UNIT A: Cell Biology
Chapter 2: The Molecules of Cells:
Sections 2.3, 2.4
Chapter 3: Cell Structure and Function
Chapter 4: DNA Structure and Gene
Expression
Chapter 5: Metabolism: Energy and
Enzymes
Chapter 6: Cellular Respiration
Chapter 7: Photosynthesis
UNIT A Chapter 2: The Molecules of Cells
Chapter 2: The Molecules of Cells
In this chapter, you will learn
how basic chemistry is used in
biology.
What life processes might be
affected by a problem with
protein structure?
How are biological
molecules involved in energy
use in the body?
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UNIT A Chapter 2: The Molecules of Cells
Section 2.3
2.3 Chemistry of Water
Organisms are composed of 70 to 90% water. Therefore, the
properties of water play an important role in our survival.
• Water is a polar molecule.
• Water molecules hydrogen bond to one another, making
them cling together.
• Without hydrogen bonding, water would change from a
solid to liquid state at −100oC and from a liquid to gas
state at −91oC.
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UNIT A Chapter 2: The Molecules of Cells
Section 2.3
Properties of Water
• Water has a high heat capacity. Most
other polar molecules require much less
than 1 calorie of energy to change their
temperature by 1oC. The temperature of
water rises and falls slowly.
• Water has a high heat of vaporization.
It requires a great deal of energy to turn
water from liquid to gas. This provides
animals in a hot environment an
efficient way to cool their body heat.
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Figure 2.8 The advantage
of water’s high heat of
vaporization.
UNIT A Chapter 2: The Molecules of Cells
Section 2.3
Properties of Water
• Water is a solvent. Due to its polarity, water facilitates
chemical reactions and dissolves many substances. A
solution contains one or more dissolved solutes, such as
sodium chloride.
• Hydrophilic molecules attract water
• Hydrophobic molecules do not attract water
When ionic salts such as
sodium chloride are put in
water, the negative ends of
the water molecules are
attracted to the sodium ions
and the positive ends of the
water molecules are
attracted to the chloride ions.
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UNIT A Chapter 2: The Molecules of Cells
Section 2.3
Properties of Water
• Water molecules are cohesive and
adhesive. Water flows freely, but the
molecules cling together. It also adheres
to polar surfaces. This makes water an
excellent transport system, inside and
outside of organisms.
• Water has a high surface tension. The
force between molecules is high.
• Frozen water (ice) is less dense than
liquid water. Water expands as it freezes,
making it less dense. This keeps ice on
bodies of water from sinking.
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Figure 2.9 Ice floats on
water.
UNIT A Chapter 2: The Molecules of Cells
Section 2.3
Acids and Bases
When water ionizes it releases an equal number of hydrogen
ions and hydroxide ions (although the number is very small).
Acidic Solutions (High H+ Concentrations)
Acids release hydrogen ions in water.
HCl
H+ + OH−
• Acidic solutions have a higher concentration of H+ than
OH−. Examples include lemon juice, vinegar, and tomatoes.
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UNIT A Chapter 2: The Molecules of Cells
Section 2.3
Basic Solutions (Low H+ Concentrations)
Bases take up hydrogen ions or release hydroxide ions.
• Basic solutions have a higher concentration of OH− than
H+. Sodium hydroxide dissociates as shown below.
NaOH
Na+ + OH-
• Dissociation is almost complete, which makes sodium
hydroxide a strong base. Other examples of bases include
baking soda and antacids.
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UNIT A Chapter 2: The Molecules of Cells
Section 2.3
pH Scale
The pH scale indicates basicity or acidity according to a scale
of 0 to 14.
• pH = 7: neutral solution ([H+] = [OH−])
• pH < 7: acidic solution
([H+] > [OH−])
• pH > 7: basic solution
([OH−] > [H+])
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Figure 2.10 The pH scale.
UNIT A Chapter 2: The Molecules of Cells
pH Scale
The H+ concentration differs by a factor of ten between
pH units.
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Section 2.3
UNIT A Chapter 2: The Molecules of Cells
Section 2.3
Buffers and pH
Buffers help keep the pH of a solution within a specific limit.
• They can take up excess H+ or OH−
In animals, the pH of body fluids must be controlled within a
narrow range. The pH of human blood should be 7.4. If it
drops to 7, acidosis results. If it rises to 7.8, alkalosis results.
Human blood contains a combination of carbonic acid and
bicarbonate ions that acts as a buffer to maintain a pH of 7.4
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UNIT A Chapter 2: The Molecules of Cells
Section 2.3
Check Your Progress
1. Compare the difference between water’s high heat
capacity and high heat of vaporization.
2. Explain why a solution with a pH of 6 contains more H+
than a solution with a pH of 8.
3. Explain why a weakly dissociating acid/base is a better
buffer than a strongly dissociating one.
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UNIT A Chapter 2: The Molecules of Cells
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Section 2.3
UNIT A Chapter 2: The Molecules of Cells
Section 2.4
2.4 Organic Molecules
Nonliving matter consists of inorganic molecules. However,
many inorganic substances, such as water and salts (such as
sodium chloride) are essential to organisms.
The molecules of life are organic molecules. Organic
molecules contain carbon (C) and hydrogen (H) atoms.
• The chemistry of carbon accounts for the numerous
organic molecules that exist. For example, it can form as
many as four bonds with other atoms, including other
carbons.
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UNIT A Chapter 2: The Molecules of Cells
Functional Groups
In many organic molecules,
carbon atoms are bonded to
functional groups. Functional
groups are specific
combinations of bonded atoms.
• Each functional group has
particular properties and
reacts in a certain way.
• Common functional groups
in biological molecules are
shown here.
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Section 2.4
UNIT A Chapter 2: The Molecules of Cells
Section 2.4
Monomers and Polymers
Many molecules of life are macromolecules, which consist
of smaller molecules joined together.
Monomers are simple organic molecules that can exist on
their own or be linked with other monomers to form
polymers.
Important polymers in cells and the
monomers they are composed of.
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UNIT A Chapter 2: The Molecules of Cells
Section 2.4
Synthesis and Degradation of Polymers
Monomers are often joined
together to form a polymer
by a dehydration reaction.
• A hydroxyl functional
group (−OH) on one
monomer and a H atom on
another monomer (the
equivalent to a water
molecule) are removed
during each reaction.
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Figure 2.11a Synthesis and degradation of
polymers. In cells, synthesis often occurs when
monomers join (bond) during a dehydration reaction
(removal of H2O).
UNIT A Chapter 2: The Molecules of Cells
Section 2.4
Synthesis and Degradation of Polymers
To degrade polymers, a
hydrolysis reaction is
carried out.
• The components of water
(an −OH group and a H
atom) are added, breaking
the bonds that connect the
monomers.
Figure 2.11b Synthesis and degradation of
polymers. Degradation occurs when the monomers in
a polymer separate during a hydrolysis reaction
(addition of H2O).
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UNIT A Chapter 2: The Molecules of Cells
Check Your Progress
1. Explain why organic molecules are considered the
molecules of life.
2. Compare and contrast dehydration and hydrolysis
reactions
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Section 2.4
UNIT A Chapter 2: The Molecules of Cells
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Section 2.4