Chapter 4: The Chemical Basis of Life
Concept 1: Life requires about 25 chemical elements.
Elements
 Matter – anything that occupies space & has mass.
o Examples: rock, wood, animals
o Matter is composed of elements.
 Element – a pure substance that cannot be broken down into other substances.
o O, C, H, & N (make up 96%)
o Ca, P, K, & S (make up 4%
 Trace elements – elements that make up more than 0.01% of your mass.
o Examples: iodine, iron, fluorine
Compounds
 Compound – a substance containing 2 or more elements that are chemically
combined in a fixed ratio.
o Most compounds in our bodies are composed of 3 or more elements.
Chapter 4: The Chemical Basis of Life
Concept 2: Chemical properties are based on the structure of atoms.
Atoms
 Atom – the smallest possible particle of an element
o Made of smaller subatomic particles: protons, electrons, and neutrons
 Protons – subatomic particle with a positive charge
 Electron – subatomic particle with a negative charge
 Neutron – subatomic particle with no charge
 An atoms chemical properties depend on the # & arrangement of its subatomic
particles.
 Nucleus – central core of the atom containing protons & neutrons.
 Atomic Number - the number of protons in an element.
o The # of protons determines the atom’s properties.
o No 2 elements have the same chemical behavior.
 Isotopes
 Isotopes – alternate forms of elements that have different numbers of neutrons
in the nucleus.
o Example: Carbon-12, Carbon-13, Carbon-14 (p. 76)
 Radioactive isotope – an isotope who’s nucleus decays over time, giving off
radiation in the form of matter and energy.
Electrons & Reactivity
 How does an atom’s structure determine how it reacts w/ other atoms?
o The electrons in the highest energy level determine how an atom
reacts.

When atoms have partially filled outer electron shells, they are
more likely to react.
Chapter 4: The Chemical Basis of Life
Concept 3: Chemical bonds join atoms to one another.
Ionic Bonds
 Ionic bond – a bond where an atom transfers an electron to another atom.
o Before bonding, atoms are neutral (equal #’s of protons and
electrons).
o After the electron transfer, they are ions.
 Ion – an electrically charges atom
o Fig. 4.8 (p. 78)
Covalent Bonds
 Covalent bond – 2 or more atoms share electrons
 Number of bonds:
o The number of bonds is determined by the number of electrons
needed to fill the outer energy level.
 Molecule – 2 or more atoms that are bonded together by a covalent bond.
o 3 types of models to represent molecules (fig. 4-10, p.79):
 Chemical formula
 Space filling
 Structural
Chemical Reactions
 Chemical reaction – the breaking of old and formation of new chemical bonds
that results in new substances. (fig. 4-11, p. 80)
o Some reactions absorb energy
o Some release energy
 Reactants – materials present before the reaction
 Products – materials present after the reaction
 Chemical reactions do not create or destroy atoms; they only rearrange them.
Chapter 4: The Chemical Basis of Life
Concept 4: Life depends on the unique properties of water.
Water
 H2O
 Why is it important to you?
o The fluid that surrounds your cells contains mostly water.
o Your cells are composed of 70% – 95% of water.
The Structure of Water
 Composed of 1 Oxygen atom covalently bonded to 2 hydrogen atoms.
 Polar molecule – a molecule in which opposite ends have opposite electric
charges. WHY?
o There is an unequal sharing of electrons.
 O’s nucleus has more protons, making it stronger.
o “V” shaped molecule
o Fig. 4-12, p. 81

o Water molecules are attracted to each other due to them having a
“positive” end & a “negative” end.
o This is a weak attraction known as a hydrogen bond.
Hydrogen bond – a chemical bond created b/w a slightly positive hydrogen atom
& a slightly negative portion of another molecule.
o Fig. 4-12. p. 81
o Not as strong as an ionic bond
Water’s Life Supporting Properties
 Cohesion
 Adhesion
 Temperature moderation
 Low density of ice
 Water’s ability to dissolve other substances
Cohesion & Adhesion
o Cohesion – the tendency of molecules of the same kind to stick to one
another
 Hydrogen bonds last for less than a second, yet they form new
bonds just as quickly.
 Surface tension – the affect of cohesion pulling water
molecules together to create a “film”. (p.82)
o Adhesion – the type of attraction that occurs between unlike
molecules.
o Important to life b/c they allow large molecules to stay together and
function properly in cells.
o See “tree” example on p.82, fig. 4-13.
Temperature Moderation
o Thermal energy – the total amount of energy associated with the
random movement of atoms & molecules in a sample of matter.
o Temperature – a measure of the average energy of random motion of
the particles in a sample of matter.
o When 2 substances differ in temperature, the warmer substance
transfers thermal energy in the form of heat to the cooler substance.
o B/c of hydrogen bonding, H2O has a better ability to resist
temperature changes than most substances. WHY?
 Heating water (metal pan vs. H2, p. 83)
 Some of the thermal energy absorbed to H2O is used to
break the hydrogen bonds, causing water to heat up
slower.
 Cooling water
 As H2O cools, hydrogen bonds are also forming.
 The formation releases heat, causing the water to cool
slowly.
o Why does this matter?
 It helps regulate air temperature.
 It helps cool our bodies when we sweat.
Low Density of Ice
o Why does ice have a lower density than water?
 Hydrogen bonds allow there to be spaces between water
molecules.
 When the H2O if in solid state, there is not enough energy to
break the hydrogen bonds.
o Why does it matter that ice floats on water?
 When ice is on top of the water, it insulates the water
underneath so that organisms can survive.
Water’s Ability to Dissolve Other Substances
 Solution – a uniform mixture of 2 or more substances
o Solvent – the substance that dissolves another substance
o Solute – the substance that is dissolved
 Aqueous Solution – a solution where water is the solvent
o Since H2O is a polar molecule, the slightly charged ends attract the
ions or atoms of the solute to break apart the bonds and dissolve
them.
Acids, Bases, and pH
 In aqueous solutions, a small percentage of water molecules break apart into
ions.
 The ions formed are positive H (H+) & negative OH (OH-).
 Acid – a compound that donates H+ ions to a solution.
 Base – a compound that removes H+ ions from an aqueous solution.
o Some add OH- ions.
 Fig. 4-17, p. 85
 pH Scale – describes how acidic or basic a solution is
o Ranges from 0 – 14.
 0 most acidic
 14 most basic

o Each pH unit represents a tenfold change in concentration of H+ ions.
 Example: lemon juice with a pH of 2 has 10 times more H+ ions
than grapefruit juice with a pH of 3.
o The pH in most living cells is around 7.
Buffers – substances that cause a solution to resist changes in pH.
o A buffer accepts H+ ions when there are too many, and donating H+
ions when there is not enough.
o Biological fluids contain buffers to regulate pH levels.