Transcript Chapter 2

Chapter 2
The Chemical Basis of Life
Nature's Chemical Language
• Chemicals play an important role in all
organisms
• The rattlebox moth provides a good example
of chemicals used in mating and defense
ELEMENTS, ATOMS, AND MOLECULES
2.1 Living organisms are composed of about 25
chemical elements
• Elements are the basic chemical units that
cannot be broken apart by typical chemical
processes
• There are 92 naturally occurring elements
– 25 are required by living organisms
– 4 make up 96.3 of the human body
CONNECTION
2.2 Trace elements are common additives to
food and water
• Trace elements are essential in minute
quantities for proper biological functioning
– Example: iodine is a trace element that
prevents goiter
– Many foods are fortified with trace elements
and vitamins (which consist of two or more
elements)
2.3 Elements can combine to form compounds
• Compounds contain two or more elements in a
fixed ratio
• Different arrangements of the atoms of
elements determine the unique properties of
each compound
• The smallest unit of an element is an atom
LE 2-3
Sodium
Chlorine
Sodium Chloride
2.4 Atoms consist of protons, neutrons, and
electrons
• Subatomic particles
– Protons and neutrons occupy the central
region (nucleus) of an atom
• A proton has a single positive charge
• A neutron is electrically neutral
– Electrons surround the nucleus
• An electron has a single negative charge
LE 2-4a
Electron
cloud
6e–
2e–
Nucleus
2
Protons
2
Neutrons
2
Electrons
Helium atom
Mass
number = 4
6
Protons
6
Neutrons
6
Electrons
Carbon atom
Mass
number = 12
• Differences in Elements
– All the atoms of a particular element have the same
number of protons
– The number of protons-the atomic number-defines
the element's unique properties
– An atom's mass number (atomic mass) is the sum of
its protons and neutrons
• Isotopes
– Isotopes are atoms with the same atomic number
but different atomic masses (different number of
neutrons)
CONNECTION
2.5 Radioactive isotopes can help or harm us
• Radioactive isotopes are useful as tracers to
study the fate of elements and molecules in
living systems
• Radioactive tracer isotopes are often used in
combination with sophisticated imaging
instruments for medical diagnosis
• Uncontrolled exposure to radioactive material
can harm living organisms
2.6 Electron arrangement determines the
chemical properties of an atom
• Electrons in an atom are arranged in electron
shells, which may contain different numbers of
electrons
• The chemical reactivity of an atom depends
on the number of electrons in the outer shell
– Atoms whose outer shells are not full share
or transfer electrons to other atoms,
forming molecules
– Two major types of chemical bonds
between atoms form compounds
• Ionic bonds
• Covalent bonds
2.7 Ionic bonds are attractions between ions of
opposite charge
• An ion is a charged atom that has lost or
gained electrons in its outer shell
– A positively charged ion (cation) is an atom
that has lost an electron
– A negatively charged ion (anion) is an atom
that has gained an electron
• An electrical attraction between ions with
opposite charges results in an ionic bond
• Example: sodium chloride (table salt) results
from an ionic bond between sodium and
chlorine
Transfer of
electron
Na
Sodium atom
Cl
Chlorine atom
LE 2-7b
Na+
Cl-
2.8 Covalent bonds join atoms into molecules
through electron sharing
• Covalently bonded atoms share one or more
pairs of outer shell electrons, forming a
molecule
• In a double bond, two pairs of electrons are
shared
• Covalent bonds can be represented in various
ways
2.9 Unequal electron sharing creates polar
molecules
• A molecule whose covalently bonded atoms
share electrons equally is nonpolar
• A molecule whose covalently bonded atoms
share electrons unequally is polar
– One part of the molecule is slightly positive,
and one part is slightly negative
LE 2-9
A water molecule
2.10 Hydrogen bonds are weak bonds important
in the chemistry of life
• The attraction between slightly positive regions
and slightly negative regions creates hydrogen
bonds
• Hydrogen bonding occurs in many biologically
important compounds
– Water
– DNA
– Proteins
LE 2-10
Hydrogen bond
WATER'S LIFE-SUPPORTING PROPERTIES
2.11 Hydrogen bonds make liquid water cohesive
• Cohesion is the tendency of molecules to stick
together
• Surface tension results from the cohesion of
water molecules
Animation: Water Transport
2.12 Water's hydrogen bonds moderate
temperature
• When water is heated, the heat energy is
absorbed, disrupting hydrogen bonds
– The water stores a large amount of heat
while warming only a few degrees
• When water is cooled, heat energy is released
as hydrogen bonds are formed
– The temperature of the water is lowered
slowly
• Water also moderates
temperature by
evaporative cooling
– The surface cools as
the hottest molecules
leave
2.13 Ice is less dense than liquid water
• Hydrogen bonds in ice create a stable, threedimensional structure
• Ice is less dense than water, because it has
fewer molecules in the same volume
Hydrogen bond
Ice
Liquid water
Hydrogen bonds are stable
Hydrogen bonds
constantly break and re-form
2.14 Water is the solvent of life
• A solution is a homogeneous mixture of a
liquid solvent and one or more dissolved
solutes
• Because water is a polar molecule, it readily
forms solutions with many other polar and ionic
compounds
• A solution in which water is the solvent is an
aqueous solution
LE 2-14
Ion in
solution
Salt
crystal
2.15 The chemistry of life is sensitive to acidic
and basic conditions
• A compound that releases H+ ions in solution
is an acid
• A compound that accepts H+ ions in solution is
a base
• Acidity is measured on the pH scale from 0
(most acidic) to 14 (most basic)
• The pH of most cells is kept close to 7 (neutral)
by buffers that resist pH change
LE 2-15
pH scale
H+
H+
H+
-
H+ OH
+
OH- H
H+
Lemon juice, gastric juice
H+
H+
Grapefruit juice, soft drink
Acidic solution
Tomato juice
Human urine
OH-
OH-
-
H+
H+ OH
OH OHH+
H+
H+
Neutral solution
NEUTRAL
[H+[OH-
Pure water
Human blood
Seawater
Milk of magnesia
OHOHOH-
Household ammonia
OH-
H+
H+
OH-
Household bleach
OHOven cleaner
Basic solution
CONNECTION
2.16 Acid precipitation threatens the environment
• Acid precipitation is formed when air pollutants
from burning fossil fuels combine with water
vapor in the air to form sulfuric and nitric acids
• Some ecosystems and structures are
threatened by acid precipitation
CHEMICAL REACTIONS
2.17 Chemical reactions change the composition
of matter
• In a chemical reaction, reactants interact,
leading to products
• Atoms are rearranged, but the number of
atoms stays constant on both sides of the
equation
+
2 H2
+
O2
2 H2O
• Living cells carry out thousands of chemical
reactions that rearrange matter in significant
ways
Beta-carotene
Vitamin A
(2 molecules)