CHAPTER 3
WATER: THE MEDIUM OF LIFE
CHAPTER OUTLINE
Molecular Structure of Water
Noncovalent Bonding
Ionic Interactions
Hydrogen Bonds
Van der Waal’s Forces
Thermal Properties of Water
Solvent Properties of Water
Hydrophilic Molecules
Hydrophobic Molecules
Amphipathic Molecules
Osmotic Pressure
Ionization of Water
Acids, Bases, and pH
Special Interest Box 3.1 Cell Volume Regulation and Metabolism
Buffers
Physiological Buffers
Biochemical Methods 3.1 Dialysis
CHAPTER SUMMARY
Water molecules (H20) are composed of two atoms of hydrogen and one of oxygen. Each hydrogen atom is linked to
the oxygen atom by a single covalent bond. The oxygen-hydrogen bonds are polar and water molecules are dipoles.
One consequence of water’s polarity is that water molecules are attracted to each other by the electrostatic force
between the oxygen of one molecule and the hydrogen of another. This attraction is called a hydrogen bond.
Noncovalent bonds are relatively weak and, therefore, easily disrupted. They play a vital role in determining the
physical and chemical properties of water and biomolecules. Ionic interactions occur between charged atoms or
groups. Although each hydrogen bond is not especially strong when compared to covalent bonds, large numbers of
them have a significant effect on the molecules involved. Van der Waal’s forces occur between permanent and/or
induced dipoles. They may be attractive or repulsive.
Water has an exceptionally high heat capacity. Its boiling and melting points are significantly higher than those of
compounds with comparable structure and molecular weight. Hydrogen bonding is responsible for this anomalous
behavior.
Water is also a remarkable solvent. Water’s dipolar structure and its capacity to form hydrogen bonds enable it to
dissolve many ionic and polar substances.
Liquid water molecules have a limited capacity to ionize to form a hydrogen ion (H +) and a hydroxide ion (OH-).
When a solution contains equal amounts of H+ and OH- ions, it is said to be neutral. Solutions with an excess of H+
are acidic, whereas those with a greater number of OH- are basic. Because organic acids do not completely
dissociate in water, they are referred to as weak acids. The acid dissociation constant (Ka) is a measure of the
strength of a weak acid. Because Ka values vary over a wide range, pKa values (-log Ka) are used instead.
The hydrogen ion is one of the most important ions in biological systems. The pH scale conveniently expresses
hydrogen ion concentration. pH has been defined as the negative logarithm of the hydrogen ion concentration.
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Because hydrogen ion concentration affects living processes so profoundly, it is not surprising that regulating pH is
a universal and essential activity of living organisms. Hydrogen ion concentration is typically kept within narrow
limits. Because buffers combine with H+ ions, they help maintain a relatively constant hydrogen ion concentration.
The ability of a solution to resist pH changes is called buffering capacity. Most buffers consist of a weak acid and its
conjugate base.
Several physical properties of liquid water change when solute molecules are dissolved. The most important of
these for living organisms is osmotic pressure, the pressure that prevents the flow of water across cellular
membranes.
LECTURE THEMES
This chapter describes the properties of water, the biological solvent. Although students will have been previously
exposed to this material (e.g., noncovalent bonding and pH), most will benefit from a review. An emphasis should
be placed on the relationship between water’s peculiar properties and its biological roles. For example, noncovalent
bonding interactions and pH (the measure of the acidity of a solution) play an important role in protein structure and
function. It should also be stressed that many biomolecules possess functional groups that are weak acids or bases.
Consequently, it is important to understand that the dissociation properties of these groups in water have an
important effect on the physiological properties of the molecules that contain them. Because living organisms are
exquisitely sensitive to changes in pH, buffers play a critical role in biochemical processes. Buffering, the tendency
of a solution to resist changes in pH, is measured quantitatively with the Henderson-Hasselbalch equation.
ANSWERS TO EVEN-NUMBERED QUESTIONS
1.
pH = - log [H+]
8.3 = - log [H+]
= antilog 0.7 + antilog -9
= 5.01 3 10-9
pH = pKa + log [Salt]/[Acid]
From a table of ionization constants the phosphate conjugate acid base pair with a pKa closest to 7.2 is chosen:
Substituting these values into the equation gives
7.2 = 7.2 + log [Salt]/[Acid]
0 = log [Salt]/[Acid]
1 = [Salt]/[Acid]
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The concentrations of the conjugate base and acid must be equal. Using simultaneous equations to determine
the concentrations of the salt and acid for this particular buffer solution:
0.1 M = [Salt] + [Acid]
[Salt] = [Acid]
substituting values gives
0.1 M = [Salt] + [Salt]
0.1 M = 2[Salt]
0.05 M = [Salt] = [Acid]
To prepare this buffer, place 0.05 mol each of the salt and acid in a 1 L volumetric flask and dilute with water to
the 1 L mark.
6.
Osmolarity = Molarity × the number of ions produced.
Na3PO4 dissociates into four ions. Assuming 85% ionization the osmolarity of a 1.3 molar solution of Na 3PO4
would therefore be 4 × 1.3 × 0.85 = 4.4.
8.
a.
b.
c.
d.
e.
water and ammonia – hydrogen bond
lactate and ammonia ions – electrostatic interactions
benzene and octane – van der Waal’s forces
carbon tetrachloride and chloroform – van der Waal’s forces
chloroform and diethyl ether – van der Waal’s forces
10. Arrows indicate atoms that would be involved in hydrogen bonding.
12. b, c, and d would all be expected to have dipole moments.
14. Carbon dioxide is present in the blood in sufficient quantities to make it an effective buffer. Phosphate
concentration in blood is too low to be an effective buffer. In cells the relatively higher phosphate
concentration makes it an effective buffer.
16. b, c and e are all weak acids because they are only partially ionized.
18. Hyperventilation drives the transfer of carbon dioxide from the blood. This process, which shifts the following
equilibrium, consumes protons thereby making the blood more alkaline.
20. No. The carbonic acid and carbonate react to produce bicarbonate. It is possible to have a buffer system of
carbonic acid and bicarbonate or bicarbonate and carbonate.
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22. The amount of hydrogen ascorbate is 0.25 mol/l × 0.3 l = 0.075 mol. The amount of hydrochloric acid is 0.2
mol/l × 0.15 l = 0.03 mol. After they are mixed, the amount of ascorbic acid is 0.03 mol and the amount of
hydrogen ascorbate is 0.75 – 0.03 = 0.045 mol. The pH of the solution is calculated using the HendersonHasselbach equation
pH = pKa + Log [salt]/[acid]
pH = 4.3 – log 0.45 mol/0.03
= 4.3 – log 1.5 = 4.3 – 0.18
= 4.12
THOUGHT QUESTIONS
2.
The regular crystal lattice of ice is more open than the more tightly hydrogen bonded liquid water. If ice were
more dense, then ice formed in lakes and oceans would sink to the bottom and eventually only a narrow layer at
the surface would be liquid, an environmental condition that is incompatible with life for most aquatic species.
4.
The blood is so highly buffered by the bicarbonate buffer and the large amounts of blood proteins that under
normal physiological conditions the transport of weak acids in the blood does not appreciably change its pH.
6.
In a liquid, the molecules are free to move past each other. In the jello solution, each water molecule forms
hydrogen bonds with two segments of protein, a circumstance which locks the protein chains and the water
together. Because the water molecules are no longer able to move freely, the mixture becomes semi-rigid.
8.
No. The structure of cells is based on the phase separation of hydrophobic and hydrophilic substances. For
example, if water dissolved every molecule, living organisms would not be able to create a barrier between
themselves and their surroundings.
10. The conversion of glycogen to glucose creates an increase in osmotic pressure. To offset this rise in osmotic
pressure, ions such as sodium and potassium are pumped out of the cell.
12. The polar water molecules crowd around the ions interacting with them and weaken the interactions with other
ions.
ESSAY QUESTIONS
1.
Explain why seawater should not be used on house plants.
2.
Explain why perspiration assists in the regulation of body temperature.
3.
Describe the important physiological buffer systems. Where in the body is each system effective?
4.
Why is water sometimes referred to as the universal solvent? Is this term literally true?
5.
Describe, in general terms, the impact of osmotic pressure on living organisms.
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