CHAPTER 2 The Chemical Basis of Life

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CHAPTER 2The Chemical Basis of Life
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ATOMS AND MOLECULES The emergence of biological
function starts at the chemical level

• Everything an organism is and does depends on
  chemistry
• Chemistry is in turn dependent on the arrangement
  of atoms in molecules

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Molecules and ecosystems are at opposite ends of
the biological hierarchy

• Each level of organization in the biological
  hierarchy builds on the one below it
• At each level, new properties emerge

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Elements

• An element is a substance that cant be broken
  down into simpler chemical substances.
• About 25 different chemical elements are
  essential to life
• Carbon, hydrogen, oxygen, and nitrogen make up
  more that 96 percent of the mass of a human body.
• Trace elements such as iron and copper, play a
  vital role in maintaining healthy cells in all
  organisms.

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Elements can combine to form compounds

• Chemical elements combine in fixed ratios to form
  compounds
• Example sodium chlorine ? sodium chloride

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Atoms The Building Blocks of Elements

• An atom is the smallest particle of an element
  that has the characteristics of that element.
• Atoms are the basic building blocks of all
  matter.
• Different elements have different types of atoms

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Atoms consist of protons, neutrons, and electrons

• An atom is made up of protons ( charge) and
  neutrons (no charge) located in a central
  nucleus
• The nucleus is surrounded by electrons ( -
  charge)

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Isotopes of an Element

• Atoms of each element are distinguished by a
  specific number of protons
• The number of neutrons may vary
• Variant forms of an element are called isotopes
• Some isotopes are radioactive

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Connection Radioactive isotopes can help or harm
us

• Radioactive isotopes can be useful tracers for
  studying biological processes
• PET scanners use radioactive isotopes to create
  anatomical images

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Electron arrangement determines the chemical
properties of an atom

• Electrons are arranged in shells
• The outermost shell determines the chemical
  properties of an atom
• In most atoms, a full outer shell holds eight
  electrons
• Atoms whose shells are not full tend to interact
  with other atoms and gain, lose, or share
  electrons

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Ionic bonds are attractions between ions of
opposite charge

• When atoms gain or lose electrons, charged atoms
  called ions are created
• An electrical attraction between ions with
  opposite charges results in an ionic bond
• Sodium and chloride ions bond to form sodium
  chloride, common table salt

Na Sodium atom
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Covalent bonds, the sharing of electrons, join
atoms into molecules

• Some atoms share outer shell electrons with other
  atoms, forming covalent bonds
• Atoms joined together by covalent bonds form
  molecules

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THE PROPERTIES OF WATER 2.9 Water is a polar
molecule

• Atoms in a covalently bonded molecule may share
  electrons equally, creating a nonpolar molecule
• If electrons are shared unequally, a polar
  molecule is created

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In a water molecule, oxygen exerts a stronger
pull on the shared electrons than hydrogen

• This makes the oxygen end of the molecule
  slightly negatively charged
• The hydrogen end of the molecule is slightly
  positively charged
• Water is therefore a polar molecule

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2.10 Overview Waters polarity leads to
hydrogen bonding and other unusual properties

• The charged regions on water molecules are
  attracted to the oppositely charged regions on
  nearby molecules
• This attraction forms weak bonds called hydrogen
  bonds

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Like no other common substance, water exists in
nature in all three physical states

• as a solid
• as a liquid
• as a gas

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2.11 Hydrogen bonds make liquid water cohesive

• Due to hydrogen bonding, water molecules can move
  from a plants roots to its leaves
• Insects can walk on water due to surface tension
  created by cohesive water molecules

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2.12 Waters hydrogen bonds moderate temperature

• It takes a lot of energy to disrupt hydrogen
  bonds
• Therefore water is able to absorb a great deal of
  heat energy without a large increase in
  temperature
• As water cools, a slight drop in temperature
  releases a large amount of heat

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Water moderates temperatures

• A water molecule takes a large amount of energy
  with it when it evaporates
• This leads to evaporative cooling

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2.13 Ice is less dense than liquid water

• Molecules in ice are farther apart than those in
  liquid water
• Ice is therefore less dense than liquid water,
  which causes it to float

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2.14 Water is a versatile solvent

• Solutes whose charges or polarity allow them to
  stick to water molecules dissolve in water
• They form aqueous solutions

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Mixtures and Solutions

• A mixture is a combination of substances in
  which the individual components retain their
  own properties
• Neither component of the mixture changes.
• A solution is a mixture in which one or more
  substances (solutes) are distributed evenly in
  another substance (solvent).

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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, and one that accepts H ions in solution
  is a base

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Dissociation of Water

• Occasionally, a hydrogen atom shared by two water
  molecules shifts from one molecule to the other
• The hydrogen atom leaves its electron behind and
  is transferred as a single proton - a hydrogen
  ion (H).
• The water molecule that lost a proton is now a
  hydroxide ion (OH-).
• H2O ltgt H OH-

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Dissociation of Water

• At equilibrium the concentration of water
  molecules greatly exceeds that of H and OH-.
• Adding certain solutes, called acids and bases,
  disrupts the equilibrium and modifies the
  concentrations of hydrogen and hydroxide ions

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pH scale

• A compound that releases H ions in solution is
  an acid, and one that accepts H ions in solution
  is a base thus increasing OH- ions
• The pH scale is used to describe how acidic or
  basic a solution is.
• 0-7 is acidic
• 8-14 is basic
• Pure water and solutions that are neither basic
  nor acidic are neutral, with a pH of 7

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Neutralization

• Neutralization occurs when an acid is mixed with
  a base (correct amount) producing a neutral
  solution
• H OH- -? H2O
• Another product ,a salt is also formed
• HCl NaOH -gt H2O NaCl

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Buffers

• pH can affect chemical reactions in an organism
• Cells are kept close to pH 7 by buffers
• Buffers are substances that resist pH change
• They accept H ions when they are in excess and
  donate H ions when they are depleted
• Buffers are not foolproof