate ion concentration, the slower the rate of calcification.

Summary of Key Concepts

• Elements and compounds are the chemical basis of life.

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

• Chemical bonds (covalent, ionic, 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: cohesion, temperature moderation, ice floating, and solvent abilities.

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

• Rising CO2 levels cause ocean acidification, threatening coral reefs and marine ecosystems.