Chapter 2Life’s Chemical Basis

Atomic NumberNumber of protonsAll atoms of an element have the same atomic

numberAtomic number of hydrogen = 1Atomic number of carbon = 6

Mass Number

Number of protons+

Number of neutrons

Isotopes vary in mass number

IsotopesAtoms of an element with different numbers

of neutrons (different mass numbers)Carbon 12 has 6 protons, 6 neutronsCarbon 14 has 6 protons, 8 neutrons

RadioisotopesHave an unstable nucleus that emits

energy and particlesRadioactive decay transforms

radioisotope into a different elementDecay occurs at a fixed rate

Radioisotopes as TracersTracer is substance with a

radioisotope attached to itEmissions from the tracer can be

detected with special devices Following movement of tracers is

useful in many areas of biology

Radioisotopes in MedicinePositron-Emission Tomography (PET) uses

radioisotopes to form images of body tissuesPatient is injected with tracer and put

through a PET scannerBody cells absorb tracer at different ratesScanner detects radiation caused by energy

from decay of the radioisotope, and radiation then forms an image

Image can reveal variations and abnormalities in metabolic activity

Other Uses of RadioisotopesDrive artificial pacemakers

Radiation therapyEmissions from some radioisotopes can destroy cells. Some radioisotopes are used to kill small cancers.

ELEMENTS IN LIVING THINGS4 major elements in all living things: O, C, H,

N (decreasing amounts)TRACE ELEMENTS—required only in small

amountsEx: iron, iodine, copper

What Determines Whether Atoms Will Interact?

The number and arrangement of their VALENCE electrons

Shell ModelFirst shell

Lowest energy

Holds 1 orbital with up to 2 electrons

Second shell4 orbitals

hold up to 8 electrons

HYDROGEN1p+ , 1e-

HELIUM2p+ , 2e-

CARBON6p+ , 6e-

OXYGEN8p+ , 8e-

SODIUM11p+ , 11e-

CHLORINE17p+ , 17e-

Electron VacanciesUnfilled shells make atoms likely

to react AND bond

Chemical Bonds, Molecules, & CompoundsBond is union between electron structures of

atomsAtoms bond to form molecules

Important Bonds in Biological MoleculesIonic BondsCovalent BondsHydrogen Bonds

Ionic BondingOne atom loses electrons, becomes

positively charged ionBetween metals & nonmetalsAnother atom gains these

electrons, becomes negatively charged ion

This type of bond is LEAST affected by water’s presence

Fig. 2-8a(2), p.24

Covalent BondingAtoms SHARE a pair or pairs of electrons to fill outermost shell

• Single covalent bond• Double covalent

bond• Triple covalent bond

Nonpolar Covalent BondsAtoms share electrons equally

Example: Hydrogen gas (H-H)

Polar Covalent Bonds

Unequal sharing of electronsEx: Water - Electrons more attracted

to O nucleus than to H nuclei

Water Is a Polar Covalent MoleculeMolecule has no net

charge

Oxygen end has a slight negative charge

Hydrogen end has a slight positive charge

+ +

HH

O

Hydrogen BondingThe weakest type of bond (20 x easier to break

than a covalent bond); are rapidly broken and made

This is what holds water molecules together

The more hydrogen bonds in a molecule= the more stable the molecule

Hydrophilic & HydrophobicSubstances

Hydrophilic substancesPolarLike water (so hydrogen bond

to it)Example: sugar

Hydrophobic substancesNonpolarRepelled by waterExample: oil

Properties of WaterPolar (dissolves polar solutes)

Temperature-Stabilizing EffectsWater can absorb much heat before its

temperature rises (has a high specific heat capacity)

How is this important for aquatic organisms?

Evaporation of WaterLots of heat must be added to break H-bonds

& vaporize water

As it vaporizes, it carries a lot of heat with it(lower the temperature)—AKA high heat of vaporization

Evaporative water loss is used by mammals to lower body temperature

WHY SOLID WATER (ICE )FLOATS IN LIQUID WATER

In ice, hydrogen bonds lock molecules in a lattice

Water molecules in lattice are spaced farther apart then those in liquid water

Ice is less dense than water(water contracts when cooled to 4 C, but expands from 4 to O C

This is why lakes freeze from top to bottom (helps insulate aquatic life below)

High Cohesion & AdhesionInvolves hydrogen bondsCohesion—water molecules stick

to each other; Adhesion—water molecules stick to other things

Creates surface tension (Ex: water striders)

Allows water to move as continuous column upward through stems of plants(AKA transpiration)

The pH ScaleMeasures H+ (hydronium ion)concentrationChange of 1 on scale means 10X change in

H+ concentrationEx: a substance with pH of 2 vs. a pH of 6 is 10,000 more acidic

Highest H+ Lowest H+

0---------------------7-------------------14Acidic Neutral Basic

Acids & BasesAcids

Donate H+ (hydronium) when dissolved in water

Acidic solutions have pH < 7

Bases

Accept H+ when dissolved in water; contain

hydroxide ion

Acidic solutions have pH > 7

More hydronium = lower pH = more acidic

Weak and Strong AcidsWeak acids

Reluctant H+ donors Can also accept H after giving it up

Carbonic acid (H2CO3) is example

Strong acidsCompletely give up H+ when dissolvedHydrochloric acid (HCl) is example

ACID RAIN

A coal-burning power plant emits sulfur dioxide, which dissolves in water vapor to form acid rain (pg. 29)

Fig. 2-13, p.29

Buffer SystemsMinimizes or helps prevent changes in pH

Partnership between weak acid and base it forms when dissolved

Two work as pair to counter shifts in pH

Carbonic Acid-Bicarbonate Buffer System—found in our blood & in the oceanWhen blood pH rises due to increased

carbon dioxide levels, carbonic acid breaks

apart to form bicarbonate and H+

H2C03 -----> HC03- + H+

When blood pH drops, bicarbonate binds H+

to form carbonic acid

HC03- + H+ -----> H2C03