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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
2
2
2
Protons
Neutrons
Electrons
Helium atom
Mass
number = 4
6
6
6
Protons
Neutrons
Electrons
Carbon atom
Mass
number = 12
Electron
cloud
Nucleus
2e–
6e–
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
Animation: Ionic Bonds
LE 2-7a-1
Transfer of
electron
Na
Sodium atom
Cl
Chlorine atom
LE 2-7a-2
Na+
Sodium ion
Cl-
Chloride ion
Sodium chloride (NaCl)
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
Animation: Covalent Bonds
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
Animation: Water Structure
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, three-dimensional structure
Ice is less dense than water, because it has fewer molecules in the same volume
LE 2-13
Hydrogen bond
Ice
Hydrogen bonds are stable
Liquid water
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
Acidic solution
OH
H+
H+
H+
H+
OH
H+
H+
H+
OH
OH
OH
OH
OH
H+
H+
H+
H+
H+
H+
Increasingly ACIDIC
(Higher concentration of H+)
Neutral solution
OH
OH
OH
OH
OH
OH
H+
H+
Basic solution
NEUTRAL
[HOH-]
pH scale
Lemon juice, gastric juice
Grapefruit juice, soft drink
Tomato juice
Human urine
Pure water
Human blood
Seawater
Milk of magnesia
Household ammonia
Household bleach
Oven cleaner
Increasingly BASIC
(Lower concentration of H+)
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
LE 2-17a
+
2 H2
O2
2 H2O
+
Living cells carry out thousands of chemical reactions that rearrange matter in significant ways
LE 2-17b
Beta-carotene
Vitamin A
(2 molecules)