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

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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Chapter 2: The Molecules of Cells

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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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.

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

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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.

• 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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Properties of Water

Figure 2.9 Ice floats on

water.

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.

•Acidic solutions have a higher concentration of H+ than OH−.

Examples include lemon juice, vinegar, and tomatoes.

HCl H+ + OH−

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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.

•Dissociation is almost complete, which makes sodium

hydroxide a strong base. Other examples of bases include

baking soda and antacids.

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NaOH Na+ + OH-

Basic Solutions (Low H+ Concentrations)

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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pH Scale

The H+ concentration differs by a factor of ten between

pH units.

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pH Scale

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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

Buffers and pH

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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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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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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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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.

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Monomers and Polymers

Important polymers in cells and the

monomers they are composed of.

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).

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.

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Synthesis and Degradation of Polymers

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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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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