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AP Biology 2005-2006
Chapter 2.
The Chemical Context of Life
AP Biology 2005-2006
Why are we studying chemistry?
Biology has chemistry at its foundation
AP Biology 2005-2006
Everything is made of matter Matter is made of atoms Atoms are made of:
protons + mass of 1 nucleus neutrons 0 mass of 1 nucleus electrons - mass <<1 orbits
Different kinds of atoms = elements
The Basics
AP Biology 2005-2006
The World of Elements
AP Biology 2005-2006
Models of atoms
(c)
AP Biology 2005-2006
Atomic structure determines behavior
The number of protons in an atom determines the element # of protons = atomic number this also tells you # of electrons
All atoms of an element have same chemical properties all behave the same properties don’t change
AP Biology 2005-2006
Life requires ~25 chemical elements
About 25 elements are essential for life Four elements make up 96% of living
matter: • carbon (C) • hydrogen (H)
• oxygen (O) • nitrogen (N) Four elements make up most of
remaining 4%: • phosphorus (P) • calcium (Ca)
• sulfur (S) • potassium (K)
AP Biology 2005-2006
AP Biology 2005-2006
Isotopes Different number of neutrons (heavier) Some are unstable
nuclear reactions / decay
Split off neutrons &/or protons radioactivity
Biological tool Biological hazard
AP Biology 2005-2006
Bonding properties
Effect of electrons chemical behavior of an
atom depends on its electron arrangement
depends on the number of electrons in its outermost shell, the valence shell
How does this atom behave?
AP Biology 2005-2006
Bonding properties
Effect of electrons chemical behavior of an atom depends on
number of electrons in its outermost shell
How does this atom behave? How does this atom behave?
AP Biology 2005-2006
Elements & their valence shells
Elements in the same row have the same number of shells
AP Biology 2005-2006
Elements & their valence shells
Elements in the same column have the same valence & similar chemical properties
AP Biology 2005-2006
Elements & their valence shells
Moving from left to right, each element has a sequential addition of electrons (and protons)
AP Biology 2005-2006
Chemical reactivity Atoms tend to
Complete a partially filled outer (valence) electron shell
or Empty a partially filled outer (valence)
electron shell
This tendency drives chemical reactions
AP Biology 2005-2006
Ionic bonds
AP Biology 2005-2006
Ionic bonds Transfer of an electron Forms + & - ions
+ = cation – = anion
Weak bond
example: salt = dissolves
easily in water
“Let’s go to the video tape!”(play movie here)
AP Biology 2005-2006
Covalent bonds Two atoms need an electron Share a pair of electrons Strong bond
both atoms holding onto the electrons Forms molecules
example: water = takes energy to separate
AP Biology 2005-2006
Double covalent bonds Two atoms can share more than one
pair of electrons double bonds (2 pairs of electrons) triple bonds (3 pairs of electrons)
Very strong bonds
AP Biology 2005-2006
Multiple covalent bonds 1 atom can form covalent bonds with
two or more other atoms forms larger molecules ex. carbon
AP Biology 2005-2006
Polar covalent bonds Pair of electrons not shared equally by
2 atoms Water = O + H
oxygen has stronger “attraction” for the shared electrons than hydrogen
oxygen has higher electronegativity
AP Biology 2005-2006
Polar covalent bonds 2 hydrogens in the water molecule form
an angle Water molecule is polar
oxygen end is – hydrogen end is +
Leads to many interesting properties of water….
“Let’s go to the video tape!”(play movie here)
AP Biology 2005-2006
Hydrogen bonds Positive H atom in
1 water molecule is attracted to negative O in another
Can occur wherever an -OH exists in a larger molecule
Weak bonds
AP Biology 2005-2006
Reductionist view of biology Matter is made of atoms Life requires ~25 chemical elements Atomic structure determines behavior of an
element Atoms combine by chemical bonding to form
molecules Weak chemical bonds play important roles in
chemistry of life A molecule’s biological function is related to its
shape Chemical reactions make & break chemical bonds
AP Biology 2005-2006
Any Questions??
AP Biology 2004-2005
Chapter 3.
Water— The Elixir of Life!
AP Biology 2004-2005
Why are we studying water?
All life occurs in water inside & outside the cell
AP Biology 2004-2005
Chemistry of water Water is polar molecule
remember polar covalent bonds + & – poles
AP Biology 2004-2005
Chemistry of water H2O molecules form H bonds with each
other + attracted to – creates a sticky
molecule
“Let’s go to the video tape!”(play movie here)
AP Biology 2004-2005
Cohesion H bonding between H2O creates
cohesion water is “sticky” surface tension drinking straw
can you suck sugar up a straw?
AP Biology 2004-2005
How does H2O get to top of tree? Transpiration
“Let’s go to the video tape!”(play movie here)
AP Biology 2004-2005
Adhesion H2O molecules form H bonds with other
substances capillary action meniscus water climbs up fiber
ex. paper towel
AP Biology 2004-2005
Water is the solvent of life H2O is a good solvent due to its polarity
polar H2O molecules surround + & – ions solvents dissolve solutes creating
aqueous solutions
AP Biology 2004-2005
Hydrophilic Hydrophilic
substances have affinity for H2O polar or non-polar? ionic
AP Biology 2004-2005
Hydrophobic Hydrophobic
substances do not have affinity for H2O polar or non-polar? non-ionic
fat (triglycerol)
AP Biology 2004-2005
The special case of ice Most (all?) substances are more
dense when they are solid But not water… Ice floats!
H bonds form a crystal with loose structure
AP Biology 2004-2005
Ice floats
AP Biology 2004-2005
Why is “ice floats” important? Oceans & lakes don’t freeze solid
if ice sank… eventually all ponds, lakes & even ocean would
freeze solid during summer, only upper few inches would
thaw surface ice insulates water below
allowing life to survive the winter seasonal turnover of lakes
cycling nutrients
AP Biology 2004-2005
Specific heat H2O has high specific heat
due to H bonding
H2O resists changes in temperature takes a lot to heat it up takes a lot to cool it down
H2O moderates temperatures on Earth
AP Biology 2004-2005
AP Biology 2004-2005
Evaporative cooling Organisms rely on
heat of vaporization to remove heat
AP Biology 2004-2005
Water forms ions Hydrogen ion (H+) splits off from water to
leave a hydroxide ion (-OH)
H20 ----> H+ + -OH
If concentration of 2 ions is equal, water is neutral
If [H+] > [-OH], water is acidic If [-OH] > [H+], water is basic pH scale = how acidic or basic a solution is
AP Biology 2004-2005
pH Scale In pure water only 1 water molecule in
every 554 million is dissociated. very small amount of ions [H+] or [OH-] is 10-7M
[H+] [OH-] = 10-14
pH scale is based on this equation
AP Biology 2004-2005
pH Scale In neutral solution
[H+] = 10-7 pH = 7 Values for pH decline
as [H+] increase Acids
adding acid increases [H+]
Bases adding base
increases [OH-]
AP Biology 2004-2005
pH & Biology pH of a neutral solution = 7 Acidic solutions = pH < 7
Basic solutions = pH > 7 Most biological fluids have pH 6 – 8
pH values in human stomach can reach 2
Each pH unit represents a 10-fold difference in H+ & OH- concentrations. small change in pH actually indicates a
substantial change in [H+] & [OH-]
AP Biology 2004-2005
Water is a polar molecule The special properties of water make
life on Earth possible The chemical behavior of water
governs how organisms function
Punchline
AP Biology 2004-2005
Any Questions??