AP Biology 2005-2006 Chapter 2. The Chemical Context of Life

AP Biology 2005-2006

Chapter 2.

The Chemical Context of Life


Why are we studying chemistry?

Biology has chemistry at its foundation


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


The World of Elements


Models of atoms

(c)


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


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)



Isotopes Different number of neutrons (heavier) Some are unstable

nuclear reactions / decay

Split off neutrons &/or protons radioactivity

Biological tool Biological hazard


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?


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?


Elements & their valence shells

Elements in the same row have the same number of shells



Elements in the same column have the same valence & similar chemical properties



Moving from left to right, each element has a sequential addition of electrons (and protons)


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


Ionic bonds


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)


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


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


Multiple covalent bonds 1 atom can form covalent bonds with

two or more other atoms forms larger molecules ex. carbon


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


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….



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


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


Any Questions??


Chapter 3.

Water— The Elixir of Life!


Why are we studying water?

All life occurs in water inside & outside the cell


Chemistry of water Water is polar molecule

remember polar covalent bonds + & – poles


Chemistry of water H2O molecules form H bonds with each

other + attracted to – creates a sticky

molecule



Cohesion H bonding between H2O creates

cohesion water is “sticky” surface tension drinking straw

can you suck sugar up a straw?


How does H2O get to top of tree? Transpiration



Adhesion H2O molecules form H bonds with other

substances capillary action meniscus water climbs up fiber

ex. paper towel


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


Hydrophilic Hydrophilic

substances have affinity for H2O polar or non-polar? ionic


Hydrophobic Hydrophobic

substances do not have affinity for H2O polar or non-polar? non-ionic

fat (triglycerol)


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


Ice floats


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


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



Evaporative cooling Organisms rely on

heat of vaporization to remove heat


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


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


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-]


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-]


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


Any Questions??

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AP Biology 2005-2006 Chapter 2. The Chemical Context of Life