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?

UNIT A Chapter 2: The Molecules of Cells

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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+])

UNIT A Chapter 2: The Molecules of Cells Section 2.3

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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 areshown 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

UNIT A Chapter 2: The Molecules of Cells Section 2.4

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