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CHAPTER 2 The Chemical Basis of Life

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Title: CHAPTER 2 The Chemical Basis of Life


1
CHAPTER 2The Chemical Basis of Life
2
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

3
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

4
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.

5
Elements can combine to form compounds
  • Chemical elements combine in fixed ratios to form
    compounds
  • Example sodium chlorine ? sodium chloride

6
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

7
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)

8
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

9
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

10
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

11
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
12
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

13
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

14
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

15
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

16
Like no other common substance, water exists in
nature in all three physical states
  • as a solid
  • as a liquid
  • as a gas

17
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

18
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

19
Water moderates temperatures
  • A water molecule takes a large amount of energy
    with it when it evaporates
  • This leads to evaporative cooling

20
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

21
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

22
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).

23
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

24
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-

25
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

26
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

27
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

28
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
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