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
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
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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?
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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
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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
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Ionic bonding – transfer of electrons
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•
•
•
Electron transfer creates ions (charged atoms)
Cation (positive ion); Anion (negative ion)
Ex: Salts (sodium chloride)
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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.
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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
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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
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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–
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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)
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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
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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
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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