The Chemical Basis of Life Chemical Basis of Life? What are we made of? What are we made of?

The Chemical Basis of Life

Chemical Basis of Life? Chemical Basis of Life? What are we made of?What are we made of?

2.1 Living organisms are composed of about 25 chemical elements

Describe the importance of chemical elements to living organisms

• Chemicals are at the base level of biological hierarchy

• Arrangement of these elements eventually leads to formation of living organisms

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2.3 Elements can combine to form compounds

Explain the formation of compounds• Compound—a substance consisting of two or more

different elements combined in a fixed ratio• Many of the compounds in living organisms contain

C, H, N, and O.– DNA, for example, contains all four of these elements

• Different arrangements of elements provide unique properties for each compound


2.4 Atoms consist of protons, neutrons, and electrons

Describe the structure of an atom• An atom is the smallest unit of matter that still

retains the properties of a element– Proton—has a single positive electrical charge– Electron—has a single negative electrical charge– Neutron—is electrically neutral

Demonstration on charge – Do opposites really attract?


Electroncloud

Protons

2e–

Nucleus

Electrons

Massnumber = 4

Neutrons

2

2

2

2.4 Atoms consist of protons, neutrons, and electrons

• Although all atoms of an element have the same atomic number, some differ in mass number– Isotopes have the same numbers of protons and

electrons but different numbers of neutrons– Unlike 12C, 14C is an unstable (radioactive) isotope that

gives off energy


Healthy brain Alzheimer’s patient

PET scan used to image the brain

2.6 Electron arrangement determines the chemical properties of an atom

• Only electrons are involved in chemical activity• Atoms want to fill their outer electron shells

– To accomplish this, the atom can share, donate, or receive electrons– This results in attractions between atoms called chemical bonds

Distinguish between ionic, hydrogen, and covalent bonds


Ionic bonding – transfer of electrons• Electron transfer creates ions (charged atoms)• Cation (positive ion); Anion (negative ion)• Ex: Salts (sodium chloride)• file:///C:/Users/Pringle/AppData/Local/Temp/

02_07AIonicBonds_A%20%282%29.html

02_07AIonicBonds_A.html

Covalent Bonding – sharing pairs of electrons

• Number of electrons required to complete an atom’s valence shell determines how many bonds will form

• Ex: Hydrogen & oxygen bonding in water; methane

02T_08CovalentBonds_A.html

Polar/nonpolar covalent bonds

• Electronegativity-attraction for electrons

• Nonpolar covalent -electrons shared equally Ex: diatomic H-H

• Polar covalent-one atom more electronegative than the other (charged)Ex: water H-O-H

Hydrogen bonds

• In water, bond formed by charge differential between hydrogen and oxygen(electronegativity)

02_10WaterStructure_A.html

2.11 Hydrogen bonds make liquid water cohesive

List and define the life-supporting properties of water

• Hydrogen bonding causes molecules to stick together, a property called cohesion– This is useful in plants that depend upon cohesion to

help transport water and nutrients up the plant

Activities with water to demonstrate cohesion, surface tension, and cohesion.


02_11WaterTransport_A.html

2.14 Water is the solvent of life

• A solution is a liquid consisting of a uniform mixture of two or more substances– The dissolving agent is the solvent– The substance that is dissolved is the solute

Demonstration of a solution


2.15 The chemistry of life is sensitive to acidic and basic conditions

• A few water molecules can break apart into ions– Some are hydrogen ions (H+)– Some are hydroxide ions (OH–)

– A balance between the two is critical for chemical processes to occur in a living organism



• Chemicals other than water can contribute H+ to a solution– They are called acids– An example is hydrochloric acid (HCl)

• An acidic solution has a higher concentration of H+ than OH–



• Some chemicals accept hydrogen ions and remove them from solution– These chemicals are called bases– For example, sodium hydroxide (NaOH) provides

OH– that combines with H+ to produce H2O (water)



Explain the pH scale and the formation of acid and base solutions

• A pH scale (pH = potential of hydrogen) is used to describe whether a solution is acidic or basic– pH ranges from 0 (most acidic) to 14 (most basic)– A solution that is neither acidic or basic is neutral

(pH = 7)

Activity -testing the pH of various solutions


Acidic solution

pH scale

Battery acid

0

1

2

3

4

5

Lemon juice, gastric juice

Grapefruit juice, soft drink,vinegar, beer

Tomato juice

Rain water

Human urine

Saliva

Pure water

6

7Human blood,tears

Seawater

8

9

10

11

12

13

Milk of magnesia

Household ammonia

Household bleach

Oven cleaner

Neutral solution

Basic solution

NEUTRAL[H+]=OH–]

Incr

easi

ngly

ACI

DIC

(Hig

her c

once

ntra

tion

of H

+ )

14

Incr

easi

ngly

BAS

IC(L

ower

con

cent

ratio

n of

H+ )

2.18 Chemical reactions make and break bonds, changing the composition of matter

Define a chemical reaction and explain how it changes the composition of matter

• The formation of water from hydrogen and oxygen is an example of a chemical reaction

• The reactants (H2 and O2) are converted to H2O, the product– Photosynthesis is an example where plants drive a

sequence of chemical reactions that produce glucose


Resources for Chapter 2Using your disc that came with your text, go to Student Home, Chapter 2: The Chemical Basis of

Life1. Take the pre test2. Complete activities.3. Test yourself4. Extend your knowledge5. Current events -On line article New Glimpses of Life’s Puzzling Origins - go to

http://www.nytimes.com/2009/06/16/science/16orig.html?_r=1&scp=9&sq=rna&st=cse and answer questions – hand in

http://www.nytimes.com/2009/06/16/science/16orig.html?_r=1&scp=9&sq=rna&st=cse

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The Chemical Basis of Life Chemical Basis of Life? What are we made of? What are we made of?