1. Covalent bonds – where electrons are shared
Typically the strongest bonds in biological systems.
Can be polar (where electrons are not equally shared) or non-polar (electrons are equally shared).

2. Formation of a covalent bond
Hydrogen atoms (2 H)
Hydrogen
molecule (H2) +
In each hydrogen
atom, the single electron
is held in its orbital by
its attraction to the
proton in the nucleus. 1
When two hydrogen
atoms approach each
other, the electron of
each atom is also
attracted to the proton
in the other nucleus. 2
The two electrons
become shared in a
covalent bond,
forming an H2
molecule. 3
Figure 2.10

3. A molecule A single bond A double bond
Consists of two or more atoms held together by covalent bonds
A single bond
Is the sharing of one pair of valence electrons
A double bond
Is the sharing of two pairs of valence electrons

4. Single and double covalent bonds
Name
(molecular
formula)
Electron-
shell
diagram
Structural
formula
Space-
filling
model
Hydrogen (H2).
Two hydrogen
atoms can form a
single bond.
Oxygen (O2).
Two oxygen atoms
share two pairs of
electrons to form
a double bond. H O
Figure 2.11 A, B
(a)
(b)

5. Covalent bonding in compounds
Name
(molecular
formula)
Electron-
shell
diagram
Structural
formula
Space-
filling
model
(c)
Methane (CH4).
Four hydrogen
atoms can satisfy
the valence of
one carbon
atom, forming
methane.
Water (H2O).
Two hydrogen
atoms and one
oxygen atom are
joined by covalent
bonds to produce a
molecule of water.
(d) H O C
Figure 2.11 C, D

6. The more electronegative an atom
Electronegativity
Is the attraction of a particular kind of atom for the electrons in a covalent bond
The more electronegative an atom
The more strongly it pulls shared electrons toward itself

7. A nonpolar covalent bond
The atoms have similar electronegativities
Share the electron equally
Common in hydrocarbons

8. A polar covalent bond The atoms have differing electronegativities
Share the electrons unequally
This results in a
partial negative
charge on the
oxygen and a
partial positive
charge on
the hydrogens.
H2O d– O H d+
Because oxygen (O) is more electronegative than hydrogen (H),
shared electrons are pulled more toward oxygen.
Figure 2.12

9. Electron transfer between two atoms creates ions Ions
Ionic Bonds
Electron transfer between two atoms creates ions
Ions
Are atoms with more or fewer electrons than usual
Are charged atoms
An anion
Is negatively charged ions
A cation
Is positively charged

10. Sodium chloride (NaCl)
An ionic bond
An attraction between anions and cations
These bonds are strong in crystal form, but weak in water
Cl–
Chloride ion
(an anion) –
The lone valence electron of a sodium
atom is transferred to join the 7 valence
electrons of a chlorine atom. 1
Each resulting ion has a completed
valence shell. An ionic bond can form
between the oppositely charged ions. 2 Na Cl +
Sodium atom
(an uncharged
atom)
Chlorine atom
Na+
Sodium on
(a cation)
Sodium chloride (NaCl)
Figure 2.13

11. Ionic compounds Are often called salts, which may form crystals Na+
Figure 2.14
Na+
Cl–

12. Weak Chemical Bonds – form due to differences in polarity
Hydrogen bonds
Form when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom
 –
 +
Water
(H2O)
Ammonia
(NH3) O H
 +
 – N
A hydrogen
bond results
from the
attraction
between the
partial positive
charge on the
hydrogen atom
of water and
the partial
negative charge
on the nitrogen
atom of
ammonia. + d+
Figure 2.15
 +

13. Van der Waals Interactions
Occur when transiently positive and negative regions of molecules attract each other
Weak chemical bonds
Reinforce the shapes of large molecules
Help molecules adhere to each other

14. BSC 2010 - Exam I Lectures and Text Pages
I. Intro to Biology (2-29)
II. Chemistry of Life
Chemistry review (30-46)
Water (47-57)
Carbon (58-67)
Macromolecules (68-91)
III. Cells and Membranes
Cell structure (92-123)
Membranes ( )
IV. Introductory Biochemistry
Energy and Metabolism ( )
Cellular Respiration ( )
Photosynthesis ( )

15. Water – The Solvent of Life (Ch. 3)
Cells are made of 70-95% water, the “SOLVENT OF LIFE”. All living things require water more than any other substance.
Solvent -
Solute -
Aqueous -

16. Three-quarters of the Earth’s surface is submerged in water
The abundance of water is the main reason the Earth is habitable
Figure 3.1

17. The water molecule is a polar molecule
The polarity of water molecules
Allows them to form hydrogen bonds with each other (negative O ends are attracted to positive H ends)
Contributes to the various properties water exhibits
Hydrogen bonds + H
 –
Figure 3.2

18. Emergent Properties of Water Contribute to Life
A. cohesion: (related properties: surface tension and adhesion)
B. Water tends to resist rupturing. (related to cohesion)
C. Water resists changes in temperature.
D. Water expands when it freezes.
E. Water is a versatile solvent.

19. Water molecules exhibit cohesion Cohesion
Is the bonding of a high percentage of the molecules to neighboring molecules
Water molecules stick together due to hydrogen bonding
Causes surface tension and adhesion.

20. Water conducting cells
Cohesion
Helps pull water up through the microscopic vessels of plants. Water molecules stick to each other and to the walls of the xylem.
Water conducting cells
100 µm
Figure 3.3

21. Is a measure of how hard it is to break the surface of a liquid.
Surface tension
Is a measure of how hard it is to break the surface of a liquid.
Figure 3.4

22. Moderation of Temperature
Water moderates air temperature
This is very important for the maintenance of homeostasis by living organisms.
Also - ~75% of the earth is covered with water, this helps stabilize climate.
Water absorbs heat from air that is warmer and releases the stored heat to air that is cooler

23. Water’s High Specific Heat
The specific heat of a substance
Is the amount of heat that must be absorbed or lost for 1 gram of that substance to change its temperature by 1ºC

24. Water’s High Specific Heat
Water has a high specific heat, which allows it to minimize temperature fluctuations to within limits that permit life.
Heat is absorbed when hydrogen bonds break.
Heat is released when hydrogen bonds form.

25. Evaporative Cooling Heat of vaporization Evaporative cooling
Is the quantity of heat a liquid must absorb for 1 gram of it to be converted from a liquid to a gas
Evaporative cooling
Is due to water’s high heat of vaporization
Allows water to cool a surface

26. The hydrogen bonds in ice
Ice Floats
The hydrogen bonds in ice
Are more “ordered” than in liquid water, making ice less dense
Liquid water
Hydrogen bonds constantly break and re-form
Ice
Hydrogen bonds are stable
Hydrogen bond
Figure 3.5

27. Insulation of Bodies of Water by Floating Ice
Solid water, or ice
Is less dense than liquid water
Floats in liquid water
Allows life to exist in frozen lakes and ponds.

28. The Solvent of Life
Water is a versatile solvent due to its polarity
It can form aqueous solutions

29. Forming solutions with ionic solutes.
The different regions of the polar water molecule can interact with ionic compounds and dissolve them.
Negative oxygen regions of polar water molecules are attracted to sodium cations (Na+). +
Cl – –
Na+
Positive hydrogen regions of water molecules cling to chloride anions (Cl–).
Cl–
Figure 3.6

30. Forming solutions with polar solutes.
Water can also interact with polar molecules such as proteins
This oxygen is attracted to a slight positive charge on the lysozyme molecule.
This hydrogen is attracted to a slight negative charge on the lysozyme molecule.
(a) Lysozyme molecule in a nonaqueous environment
(b) Lysozyme molecule (purple) in an aqueous environment such as tears or saliva
(c) Ionic and polar regions on the protein’s Surface attract water molecules. + –
Figure 3.7

31. Hydrophilic and Hydrophobic Substances
Some substances are attracted to water and others are not.
A hydrophilic substance
Has an affinity for water. Ions and polar molecules.
A hydrophobic substance is not attracted to water.
Nonpolar molecules.

32. Life is sensitive to pH (Acids and Bases)
Water can dissociate
Into hydronium ions and hydroxide ions
Changes in the concentration of these ions
Can have a great affect on living organisms H
Hydronium
ion (H3O+)
Hydroxide
ion (OH–) + –
Figure on p. 53 of water dissociating

33. Acids and Bases An acid A base
Is any substance that increases the hydrogen ion concentration of a solution (donates protons)
A base
Is any substance that reduces the hydrogen ion concentration of a solution (accepts protons)

34. The pH Scale The pH of a solution
Is determined by the relative concentration of hydrogen ions
Is low in an acid
Is high in a base
Most biological solutions range from pH of 6-8, but there are exceptions (stomach acids pH 1-2)

35. The pH scale and pH values of various aqueous solutions
Increasingly Acidic
[H+] > [OH–]
Increasingly Basic
[H+] < [OH–]
Neutral
[H+] = [OH–]
Oven cleaner 1 2 3 4 5 6 7 8 9 10 11 12 13 14
pH Scale
Battery acid
Digestive (stomach)
juice, lemon juice
Vinegar, beer, wine,
cola
Tomato juice
Black coffee Rainwater
Urine
Pure water
Human blood
Seawater
Milk of magnesia
Household ammonia
Household bleach
Figure 3.8

36. The internal pH of most living cells
Buffers
The internal pH of most living cells
Must remain close to pH 7

37. Buffers
Are substances that minimize changes in the concentrations of hydrogen and hydroxide ions in a solution
Consist of a weak acid-base pair that reversibly combines with hydrogen ions