The%20Chemical%20Basis%20of%20Life

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Chapter 2

• The Chemical Basis of Life

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• 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

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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

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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)

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• 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

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LE 2-3
Sodium
Chlorine
Sodium Chloride
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• 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

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LE 2-4a
Electron cloud
6e
2e
Nucleus
2
Protons
6
Protons
Mass number 4
Mass number 12
2
Neutrons
6
Neutrons
2
Electrons
6
Electrons
Helium atom
Carbon atom
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• 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)

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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

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• 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

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• 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

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• 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

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• 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

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LE 2-7b
Na
Cl-
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• 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

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• 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

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LE 2-9
A water molecule
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• 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

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LE 2-10
Hydrogen bond
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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
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• 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

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• Water also moderates temperature by evaporative
  cooling
• The surface cools as the hottest molecules leave

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• 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

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• 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

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LE 2-14
Salt crystal
Ion in solution
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• 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

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LE 2-15
pH scale
H
H
H
OH?
H
Lemon juice, gastric juice
H
OH?
H
H
H
Grapefruit juice, soft drink
Increasingly ACIDIC (Higher concentration of H)
Acidic solution
Tomato juice
Human urine
OH?
OH?
NEUTRAL H????OH-
Pure water
OH?
H
H
Human blood
OH?
OH?
H
H
H
Seawater
Neutral solution
Increasingly BASIC (Lower concentration of H)
Milk of magnesia
Household ammonia
OH?
OH?
OH?
OH?
H
Household bleach
OH?
OH?
H
Oven cleaner
Basic solution
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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

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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

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• Living cells carry out thousands of chemical
  reactions that rearrange matter in significant
  ways