1. Water and the Fitness of the Environment
Chapter 3
Water and the Fitness of the Environment

2. Overview: The Molecule That Supports All of Life
Water is the biological medium on Earth
All living organisms require water more than any other substance
Most cells are surrounded by water, and cells themselves are about 70–95% water
Harsh Environment
~3 days without water
~ 3 weeks without food
The abundance of water is the main reason the Earth is habitable
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

3. Concept 3.1: The polarity of water molecules results in hydrogen bonding
The structure of water is key to its special properties. Water molecules are made up of:
One Oxygen Atom
Two Hydrogen Atoms
What about the structure makes it so special?
Polar molecule
Why?
Oxygen atom has a slightly (-ve) charge
Hydrogen atoms have slightly (+ve) charges
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. Polarity allows water molecules to form hydrogen bonds with each other The structure of water is key to its special properties. Water molecules are made up of:
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

5. Each water molecule forms a maximum of four hydrogen bonds at a time
Polarity allows water molecules to form hydrogen bonds with each other.
Each water molecule forms a maximum of four hydrogen bonds at a time
Hydrogen
bond
 – H
 + —— O ——
Figure 3.2 Hydrogen bonds between water molecules
Fig. 3-2

6. Bond formation and breaking lasts only a few trillionths of a second
So if wat molecules from bonds between each other why is water in a liquid form?
Hydrogen bonds are weak bonds which form, break and re-form with great frequency
Bond formation and breaking lasts only a few trillionths of a second
Figure 3.2 Hydrogen bonds between water molecules

7. Concept 3.2: Four emergent properties of water contribute to Earth’s fitness for life
Four of water’s properties that facilitate an environment for life are:
Cohesive behavior
Ability to moderate temperature
Expansion upon freezing
Versatility as a solvent
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

8. Cohesion Cohesion: The linking of water molecules via hydrogen bonds.
Think about water transport in plants
Cohesion helps the transport of water against gravity in plants
Water-conducting
cells
Adhesion
Cohesion
150 µm
Direction
of water
movement
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

9. Cohesion
Surface Tension: A measure of how difficult it is to stretch or break the surface of a liquid.
Surface Tension is related to cohesion because of similar mechanism of hydrogen bonding
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

10. Cohesion vs. Adhesion
Adhesion: An attraction between different substances.
For example: Between water and plant cell walls or water molecules adhering to a glass windshield
Occurs because of hydrogen bonds
Example: of Cohesion & Adhesion
Transpiration: Movement of clinging water molecules (Cohesion) up very thin xylem tubes via (adhesion)
How to Remember the difference between the two
Link the word adhesion to adhesive… adhesives stick to things
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

11. Moderation of Temperature
Water moderates air temperature by:
Absorbing heat from warmer air and releasing stored heat to cooler air
Water can absorb or release a large amount of heat with only a slight change in its own temperature High Specific Heat
Specific Heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1ºC
The specific heat of water is 1 cal/g/ºC
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

12. Water’s high specific heat can be traced to Hydrogen Bonding
Heat is absorbed when hydrogen bonds break
Heat is released when hydrogen bonds form
The high specific heat of water minimizes temperature fluctuations to within limits that permit life
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

13. Evaporative Cooling
Evaporation: Transformation of a substance from liquid to gas
Heat of vaporization: is the heat a liquid must absorb for 1 g to be converted to gas
As a liquid evaporates, its remaining surface cools, a process called Evaporative cooling
Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water
Example: The human skin
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

14. Insulation of Bodies of Water by Floating Ice
Ice floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less dense
The crystal lattice keeps molecules far apart and ice is less dense because there is less water molecules per volume
Hydrogen
bond
Ice
Hydrogen bonds are stable
Liquid water
Hydrogen bonds break and re-form
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

15. Insulation of Bodies of Water by Floating Ice
So when ice forms on the surface this ice insulates the water below preventing it from freezing
If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth because during the thawing process in summer only the upper few inches of a body of water would thaw out
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

16. A solution is a liquid that is a homogeneous mixture of substances
The Solvent of Life
A solution is a liquid that is a homogeneous mixture of substances
Ex. Sugar and water
A solvent is the dissolving agent of a solution
Ex. Water
The solute is the substance that is dissolved
Ex. Salt
An aqueous solution is one in which water is the solvent
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

17. Why is water is a versatile solvent?
Due to its polarity, which allows it to form hydrogen bonds easily
When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

18. Fig. 3-7
Cl– Na + –
Figure 3.7 Table salt dissolving in water

19. Water can also dissolve compounds made of nonionic polar molecules
Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

20. (b) Lysozyme molecule (purple) in an aqueous environment
Fig. 3-8bc
Figure 3.8 A water-soluble protein
(b) Lysozyme molecule (purple) in an aqueous
environment
(c) Ionic and polar regions
on the protein’s surface
attract water molecules.

21. Hydrophilic and Hydrophobic Substances
A hydrophilic substance is one that has an affinity for water
AKA Water loving
Hydrophilic substances are water soluble
Example: Ionic compounds, polar molecules, and some proteins
A hydrophobic substance is one that does not have an affinity for water
AKA phobia = fear or dislike
Example: Oil molecules are hydrophobic because they have relatively nonpolar bonds
A colloid is a stable suspension of fine particles in a liquid
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

22. Concept 3.3: Acidic and basic conditions affect living organisms
In order for some organisms to survive they have very specific conditions that they live in Basic or Acidic
An acid is any substance that increases the H+ concentration of a solution
A base is any substance that reduces the H+ concentration of a solution
pH is a measure of how much H+ ions are in a solution
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

23. For a neutral aqueous solution [H+] is 10–7 = –(–7) = 7
The pH Scale
In any aqueous solution at 25°C the product of H+ and OH– is constant and can be written as [H+][OH–] = 10–14
The pH of a solution is defined by the negative logarithm of H+ concentration, written as pH = –log [H+]
For a neutral aqueous solution [H+] is 10–7 = –(–7) = 7
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

24. Most biological fluids have pH values in the range of 6 to 8
Fig. 3-9
Most biological fluids have pH values in the range of 6 to 8
Acidic solutions have pH values less than 7
Basic solutions have pH values greater than 7
solution
Neutral
Acidic
Basic
OH– H+
[H+] = [OH–]
Increasingly Acidic
[H+] > [OH–]
Increasingly Basic
[H+] < [OH–]
pH Scale 1 2 3 4 5 6 7 8
Battery acid
Gastric juice,
lemon juice
Vinegar, beer,
wine, cola
Tomato juice
Black coffee
Rainwater
Urine
Saliva
Pure water
Human blood, tears
Seawater 9 10
Milk of magnesia
Household ammonia
Household
bleach
Oven cleaner 11 12 13 14
Figure 3.9 The pH scale and pH values of some aqueous solutions

25. Buffers The internal pH of most living cells must remain close to pH 7
Buffers are substances that minimize changes in concentrations of H+ and OH– in a solution
Example: Carbonic Acid (H2CO3) is an important buffer in living systems. It moderates pH changes in blood plasma
CO2 + H2O H2CO HCO-3 + H+
Muscles & Lactic Acid: Shifts the equation to the left
Respiratory Acidosis & COPD: Shifts
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

26. Buffers
Example: Carbonic Acid (H2CO3) is an important buffer in living systems. It moderates pH changes in blood plasma
CO2 + H2O H2CO HCO-3 + H+
Ex: Muscles & Lactic Acid: Shifts the equation to the left
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

27. Buffers
Example: Carbonic Acid (H2CO3) is an important buffer in living systems. It moderates pH changes in blood plasma
CO2 + H2O H2CO HCO-3 + H+
Ex: COPD & Respiratory Acidosis
Smoking  Scaring of the Alveoli  Shallow breathing (hypoventilation)  less CO2 released  Shifts equation to the right
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

28. Threats to Water Quality on Earth
Acid precipitation refers to rain, snow, or fog with a pH lower than 5.6
Acid precipitation is caused mainly by the mixing of different pollutants with water in the air and can fall at some distance from the source of pollutants
Acid precipitation can damage life in lakes and streams
Effects of acid precipitation on soil chemistry are contributing to the decline of some forests
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

29. More acidic 1 2 3 Acid rain Acid rain 4 5 Normal rain 6 7 8 9 10 11 12
Fig. 3-10
More
acidic 1 2 3
Acid
rain
Acid
rain 4 5
Normal
rain 6 7 8 9 10
Figure 3.10 Acid precipitation and its effects on a forest 11 12 13
More
basic 14

30. Human activities such as burning fossil fuels threaten water quality
CO2 is released by fossil fuel combustion and contributes to:
A warming of earth called the “greenhouse” effect
Acidification of the oceans; this leads to a decrease in the ability of corals to form calcified reefs
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

31. You should now be able to:
List and explain the four properties of water that emerge as a result of its ability to form hydrogen bonds
Distinguish between the following sets of terms: hydrophobic and hydrophilic substances; a solute, a solvent, and a solution
Define acid, base, and pH
Explain how buffers work
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings