Transcript Slide 1

Unit 2 –
The Chemical
Basis of Life
I. Life requires about 25 Chemical Elements
A. Elements
1. Matter – anything that occupies
space and has mass
2. The various forms of matter are
composed of one or more chemical
elements
a) Element- a pure substance
that cannot be broken down
into other substances by
chemical means
3. About 25 elements are essential to life.
a) Oxygen (O), Carbon (C), Hydrogen
(H), and Nitrogen (N) make up about
96% of the living matter in your body.
b) Calcium (Ca), Phosphorus (P),
Potassium (K), Sulfur (S), and a few other
elements account for most of the
remaining 4%.
c) Trace elements are elements that make up
less than 0.01% of your body mass, but are
critical to your health
1) Example: Iron (Fe); essential for
carrying oxygen in your blood
B. Compounds
1. A compound is a substance
containing two or more elements
that are chemically combined in a
fixed ratio.
a) Example: water (H2O)
2. A compound’s properties may differ
greatly from those of its
component
elements.
a) Example: Salt (NaCl)
II. Chemical properties are based on the
structure of atoms
A. Atoms
1. An atom is the smallest
possible particle of an element
a) comes from the Greek
word atomos meaning
“indivisible”
2. Structure of the atom
a) Proton- a subatomic particle with a
positive charge (+); found in the nucleus
b) Electron- a subatomic particle with a
negative charge (-); found outside the
nucleus in energy levels
c) Neutron- a subatomic particle that is
neutral (no charge); found in the nucleus
d) Nucleus- the center of the atom
containing protons and neutrons
Neutrons
Color Purple
Electrons (E-)
E-
E-
Protons +
P+
Color Green
N
E-
E-
P+
N
P+
N
N
Color Red
P+
N
P+
N
Nucleus
E-
Energy Level – Color Blue
3. Atomic Number- the number of protons in
an atom’s nucleus; also the number of
electrons
a) Left alone, an atom tends to hold as
many electrons as protons
b) The number of electrons is NOT
constant like the number of protons
1) The number of electrons
determines how the atom interacts
with other atoms
2) The number of protons
determines the atom’s properties
4. Atomic Mass- the number of protons plus
the number of neutrons
B. Isotopes
1. Isotopes of an element have the
same number of protons in their atoms
but different numbers of neutrons.
a) Example:
C. Electrons and Reactivity
1. An atom’s electrons determine
how it reacts with other atoms.
a) Electrons differ in the amount
of energy they have and how
tightly they are held by the
protons in the nucleus.
b) Electrons in the highest energy level
of an atom determine how that atom
reacts.
1) The first or lowest energy
level (nearest the nucleus) can
hold two electrons.
2) Every energy level after the
first can hold up to 8 electrons.
III. Chemical bonds join atoms to one
another
A. Ionic Bonds
1. An ionic bond occurs when an
atom transfers an electron to
another atom
a) Example:
2. Ions- atoms (or groups of atoms)
that have become electrically
charged as a result of gaining or
losing electrons
B. Covalent Bonds
1. A covalent bond forms when two
atoms share electrons
2. The number of bonds an atom can
form usually equals the number of
additional electrons that will fill its
highest energy level
a) Example:
3. Molecules- two or more atoms held
together by covalent bonds
a) chemical formula- tells you the
number and types of atoms in a
molecule
b) structural formula- indicates how
atoms in a molecule are linked by
bonds
c) space-filling model- a drawing
that depicts a 3-D model of a
molecule
C. Chemical Reactions
1. A chemical reaction is the breaking
of old bonds and the formation of new
bonds that result in new substances
a) Example:
b) Reactants- the starting materials for a
chemical reaction
c) Products- the ending materials in a
chemical reaction
FYI…
2. Exothermic reactions- chemical reactions
that release energy
3. Endothermic reactions- chemical
reactions that absorb energy
IV. Life depends on the unique properties of
water
A. The structure of water
1. Water is made up of 2 hydrogen
atoms and one oxygen atom
a) Oxygen pulls electrons
much more strongly than does
hydrogen. This unequal pull
results in the shared electrons
spending more of their time
with the oxygen atom (creates
a V shape).
b) The oxygen end has a slight negative
charge, while the hydrogen atoms are
slightly positive. This is called a polar
molecule.
2. Hydrogen bond- a bond created
by the weak attraction of a slightly
positive hydrogen atom to a slightly
negative portion of another
molecule.
B. Water’s Life-Supporting Properties
1. Cohesion and Adhesion
a) Cohesion – the tendency of
molecules of the same kind to stick
to one another
1) Example: beads of water on
the outside of a glass
b) Adhesion- the attraction between unlike
molecules
1) Example:
Figure 4-13
Cohesion and adhesion contribute to the rise of water molecules within a tree's
water transport system. The dotted lines in the diagram indicate hydrogen bonds.
3. Low Density of Ice
a) Density is the amount of matter in a
given volume.
b) The solid state is more
dense than the liquid state.
c) This is important to living things
because if ice didn’t float, bodies of
water would freeze from bottom to top,
trapping the fish and other organisms in
a shrinking layer of water without access
to nutrients.
Figure 4-15
Ice floats because its molecules are less densely packed
than those in liquid water.
C. Water’s ability to Dissolve other
substances
a) Solution- a uniform mixture of
two or more substances
1) Solvent- the substance
that dissolves the other
substance
2) Solute- the substance
that is dissolved
b) Aqueous solution- a solution in which
water is the solvent
1) Water is the main solvent in all
cells, blood, and plant sap
Figure 4-16
Sodium chloride dissolves as Na+ and Cl- ions become
attracted to water molecules and break away from the
surface of the solid.
D. Acids, Bases, and pH
1. Acid- A compound that donates
H+ ions to a solution
2. Base- A compound that removes
H+ ions from an aqueous solution
3. pH scale-describes how acidic or
basic a solution is
a) pH scale ranges from 0 to
14, with 0 being the most
acidic and 14 being the
most basic.
b) Pure water and aqueous
solutions that have equal
amounts of H+ and OH- ions
are said to be neutral (pH of
7).
4. Buffers- substances that cause a
solution to resist changes in pH;
works by accepting H+ ions when
their levels rise and donating H+
ions when their levels fall
V. Carbon is the main ingredient of Organic
molecules
A. Carbon Skeletons and Functional
Groups
1. Carbon has 4 electrons in its
highest energy level, which means
they can form up to 4 bonds with
other atoms.
2. Most carbon-based molecules
are organic molecules; noncarbon-based molecules are
classified as inorganic molecules.
3. Carbon can also bond with atoms of
other elements
a) Hydrocarbons- organic
molecules that are composed of
only carbon and hydrogen
4. Functional group- a group of atoms
within a molecule that interacts in
predictable ways with other molecules
a) Hydrophilic- molecules that
attract water molecules
(hydroxyl groups)
B. Monomers and Polymers
1. Monomers – small molecular unit that
is the building block of a larger
molecule
2. Polymers- long chains of small
molecular units (monomers)
a) Every living cell has thousands of
different kinds of polymers and yet
all of these polymers are built from
a collection of fewer than 50 kinds
of monomers.
C. Building and Breaking Polymers
1. Each time a monomer is added
to a chain, a water molecule is
released; this is called dehydration
synthesis.
2. Cells break bonds between
monomers by adding water to
them; this is called hydrolysis
reaction.
Figure 5-4
In the dehydration reaction, two monomers bond to each
other, making a polymer chain longer. The hydroxyl group of
one monomer reacts with a hydrogen atom from the other
monomer. The reactions involved ultimately release a water
molecule.
Figure 5-5
In the hydrolysis reaction, the addition of a water
molecule breaks the polymer chain.
VI. Carbohydrates provide fuel and building
material
A. Sugars
1. Carbohydrate- an organic
compound made up of sugar
molecules
a) contains the elements
carbon, hydrogen, and oxygen
in the ratio of 1 carbon: 2
hydrogen: 1 oxygen
b) the carbon skeletons of
carbohydrates have a ring
shape
2. Monosaccharides- simple sugars that
contain just one sugar unit
a) Examples: glucose, fructose, and
galactose
b) Sugar molecules, particularly
glucose, are the main fuel supply
for cellular work
Figure 5-6
The complete structural diagram of the
monosaccharide glucose (left) shows all its atoms.
The simplified representation (right) shows just the
core ring formed by some of the carbon and oxygen
atoms. Ring shapes are common in sugar molecules
found in nature.
3. Disaccharides- consists of 2
monosaccharides (double sugar)
a) the most common disaccharide
is sucrose; consists of a glucose
molecule linked to a fructose
molecule
1) major carbohydrate in
plant sap
Figure 5-7
Sucrose is a disaccharide (double sugar) consisting
of two monosaccharides linked together.
B. Polysaccharides
1. Polysaccharides are long polymer
chains made up of simple sugar
monomers
a) Starch is a polysaccharide found
in plant cells
1) Examples: potatoes, rice,
and corn are all rich in starch
b) Animal cells store excess sugar in the
form of glycogen; this is stored as
granules in liver and muscle cells.
c) Cellulose serves as a building
material in plants; they protect cells
and stiffen the plant.
1) Most animals cannot digest
cellulose because they lack
the molecule necessary to
break the bonds between the
glucose monomers in
cellulose.
2) Cellulose is referred to as fiber and
serves to keep the digestive system
healthy.
d) Almost all carbohydrates are
hydrophilic; this is due to the
many hydroxyl groups in their
sugar units. Therefore,
monosaccharides and
disaccharides dissolve readily
in water.
Figure 5-8
Glycogen, cellulose, and starch are three types of
polysaccharides found in food. Though all three polymers are
composed of the same monomer, glucose, the way the
glucose monomers link together is different for each.
VII. Lipids include Fats and Steroids
A. Characteristics of Lipids
1. One of a class of water-avoiding
compounds
B. Fats
a) Water-avoiding molecules
are said to be hydrophobic
1. Consists of a three-carbon
backbone called glycerol attached
to three fatty acids
a) Saturated fat- a fat in which all 3 fatty acid
chains contain the maximum possible
number of hydrogen atoms
1) Diets rich in this type of fat are
said to be unhealthy because they
promote the buildup of lipid-containing
deposits called plaques, within the walls
of blood vessels.
2) They are solid at room temperature
3) Examples: lard, butter
b) Unsaturated fat- a fat that
contains less than the maximum
number of hydrogen atoms in one
or more of its fatty acid chains
because some of its carbon atoms
are double-bonded to each other
1) Examples: Vegetable oil, corn
oil, olive oil; fats in fruits,
vegetables, and fish
C. Steroids
1. Classified as lipids because they are
hydrophobic, but they are different from
fats in structure and function.
a) Example: estrogen, testosterone,
cholesterol
2. The best-known steroid is cholesterol.
This is an essential molecule found in the
membranes that surround your cells.
Figure 5-10
The only difference in these two steroid hormones is
the location of their functional groups. Yet, these
two molecules contribute to major differences in the
appearance and behavior of male and female
mammals.
VIII. Proteins perform most functions in cells
A. The Functions of Proteins
1. A protein is a polymer
constructed from a set of just 20
kinds of monomers called amino
acids.
a) responsible for almost all of
the day-to-day functioning of
organisms
1) example: hair, fur,
make up muscles, and
provide long-term nutrient
storage
B. Amino Acids
1. An amino acid monomer consists of
central carbon atom bonded to four
partners. One partner is a hydrogen
atom. Two others are a carboxyl group
and an amino group.
2. What is different about each amino
acid is the “side group” called the Rgroup.
Figure 5-12
All amino acids consist of a central carbon bonded
to an amino group, a carboxyl group, and a
hydrogen atom. The fourth bond is with a unique
side group. The differences in side groups convey
different properties to each amino acid.
C. Building a Protein
1. Polypeptide- a chain of linked amino
acids
2. Each link is created by a dehydration
reaction between the amino group of
one amino acid and the carboxyl group
of another amino acid.
3. The protein “alphabet” consists of 20
“letters” or amino acids.
a) each protein has its own unique
sentence of amino acids
Figure 5-13
The order of amino acids makes each polypeptide
unique. There are 129 amino acids in this protein,
called lysozyme. The three-letter symbols are
abbreviations for the amino acid names.
D. Protein Shape
1. The shape of a protein is influenced
by the following:
a) temperature
b) pH
c) other qualities of its environment
1) denaturation- loss of normal
shape of a protein due to heat
or other factor
IX. Enzymes are proteins that speed up
specific reactions in cells
A. Enzymes and Activation Energy
that
1. To start a chemical reaction, it is
first necessary to weaken chemical
bonds in the reactant molecules.
This activation process requires
the molecules absorb energy.
a) activation energy- the
minimum amount of energy
required to trigger a chemical
reaction
2. Catalysts- compounds that speed up
chemical reactions
a) The main catalysts of chemical
reactions in organisms are specialized
proteins called enzymes.
1) An enzyme doesn’t supply
activation energy, but instead
lowers the energy requirement
barrier so that the reaction can
proceed at normal cell
temperatures
2) Each enzyme catalyzes a a
specific kind of chemical reaction
Figure 5-15
The activation energy barrier is like a wall between
two parts of a pond. If an enzyme lowers the wall,
more frogs have enough energy to reach the other
side.
B. How enzymes work
1. The substrate of each enzyme fits
the active site (like a key in a lock)
a) substrate- specific reactant
acted on by an enzyme
b) active site- region of an
enzyme into which a particular
substrate fits
Figure 5-16
A substrate binds to an enzyme at an active site. The enzymesubstrate interaction lowers the activation energy required
for the reaction to proceed. In this example, water is added to
the weakened bond in sucrose, breaking sucrose into
glucose and fructose.