chapter 2 class lecture_1

8/9/2019 Chapter 2 Class Lecture_1

1/33

1

Biology

Biology the study of life

Is founded on the principles of

chemistry and physics

All living organisms are acollection of atoms and molecules


2/33

2

Atoms

Smallest functional units of matter

Form all chemical substances and

Cannot be further broken down into othersubstances

Each specific type

of atom is a

chemical element


3/33

3

Electrons occupy orbitals

Scientists initially visualized an atom asa mini solar system

Electrons travel within regionssurrounding the nucleus (orbitals) in

which the probability is high of findingthat electron

Can be depicted as a cloud


4/33

4

Atoms with progressively moreelectrons have orbitals within

electron shells that are at greater andgreater distances from the center ofthe nucleus

1st shell - 1 spherical orbital (1s) - holds 2 electrons 2nd shell - 1 spherical orbital (2s) and 3 dumbbell-

shaped orbitals (2p) can hold 4 pairs of electrons

Atomic shells


5/33

5

Nitrogen example

An atom of nitrogen has seven protonsand seven electrons

2 electrons fill 1st shell

5 electrons in 2nd shell2 fill 2s orbital

1 each in the 3 p orbitals

Outer 2nd shell is not full


6/33

6

Atomic Number

Number of protons in an atom is itsatomic number

The atomic number is also equal to thenumber of electrons in the atom so that

the net charge is zero


7/33

7

Periodic table

Organized by atomic number

Rows correspond to number of electron shells

Columns, from left to right, indicate the numbers of

electrons in the outer shell

Similarities of elements within a column occur

because they have the same number of electrons in

their outer shells, and therefore they have similarchemical bonding properties


8/33

8

Mass Number

The second number associated with each atom is the

mass number. The mass number expresses the sum

of the masses of the particles in the atom.

proton has a mass number of 1, electron 0, therefore the

mass number indicates the number of proton and neutrons in

the nucleus.


9/33

9

Mass Number

Atomic mass scale indicates an atoms mass relativeto the mass of other atoms

Most common form of carbon has six protons and sixneutrons, is assigned an atomic mass of exactly 12

A hydrogen atom has an atomic mass of 1, indicating that

it has 1/12 the mass of a carbon atom A magnesium atom, with an atomic mass of 24, has twice

the mass of a carbon atom


10/33

10

Mass or weight?

Weight is derived from the gravitational pullon a given mass

If a man who weighs 154 pounds on earthwere standing on the moon, for example, hewould only weigh about 25 pounds, but hewould weigh 21 trillion pounds if he couldstand on a neutron star. However, his mass isthe same in all locations.


11/33

11

Units

Dalton

Unit of measurement for atomic mass

One dalton (d) equals 1/12 the mass of acarbon atom

Carbon has an atomic mass of 12 Daltons

Mole1 mole of any element contains the same

number of atoms6.022 x 1023

Avogadros number


12/33

12

Isotopes

Multiple forms of an element that differ

in the number of neutrons

12C contains 6 protons and 6 neutrons

14C contains 6 protons and 8 neutrons

Atomic masses are averages of the

weights of different isotopes of an

element


13/33

13

Hydrogen, oxygen, carbon, and

nitrogen Typically make up about 95% of the atoms in living

organisms

Hydrogen and oxygen occur primarily in water

Nitrogen is found in proteins

Carbon is the building block of all living matter

Mineral elements - less than 1%

Trace elements - less than 0.01%

Essential for normal growth and function


14/33

14

Chemical bonds and molecules

Molecule

2 or more atoms bonded together

Molecular formula

Contains chemical symbols of elementsfound in a molecule

Subscript indicates how many of eachatom are present


15/33

15

3 types of bonds

1. Covalent

Polar covalent

2. Hydrogen

3. Ionic


16/33

16

Covalent bonds

Atoms share a pair of electrons

Occurs between atoms whose outer electron shellsare not full

Covalent bonds are often the strongest of allchemical bonds, because the shared electronsbehave as if they belong to each atom

Can share

1 pair of electrons single bondH-F 2 pairs of electrons double bond O=O

3 pairs of electrons triple bond


17/33

17

Octet rule

Atoms are stable when their outer

shell is full

For many atoms, the outer shell

fills with 8 electrons

One exception is hydrogen, whichfills its outer shell with 2 electrons


18/33

18

Polar covalent bonds

When two atoms with different electronegativitiesform a covalent bond, the shared electrons aremore likely to be in the outer shell of the atom of

higher electronegativity rather than the atom oflower electronegativity

Polar covalent bonds occur because the

distribution of electrons around the atoms createsa polarity, or difference in electric charge, acrossthe molecule


19/33

19

Water

Classic example of polar covalent

bonds

Electrons tend to be in the moreelectronegative oxygen atom rather

than either of the less electronegative

hydrogen atoms

Molecule has partial negative charge

region and a partial positive charge

region


20/33

20

Hydrogen bonds

Weak polar covalent bonds

Represented as dashed or dotted lines

Collectively, can form strong bond overall Holds DNA strands together

Individually, weak bonds can form and break

easily Substrate and enzyme bonding


21/33

21

Ionic bonds

An ion is an atom or molecule that has gainedor lost one or more electrons

It now has a net electric charge

Cations- net positive charge

Anions- net negative charge

Ionic bond occurs when a cation binds to an

anion


22/33

22

Chemical Reactions

Occurs when one or more substances arechanged into other substances Reactants products

Share many properties All require a source of energy

Reactions in living organisms often require acatalyst (enzymes)

Tend to proceed in a particular direction but willeventually reach equilibrium

Occur in liquid environment - water


23/33

23

Properties of water

A solution is made up of the

Solvent- liquid

Solutes- substances dissolved in solvent

Aqueous solution- water is the solvent

Ions and molecules that contain polar covalent

bonds will dissolve in water


24/33

24

Hydrophillic- water-loving Readily dissolve in water Ions and molecules that contain polar covalent bonds

Hydrophobic- water-fearing

Do not readily dissolve in water Nonpolar molecules like hydrocarbon

Amphipathic molecule Have both polar or ionized regions at one or more sites

and nonpolar regions at other sites

May form micelles in water Polar (hydrophilic) regions at the surface of the micelle and

nonpolar (hydrophobic) ends are oriented toward the interior ofthe micelle


25/33

25

Solutions

Concentration

Amount of a solute dissolved in a unit volume of solution

1 gram of NaCl was dissolved in 1 liter of water = 1 g/L

Molarity

Number of moles of a solute dissolved in 1 L of water

1 mole of a substance is the amount of the substance in

grams equal to its atomic or molecular mass


26/33

26

H2O in 3 states of matter

Solid (ice), liquid (water), and gas (water

vapor)

Changes in state, such as changes betweenthe solid, liquid, and gas states of H2O, involve

an input or release of energy

Water is extremely stable as a liquid


27/33

27

Colligative properties of water

Depend strictly on the concentration of dissolvedsolute particles and not on the specific type ofparticle

Addition of solutes to water lowers its freezing pointbelow 0C and raises its boiling point above 100C

Some animals produce antifreeze molecules thatdissolve in their body fluids, thereby lowering the

freezing point of the fluids and preventing theirblood and cells from freezing in the extreme cold


28/33

28

Not just a solvent

Water has many important functions in livingorganisms

Participates in chemical reactions

Hydrolysis or dehydration

Provides force or support

Remove toxic waste components

Evaporative cooling

Cohesion and adhesion


29/33

29

Acids and Bases

Pure water has the ability to ionize to a very

small extent into hydrogen ions (H+) and

hydroxide ions (OH-)

In pure water

[H+][OH-] = [10-7 M][10-7 M] 10-14 M


30/33

30

Acids are molecules that release hydrogen

ions in solution

A strong acid releases more H+ than a weak acid

Bases lower the H+ concentration

Some release OH-

Others bind H+


31/33

31

pH

pH = log10 [H+]

Acidic solutions are pH 6 or below

pH 7 is neutral

Alkaline solutions are pH 8 or above


32/33

32

The pH of a solution can affect

The shapes and functions of molecules

The rates of many chemical reactions

The ability of two molecules to bind to each other

The ability of ions or molecules to dissolve in

water


33/33

33

Buffers

Organisms usually tolerate only small changes

in pH

Buffers help to keep a constant pH

An acid-base buffer system can shift to

generate or release H+ to adjust for changes in

pH

chapter 2 class lecture_1

Documents