Transcript The Chemical Basis of Life

The Chemical Basis of Life
Chemical Basis of Life?
What are we made of?
2.1 Living organisms are composed of about 25
chemical elements
• Arrangement of these elements eventually leads
to formation of living organisms
Copyright © 2009 Pearson Education, Inc.
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
• Different arrangements of elements provide
unique properties for each compound
Copyright © 2009 Pearson Education, Inc.
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?
Copyright © 2009 Pearson Education, Inc.
Electron
cloud
Nucleus
2e–
2
Protons
2
Neutrons
2
Electrons
Mass
number = 4
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
Copyright © 2009 Pearson Education, Inc.
PET scan used to image the brain
Healthy brain
Alzheimer’s patient
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
Copyright © 2009 Pearson Education, Inc.
Ionic bonding – transfer of electrons
• Electron transfer creates ions (charged atoms)
• Cation (positive ion); Anion (negative ion)
• Ex: Salts (sodium chloride)
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
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)
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.
Copyright © 2009 Pearson Education, Inc.
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
Copyright © 2009 Pearson Education, Inc.
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
Copyright © 2009 Pearson Education, Inc.
2.15 The chemistry of life is sensitive to acidic and basic
conditions
• 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)
Copyright © 2009 Pearson Education, Inc.
2.15 The chemistry of life is sensitive to acidic and basic
conditions
• 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–
Copyright © 2009 Pearson Education, Inc.
2.15 The chemistry of life is sensitive to acidic and basic
conditions
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
Copyright © 2009 Pearson Education, Inc.
pH scale
0
1
Acidic solution
Increasingly ACIDIC
(Higher concentration of H+)
Battery acid
2
Lemon juice, gastric juice
3 Grapefruit juice, soft drink,
vinegar, beer
4 Tomato juice
5
Rain water
6
Human urine
Saliva
NEUTRAL
[H+]=OH–]
7
Pure water
Human blood,
tears
8
Seawater
Increasingly BASIC
(Lower concentration of H+)
Neutral solution
9
10
Milk of magnesia
11
Household ammonia
12
Household bleach
13
Oven cleaner
Basic solution
14
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
Copyright © 2009 Pearson Education, Inc.
Resources for Chapter 2
Using your disc that came with your text, go to Student Home, Chapter 2: The Chemical Basis of
Life
1. Take the pre test
2. Complete activities.
3. Test yourself
4. Extend your knowledge
5. 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