1. Chapter 2 Water: the Medium of Life

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

3. 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?

4. 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 H-bond acceptor
Potential to form four H-bonds per water molecule

5. 2.1 What Are the Properties of Water?

6. 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 = sec)

7. 2.1 What Are the Properties of Water?

8. 2.1 What Are the Properties of Water?
The fluid network of H bonds linking water molecules in the liquid state.

9. 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"

10. The Solvent Properties of Water Derive from Its Polar Nature
Hydration shells surrounding ions in solution.

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

12. 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

13. Amphiphilic/Amphipathic Molecules
“Amphiphilic” and “amphipathic” are essentially synonymous terms
Amphiphilic molecules interact favorably with both polar and nonpolar environments
Amphipathic molecules contain both polar and nonpolar groups
Good examples - fatty acids

14. The Solvent Properties of Water Derive from Its Polar Nature
(left) A disordered network of H-bonded water molecules.
(right) A clathrate cage of ordered, H-bonded water molecules around a nonpolar solute molecule.

15. The Solvent Properties of Water Derive from Its Polar Nature
Nonpolar molecules decrease the entropy of solvent water (left). When nonpolar molecules coalesce (arrow), the entropy of the solvent increases.

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

17. The Solvent Properties of Water Derive from Its Polar Nature
Micelle formation by amphiphilic molecules in aqueous solution.

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

19. Osmotic Pressure
Osmotic Pressure = ∏ = the force required to resist water movement
van‘t Hoff equation: ∏ = icRT
ic = osmolarity of the solution
i = number of solutes per molecule
(NaCl = 2)
c = concentration in molarity
R = gas constant J/mol
T = temperature in Kelvin

20. Practice Which of the following solutions has an osmolarity of 3?
3M Na3PO4
0.43M Na3PO4
0.75 M Na3PO4
3 M NaCl
1.5 M NaCl

21. Practice
You want to isolate a lysosome to study it in the lab. Assuming that the only components inside a lysosome are KCl (0.1M) and NaCl (0.03M), how much sucrose (342 g/mol) do you need to make 1 liter of an isotonic solution to isolate the lysosomes?

22. Practice
You want to isolate a lysosome to study it in the lab. Assuming that the only components inside a lysosome are KCl (0.1M) and NaCl (0.03M), how much glycogen (18,000 g/mol) do you need to make 1 liter of an isotonic solution to isolate the lysosomes?

23. 2.1 What Are the Properties of Water?
Water Can Ionize to Form H+ and OH-
H2O ⇄ H+ + OH-

24. Water Can Ionize to Form H+ and OH-
The hydration of H3O+.

25. 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 M
Then pH = 7

26. 2.2 What is pH?

27. 2.2 What is pH?

28. Dissociation of Weak Electrolytes
Consider a weak acid, HA
The acid dissociation constant is given by:
HA ⇄ H+ + A-

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. The Dissociation Behavior of Weak Electrolytes

32. Consider the Dissociation of Acetic Acid
Assume 0.1 equivalents (eq) of 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:

33. 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:

34. 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:

35. Practice
What is the buffer concentration and pH of a mixture of 0.042M NaH2PO4 (pK = 6.86) and M Na2HPO4?
What is the pH of a mixture of 75 mL of 0.042M NaH2PO4 (pK = 6.86) and 150 mL of M Na2HPO4?

36. The Dissociation Behavior of Weak Electrolytes
The titration curves of several weak acids.

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

38. 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
Buffers can only be used reliably within a pH unit of their pKa

39. 2.3 What Are Buffers, and What Do They Do?
A buffer system consists of a weak acid, HA and its conjugate base, A-

40. Enzyme Activity is Influenced by pH
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.

41. 2.3 What are Buffers and What Do They Do?
Anserine is a dipeptide buffer that helps maintain intracellular pH in some tissues.

42. 2.3 What are Buffers and What Do They Do?
The structure of HEPES, in its fully protonated form.

43. 2.4 What Properties of Water Give It a Unique Role in the Environment?
Water is a very good solvent for ionic and polar 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

44. Real World Practice
You are working in the lab and need to make a physiological phosphate buffer (pH=7.2). You have the following chemicals at your disposal: Phosphoric acid, monosodium phosphate, and disodium phosphate whose corresponding pKa’s are 2.15, 6.86, and The molecular weight of each species is 98.0 g/mol, g/mol, and g/mol. Calculate the mass of each species that you would use to make a 100 mL of a 0.5M phosphate buffer at pH=7.2.