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TRANSCRIPT
Life’s Chemical Basis
Chapter 2
Why are we studying chemistry?
Chemistry is the foundation of biology
Atoms
Determine the behavior of elementsmake up all living things
Building blocks of all substancesprotons, neutrons, electrons
Chargeelectrical property
attracts or repels other subatomic particles
Subatomic Particles
Electronsnegative chargemove around the nucleus
Protonspositive chargein the nucleus
Neutronsno chargein the nucleus
Characteristics of Atoms
Atoms differ in the # of subatomic particles:
Atomic Numbernumber of protons
determines the element
Mass Numbernumber of protons and neutrons
determines isotopes
Matter
Proton Neutron Electron
Hydrogen1 proton1 electron
Oxygen8 protons8 neutrons8 electrons
+ 0 –
Everything is made of matter
Matter is made of atoms
The World of Elements
Periodic Table
An arrangement of the elements based on their atomic number and chemical properties
Essential Elements of Life
~25 elements are essential to life
4 make up 96% of living matterCarbon
Hydrogen
Oxygen
Nitrogen
4 make up almost the rest
Phosporus
Sulfur
Calcium
Potassium
Bonding Properties
Electrons Determine the chemical behavior of an atom
Depends on # of valence e- (outer shell)
How does this atom behave?
Bonding Properties
How does this atom behave? How does this atom behave?
Why Atoms Interact
The shell model of electron orbitals diagrams electron vacancies; filled from inside outFirst shell: one orbital (2 electrons)
Second shell: four orbitals (8 electrons)
Third shell: four orbitals (8 electrons)
Atoms with vacancies in their outer shell tend to give up, acquire, or share electrons
Elements & Their Valence Shells
Elements in the same row have the same number of shells
Moving from left to right, each element has a sequential addition of electrons (& protons)
Elements & Their Valence Shells
Elements in the same column have the same valence & similar
chemical properties
Atoms & Ions
IonAn atom with a positive or negative charge due
to loss or gain of electrons in its outer shell
Examples: Na+, Cl-
ElectronegativityA measure of an atom’s ability to pull electrons
from another atom
Chemical Reactivity
Atoms tend to either:1. Complete a partially filled valence shell
-or-
1. Empty a partially filled valence shell
–
–
–
This tendency drives chemical reactions…
and creates bonds
From Atoms to Molecules
Weak bonds:Hydrogen bonds Attraction b/w + and –
Van der Waals forcesAttraction b/w hot spots
of + and – due to asymmetrical electron distribution
Ionic bondsAttraction b/w anion and
cation
Ionic Bonds
A strong mutual attraction between two oppositely charges ions with a large difference in electronegativity (an electron is not transferred)
Example: NaCl (table salt)
Covalent Bonds
Strong bonds2 atoms share 1, 2, or 3 pairs of electrons
Very stable
Form molecules
–
–
H2 (hydrogen gas)
H — H
H
H
OH
HO
Multiple Covalent Bonds
2 atoms can share >1 pair of electronsDouble bonds2 pairs of e-
Triple bonds3 pairs of e-
VERY strong bonds
More isbetter!
Nonpolar Covalent Bond
Pair of electrons shared equally by 2 atomsEx: methane (CH4)
balanced, stable,good building block
Polar Covalent Bond
Pair of electrons shared unequally by 2 atomsEx: water (H2O)Oxygen has a stronger
attraction to the e-
More electronegative
Water is a polar molecule+ vs – poles
leads to many interesting properties of water
H
H
Oxygen
Hydrogen Bonds
Polar water creates molecular attractionsAttraction b/w one +H of
one molecule and one –O of another molecule
Remember, it is a weak bond
Stabilize the structures of large biological molecules
Chemical Reactions
Make and break chemical bonds
ReactantsStarting molecules
ProductsFinal molecules
Ex: 6 CO2 + 6 H20 → C6H12O6 + 6 O2
Why Are We Studying Water?
All life occurs in waterInside and outside the cell
Chemistry of Water
H2O is a polar moleculePolar covalent bonds
+ and - poles
Chemistry of Water
Hydrogen bondsform between 2 H2O molecules
+ attracted to –
Sticky molecule
4 Properties of Water
Facilitate an environment for life:Cohesive behavior
Ability to moderate temperature
Expansion upon freezing
Versatility as a solvent
Cohesion
Created by H-bonding between H2O moleculesH2O is “sticky”
Surface tension
Adhesion
H2O molecules form bonds with other substancesCapillary action
Meniscus
H2O climbs up
fiberEx: paper towel
Transpiration
How water gets to the top of a tree
Moderation of Temperature
H2O resists changes in temp.High specific heat
Takes a lot to heat it up
Takes a lot to cool it down
H2O moderates temperatures on Earth
Heat of Vaporization
Evaporative Cooling
Organisms rely on heat of vaporization to remove heat
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
And this hasmade all the difference!
Ice Floats
Why is “ice floats” important?
Oceans & lakes don’t freeze solidSurface ice insulates water belowAllows life to survive the winterIf ice sank…Ponds, lakes, & oceans would freeze solidIn summer, only a few inches would thawSeasonal turnover of lakesnutrient cycling
Water is the Solvent of Life
SolventThe dissolving agent
SoluteThe substance that is
dissolved
SolutionA liquid that is a
homogenous mixture of substances
Water is the Solvent of Life
Polarity makes H2O a good solventPolar H2O molecules surround + and – ions
What Dissolves in Water?
Hydrophilic moleculesSubstances have an attraction to H2O
Polar or nonpolar?
What Doesn’t Dissolve in H2O?
Hydrophobic substancesDon’t have an affinity for H2O
Polar or nonpolar? Oh, lookhydrocarbons!
fat (triglycerol)
Ionization of Water and pH
Water ionizesH+ splits off from H2O, leaving OH–if [H+] = [-OH], water is neutral
if [H+] > [-OH], water is acidic
if [H+] < [-OH], water is basic
pH scalehow acid or basic solution is
1 7 14
H2O H+ + OH–
tenfold changein H+ ions
pH1 pH210-1 10-2
10 times less H+
pH8 pH710-8 10-7
10 times more H+
pH10 pH810-10 10-8
100 times more H+
Buffers & Cellular Regulation
pH of cells must be kept ~7pH affects shape of molecules
shape of molecules affect function
pH affects cellular function
Control pH by buffersreservoir of H+donate H+ when
[H+] falls
absorb H+ when [H+] rises
100
1
2
3
4
5
6
7
8
9
3Amount of base added
Bufferingrange
4 52
pH
Review!!
1. Many mammals control their body temperature by sweating. Which property of water is most directly responsible for the ability of sweat to lower body temperature?
a) Water’s change in density when it condenses
b) Water’s ability to dissolve molecules in the air
c) The release of heat by the formation of hydrogen bonds
d) The absorption of heat by the breaking of hydrogen bonds
e) Water’s high surface tension
2. The following are pH values: cola-2; orange juice-3; beer-4; coffee-5; human blood-7.4. Which of these liquids has the highest molar concentration of OH-?
a) cola
b) orange juice
c) beer
d) coffee
e) human blood
3. Based on your knowledge of the polarity of water, the solute molecule is most likely _____.
a) positively charged
b) negatively charged
c) neutral in charge
d) hydrophobic
e) nonpolar
4. If the pH of a solution is increased from pH 8 to pH 9, it means that the
a) concentration of H+ is 10 times greater than what it was at pH 8.
b) concentration of H+ is 100 times less than what it was at pH 8.
c) concentration of OH- is 10 times greater than what it was at pH 8.
d) concentration of OH- is 100 times less than what it was at pH 8.
e) concentration of H+ is greater and the concentration of OH- is less than at pH 8.
Questions???