Chapter 2

Biology and Society: More Precious than Gold

           A drought is

            a period of abnormally dry weather that changes the environment and

            one of the most devastating disasters.

            Droughts can cause

            severe crop damage,

            shortages of drinking water,

            dust storms,


            habitat loss, and

            mass migration.

           Throughout human history, droughts have helped wipe out societies and even whole civilizations.

           Droughts are catastrophic because life cannot exist without water.


           Take any biological system apart, and you eventually end up at the chemical level.

           Chemical reactions are always occurring in the human body.

Matter: Elements and Compounds

           Matter is anything that occupies space and has mass.

           Matter is found on Earth in three physical states:


            liquid, and


           Matter is composed of chemical elements.

            An element is a substance that cannot be broken down into other substances by chemical reactions.

            There are 92 naturally occurring elements on Earth.

           All of the elements are listed in the periodic table.

           Twenty-five elements are essential to people.

           Four elements make up about 96% of the weight of most cells:



            hydrogen, and


           Trace elements are

            required in only very small amounts and

            essential for life.

           An iodine deficiency causes goiter.


            is added to dental products and drinking water and

            helps to maintain healthy bones and teeth.

           Elements can combine to form compounds.

            Compounds are substances that contain two or more elements in a fixed ratio.

            Common compounds include

             NaCl (table salt) and
             H2O (water).


           Each element consists of one kind of atom.

            An atom is the smallest unit of matter that still retains the properties of an element.

The Structure of Atoms

           Atoms are composed of subatomic particles.

            A proton is positively charged.

            An electron is negatively charged.

            A neutron is electrically neutral.

           Most atoms have protons and neutrons packed tightly into the nucleus.

            The nucleus is the atoms central core.

            Electrons orbit the nucleus.

           Elements differ in the number of subatomic particles in their atoms.

            The number of protons, the atomic number, determines which element it is.

            Mass is a measure of the amount of material in an object.

            An atoms mass number is the sum of the number of protons and neutrons in its nucleus.


           Isotopes are alternate mass forms of an element.


            have the same number of protons and electrons but

            differ in their number of neutrons.

           The nucleus of a radioactive isotope decays spontaneously, giving off particles and energy.

           Radioactive isotopes have many uses in research and medicine.

            They can be used to determine the fate of atoms in living organisms.

            They are used in PET scans to diagnose heart disorders and some cancers.

           Uncontrolled exposure to radioactive isotopes can harm living organisms by damaging DNA.

            The 1986 Chernobyl nuclear accident released large amounts of radioactive isotopes.

            Naturally occurring radon gas may cause lung cancer.

Electron Arrangement and the Chemical Properties of Atoms

           Of the three subatomic particles, only electrons are directly involved in the chemical activity of an atom.

           Electrons orbit the nucleus of an atom in specific electron shells.

           The farther an electron is from the nucleus, the greater its energy.

           The number of electrons in the outermost shell determines the chemical properties of an atom.

Chemical Bonding and Molecules

           Chemical reactions enable atoms to give up or acquire electrons, completing their outer shells.

           Chemical reactions usually result in atoms

            staying close together and

            being held together by attractions called chemical bonds.

Ionic Bonds

           When an atom loses or gains electrons, it becomes electrically charged.

            Charged atoms are called ions.

            Ionic bonds are formed between oppositely charged ions.

Covalent Bonds

           A covalent bond forms when two atoms share one or more pairs of outer-shell electrons.

           Covalent bonds are the strongest of the various bonds.

           Covalent bonds hold atoms together in a molecule.

           The number of covalent bonds an atom can form is equal to the number of additional electrons needed to fill its outer shell.

Hydrogen Bonds

           Water is a compound in which the electrons in its covalent bonds are not shared equally.

            This causes water to be a polar molecule, one with an uneven distribution of charge.

           The polarity of water results in weak electrical attractions between neighboring water molecules.

            These weak attractions are called hydrogen bonds.

Chemical Reactions

           Cells constantly rearrange molecules by breaking existing chemical bonds and forming new ones.

            Such changes in the chemical composition of matter are called chemical reactions.

            A simple example is the reaction between oxygen gas and hydrogen gas that forms water.

           Chemical reactions include

            reactants, the starting materials, and

            products, the end materials. 

           Chemical reactions

            can rearrange matter

            but cannot create or destroy matter.


           Life on Earth began in water and evolved there for 3 billion years.

            Modern life remains tied to water.

            Your cells are composed of 70–95% water.

           The abundance of water is a major reason Earth is habitable.

Waters Life-Supporting Properties

           The polarity of water molecules and the hydrogen bonding that results explain most of waters life-supporting properties.

            Water molecules stick together.

            Water has a strong resistance to change in temperature.

            Frozen water floats.

            Water is a common solvent for life.

The Cohesion of Water

           Water molecules stick together as a result of hydrogen bonding.

            This tendency of molecules of the same kind to stick together is called cohesion.

            Cohesion is vital for the transport of water from the roots to the leaves of plants.

           Surface tension is the measure of how difficult it is to stretch or break the surface of a liquid.

            Hydrogen bonds give water an unusually high surface tension.

How Water Moderates Temperature

           Because of hydrogen bonding, water has a strong resistance to temperature change.

           Heat and temperature are related, but different.

            Heat is the amount of energy associated with the movement of the atoms and molecules in a body of matter.

            Temperature measures the intensity of heat.

           Water can absorb and store large amounts of heat while only changing a few degrees in temperature.

           Water can moderate temperatures.

            Earths giant water supply causes temperatures to stay within limits that permit life.

            Evaporative cooling occurs when a substance evaporates and the surface of the liquid remaining behind cools down.

The Biological Significance of Ice Floating

           When water molecules get cold enough, they move apart, forming ice.

           A chunk of ice has fewer water molecules than an equal volume of liquid water.

           Ice floats because it is less dense than liquid water.

           If ice did not float, ponds, lakes, and even the oceans would freeze solid.

           Life in water could not survive if bodies of water froze solid.

Water as the Solvent of Life

           A solution is a liquid consisting of a homogeneous mixture of two or more substances.

            The dissolving agent is the solvent.

            The dissolved substance is the solute.

           When water is the solvent, the result is an aqueous solution.

Acids, Bases, and pH

           A chemical compound that releases H+ to a solution is an acid.

           A compound that accepts H+ and removes them from solution is a base.

           To describe the acidity of a solution, chemists use the pH scale.


           Buffers are substances that resist pH change.


            accept H+ ions when they are in excess and

            donate H+ ions when they are depleted.

           Increases in global CO2 concentrations may lead to

            the acidification of the oceans and

            ecological disasters.