Chapter 2 Water: the Medium of Life

Reginald H. GarrettCharles M. Grisham

Chapter 2 Water: the Medium of Life

Chapter 2

“If there is magic on this planet, it is contained in water.”

Loren Eisley

Where there’s water, there’s life.

Essential Question

• What are the properties of water that render it so suited to its role as the medium of life?

Outline

• What are the properties of water?• What is pH?• What are buffers, and what do they do?• Does water have a unique role in the fitness of the

environment?

2.1 What Are the Properties of Water?

● Water has unusual properties:• High b.p., m.p., heat of vaporization, surface

tension• Bent structure makes it polar• Non-tetrahedral bond angles• H-bond donor and acceptor• Potential to form four H-bonds per water

molecule


Figure 2.1 The structure of water.


A comparison of ice and water, in terms of H-bonds and Motion

• Ice: 4 H-bonds per water molecule• Water: 2.3 H-bonds per water molecule• Ice: H-bond lifetime - about 10 microsec• Water: H-bond lifetime - about 10 psec• (10 psec = 0.00000000001 sec)• That's "one times ten to the minus eleven

second"!


Figure 2.2 The structure of normal ice.


Figure 2.3 The fluid network of H bonds linking water molecules in the liquid state.

The Solvent Properties of Water Derive from Its Polar Nature

• Water has a high dielectric constant• Ions are always hydrated in water and

carry around a "hydration shell"• Water forms H-bonds with polar solutes• Hydrophobic interactions - a "secret of life"


Figure 2.4 Hydration shells surrounding ion in solution.

Hydrophobic Interactions

• A nonpolar solute "organizes" water• The H-bond network of water reorganizes to

accommodate the nonpolar solute• This is an increase in "order" of water• This is a decrease in ENTROPY

Amphiphilic/Amphipathic Molecules

• “Amphiphilic” and “amphipathic” are essentially synonymous terms

• Amphiphilic molecules are attracted to both polar and nonpolar environments

• Amphipathic molecules that contain both polar and nonpolar groups

• Good examples - fatty acids


Figure 2.5(left) A disordered network of H-bonded water molecules.(right) A clathrate cage of ordered, H-bonded water molecules around a nonpolar solute molecule.


Figure 2.6 Nonpolar molecules decrease the entropy of solvent water.


Figure 2.7 (a) Sodium palmitate is an amphiphilic molecule.


Figure 2.7 (b) Micelle formation by amphiphilic molecules in aqueous solution.


Figure 2.8 The osmotic pressure of a 1 molal (m) solution is equal to 22.4 atmospheres.


• Water Can Ionize to Form H+ and OH-

Water Can Ionize to Form H+ and OH-

Figure 2.9 The ionization of water.

Water Can Ionize to Form H+ and OH-

Figure 2.10 The hydration of H3O+.

2.2 What is pH?• Søren Sørensen of Denmark devised the pH scale• pH is the negative logarithm of the hydrogen ion

concentration• If [H+] = 1 x 10 -7 M• Then pH = 7

10log [ ]14w

pH HpK pH pOH

2.2 What is pH?

Dissociation of Weak ElectrolytesConsider a weak acid, HA

• The acid dissociation constant is given by:

The Henderson-Hasselbalch Equation

Know this! You'll use it constantly.

• For any acid HA, the relationship between the pKa, the concentrations existing at equilibrium and the solution pH is given by:

2.2 What is pH?

Titration curves illustrate the progressive dissociation of a weak acid

Consider the Dissociation of Acetic Acid

• Assume 0.1 eq base has been added to a fully protonated solution of acetic acid

• The Henderson-Hasselbalch equation can be used to calculate the pH of the solution:With 0.1 eq OH¯ added:

• pH = 4.76 + (-0.95)• pH = 3.81

10(0.1)log(0.9)apH pK


Another case:• What happens if exactly 0.5 eq of base is

added to a solution of the fully protonated acetic acid?

• With 0.5 eq OH¯ added:

• pH = 4.76 + 0• pH = 4.76 = pKa

10(0.5)log(0.5)apH pK


A final case to consider:• What is the pH if 0.9 eq of base is added to a

solution of the fully protonated acid?• With 0.9 eq OH¯ added:

• pH = 4.76 + 0.9• pH = 5.71

10(0.9)log(0.1)apH pK

The Dissociation Behavior of Weak Electrolytes

The Dissociation Behavior of Weak Electrolytes

Figure 2.12 The titration curves of several weak acids.

Titration Curves Illustrate the Progressive Dissociation of a Weak Acid

Figure 2.13 The titration curve for phosphoric acid.

2.3 What Are Buffers, and What Do They Do?

• Buffers are solutions that resist changes in pH as acid and base are added

• Most buffers consist of a weak acid and its conjugate base

• Note in Figure 2.14 how the plot of pH versus base added is flat near the pKa

• Buffers can only be used reliably within a pH unit of their pKa

2.3 What Are Buffers, and What Do They Do?

Figure 2.14 A buffer system consists of a weak acid, HA and its conjugate base, A-

Enzyme Activity is Influenced by pH

Figure 2.15 pH versus enzymatic activity. Pepsin is a protein-digesting enzyme active in gastric fluid. Fumarase is a metabolic enzyme found in mitochondria. Lysozyme digests the cell walls of bacteria. It is found in tears.

2.3 What are Buffers and What Do They Do?

Figure 2.16 Anserine is an important dipeptide buffer in the maintenance of intracellular pH in some tissues.

2.3 What are Buffers and What Do They Do?

Figure 2.17 The structure of HEPES, in its fully protonated form.

2.4 What Properties of Water Give It a Unique Role in the Environment?• Water is a very good solvent for a variety of

substances• Water is a very poor solvent for nonpolar substances• Due to hydrophobic interactions, lipids coalesce,

membranes form, and the cellular nature of life is established

• Due to its high dielectric constant, water is a suitable medium for the formation of ions

• The high heat capacity of water allows effective temperature regulation in living things

Chapter 2 Water: the Medium of Life

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