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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???