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Chapter 2 Water: the Medium of Life

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.

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Chapter 2 Water: the Medium of Life

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  1. Chapter 2 Water: the Medium of Life

  2. Chapter 2 “If there is magic on this planet, it is contained in water.” Loren Eisley Where there’s water, there’s life.

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

  4. 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?

  5. 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

  6. 2.1 What Are the Properties of Water? Figure 2.1 The structure of water.

  7. 2.1 What Are the Properties 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"!

  8. 2.1 What Are the Properties of Water? Figure 2.2 The structure of normal ice.

  9. 2.1 What Are the Properties of Water? Figure 2.3 The fluid network of H bonds linking water molecules in the liquid state.

  10. 2.1 What Are the Properties of Water? Figure 2.3 The fluid network of H bonds linking water molecules in the liquid state.

  11. 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"

  12. The Solvent Properties of Water Derive from Its Polar Nature Figure 2.4 Hydration shells surrounding ion in solution.

  13. The Solvent Properties of Water Derive from Its Polar Nature

  14. 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

  15. 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

  16. The Solvent Properties of Water Derive from Its Polar Nature 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.

  17. The Solvent Properties of Water Derive from Its Polar Nature Figure 2.6 Nonpolar molecules decrease the entropy of solvent water.

  18. The Solvent Properties of Water Derive from Its Polar Nature Figure 2.7 (a) Sodium palmitate is an amphiphilic molecule.

  19. The Solvent Properties of Water Derive from Its Polar Nature Figure 2.7 (b) Micelle formation by amphiphilic molecules in aqueous solution.

  20. The Solvent Properties of Water Derive from Its Polar Nature Figure 2.8 The osmotic pressure of a 1 molal (m) solution is equal to 22.4 atmospheres.

  21. 2.1 What Are the Properties of Water? • Water Can Ionize to Form H+ and OH-

  22. Water Can Ionize to Form H+ and OH- Figure 2.9 The ionization of water.

  23. Water Can Ionize to Form H+ and OH- Figure 2.10 The hydration of H3O+.

  24. 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

  25. 2.2 What is pH?

  26. 2.2 What is pH?

  27. Dissociation of Weak Electrolytes Consider a weak acid, HA • The acid dissociation constant is given by:

  28. 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:

  29. 2.2 What is pH? Titration curves illustrate the progressive dissociation of a weak acid

  30. 2.2 What is pH? Titration curves illustrate the progressive dissociation of a weak acid

  31. 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 Consider the Dissociation of Acetic Acid

  32. Consider the Dissociation of Acetic Acid 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

  33. Consider the Dissociation of Acetic Acid • 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

  34. The Dissociation Behavior of Weak Electrolytes

  35. The Dissociation Behavior of Weak Electrolytes Figure 2.12 The titration curves of several weak acids.

  36. Titration Curves Illustrate the Progressive Dissociation of a Weak Acid Figure 2.13 The titration curve for phosphoric acid.

  37. 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

  38. 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-

  39. 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.

  40. 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.

  41. 2.3 What are Buffers and What Do They Do? Figure 2.17 The structure of HEPES, in its fully protonated form.

  42. 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

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