1. FALL, 2016
Chemical Principles:
Water and Polarity

2. Overview: A Chemical Connection to Biology
Living organisms are subject to basic laws of physics and chemistry
To understand living organisms you have to understand their physics and chemistry
“Reductionist” approach
Life is an “emergent property” of atoms and molecules
Atoms link to form molecules via chemical bonds
Covalent bonds = sharing electrons
Non-covalent bonds = other forces

3. All covalent bonds are not the same
In a nonpolar covalent bond, the atoms share the electron equally
In a polar covalent bond, one atom is more electronegative, and the atoms do not share the electron equally
Unequal sharing of electrons causes a partial positive or negative charge for each atom or molecule

4. Water is an excellent example of a compound with polar covalent bonds
 – O H H + +
H2O
Figure 2.13 Polar covalent bonds in a water molecule
The oxygen end is slightly negative and the hydrogen ends are slightly positive
Oxygen’s electron distribution causes the bent shape

5. Methane is an example of a compound with non-polar covalent bonds
Roughly equal sharing of electrons between C and H
Electron distribution is even

6. Weak Chemical Bonds
Strong chemical bonds such as covalent bonds are not the only important bonds in living systems.
Weak chemical bonds are also important
Weak chemical bonds reinforce shapes of large molecules, help molecules attach to each other and provide on/off switches
Living systems are smart! They don’t waste energy making things stronger than they need to be.

7. Hydrogen Bonds-a key type of weak bond
A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom
The hydrogen atom is shared between two more electronegative atoms
In living cells, the electronegative partners are usually oxygen or nitrogen atoms

8.   +
Hydrogen bonding between water and ammonia-a simple and non-biological example
Water (H2O) +
Hydrogen bond
H-bonds are
weak,
short-range,
temporary,
directional
 
Figure 2.16 A hydrogen bond
Ammonia (NH3) + + +

9. The polarity of water molecules results in hydrogen bonding in water
The H bonds make water molecules stick together
They will also stick to other polar molecules (but not nonpolar ones)

10. Five emergent properties of water result from H bonding and contribute to water’s fitness for life
Cohesion/adhesion/surface tension
Ability to moderate temperature
High specific heat
Evaporative cooling
Expansion upon freezing
Properties as a solvent
Dissociation

11. Cohesion, Adhesion, Surface Tension
Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion
Adhesion is an attraction between different substances, for example, between water and plant cell walls
Both help water flow inside organisms for example the transport of water against gravity in plants

12. Adhesion Water-conducting cells Cohesion Direction of water movement
Figure 3.3 Water transport in plants
Cohesion
Direction
of water
movement

13. Cohesion of water molecules leads to a property called surface tension
Surface tension is a measure of how hard it is to break the surface of a liquid

14. Ability to Moderate Temperature-Important
Water can absorb or release a large amount of heat with only a slight change in its own temperature-it has a high specific heat.
Water absorbs heat from warmer air and releases stored heat to cooler air-it moderates temperature changes
It protects against large changes in temperature

15. Water’s High Specific Heat
The 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
Water resists changing its temperature because of its high specific heat
Note: The “calories” on food packages are actually kilocalories (kcal), where 1 kcal = 1,000 cal

16. 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
Added heat breaks hydrogen bonds in water without raising water’s temperature very much.
high specific heat of water minimizes temperature fluctuations to within limits that permit life

17. Water acts as a temperature moderator-global example
The Gulf Stream is an ocean current that carries heat from the equator to the northern latitudes of Europe

18. This hydrocarbon resembles methane. It always heats up faster
than water. Why?

19. Efficient evaporative cooling by water is also a consequence of hydrogen bonding
Evaporation is 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

20. Evaporative cooling in action
This is the most effective way for many animals to maintain body temperature because turning water into steam gets rid of so much excess heat.

21. Expansion Upon Freezing-Insulation of Bodies of Water by Floating Ice
When water solidifies, each molecule is H-bonded to four others in a rigid array: ice
All the bonding holds the molecules relatively motionless at a certain distance apart
In liquid water molecules move more freely and get closer together
Water reaches its greatest density as a liquid at 4°C not as a solid at 0°C

22. H Bonds in Ice and Liquid Water
Hydrogen
bond
Figure 3.6 Ice: crystalline structure and floating barrier
Hydrogen bonds break
and re-form
Molecules can approach
more closely
Hydrogen bonds are stable
Molecules do not move easily

23. Solvent properties-The Solvent of Life
Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily with many solutes.
When a polar or ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell or sphere of hydration.
Non-ionic or non-polar substances cannot form a hydration shell with water.

24. Water can dissolve compounds made of polar or ionic molecules: like dissolves like.
Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions

25. Hydrophilic and Hydrophobic Substances
A hydrophilic substance is one that has an affinity for water (water-loving)
A hydrophobic substance is one that does not have an affinity for water (water hating)
Oil molecules are hydrophobic because they have relatively nonpolar bonds; carbohydrates and proteins tend to be hydrophilic because of polar covalent bonds.
IMPORTANT!!!!!

26. Water dissociates or ionizes
A hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other:
The hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H+)
The molecule with the extra proton is now a hydronium ion (H3O+), though it is often represented as H+
The molecule that lost the proton is now a hydroxide ion (OH–)

27. This reaction is called the dissociation of water.
Hydronium
ion (H3O+)
Hydroxide
ion (OH–)
This reaction is called the dissociation of water.
Water is in a state of dynamic equilibrium in which water molecules dissociate at the same rate at which they are being reformed
But even pure water has some hydronium and hydroxide in it.

28. Though statistically rare the dissociation of water molecules has a great effect on organisms
Changes in concentrations of H+ and OH– can drastically affect the chemistry of a cell
In pure water hydronium=hydroxide
Some substances shift the balance so there is more hydronium = acids.
Others shift the balance so there is more hydroxide = bases.

29. Biologists use something called the pH scale to describe whether a solution is acidic or basic
Pure water has a pH of 7.0
Acidic solutions have pH values less than 7
Basic solutions have pH values greater than 7

30. Therefore living systems must maintain their internal pH close to pH 7
pH Scale
Most
liological
fluids have
pH values in
the range of
6 to 8
Life unctions
best near
neutral pH 1
Battery acid
Gastric juice,
lemon juice 2 H+ H+ H+
OH– H+ 3
Vinegar, beer,
wine, cola
OH– H+ H+
Increasingly Acidic
[H+] > [OH–] H+ H+ 4
Acidic
solution
Tomato juice
Black coffee 5
Rainwater 6
Urine
OH–
Saliva
OH–
Neutral
[H+] = [OH–] H+ H+
OH– 7
Pure water
OH–
OH– H+
Human blood, tears H+ H+
Therefore living systems must maintain their internal pH close to pH 7 8
Seawater
Neutral
solution 9
Figure 3.9 The pH scale and pH values of some aqueous solutions 10
Increasingly Basic
[H+] < [OH–]
Milk of magnesia
OH–
OH– 11
OH– H+
OH–
OH–
OH–
Household ammonia H+
OH– 12
Basic
solution
Household
bleach 13
Oven cleaner 14

31. Buffers
Buffers are substances that minimize changes in concentrations of H+ and OH– in a solution
Living systems have to contain buffers
Most buffers consist of an acid-base pair that reversibly combines with H+