Covalent bonds

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

Figure 2.10
3

• A molecule
• 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

(a)
(b)
5
Covalent bonding in compounds
6

• 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

Figure 2.12
9
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
An ionic bond

• An attraction between anions and cations
• These bonds are strong in crystal form, but weak
  in water

Figure 2.13
11
Ionic compounds

• Are often called salts, which may form crystals

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

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13
Van der Waals Interactions

• 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 (124-140)
• IV. Introductory Biochemistry
• Energy and Metabolism (141-159)
• Cellular Respiration (160-180)
• Photosynthesis (181-200)

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 Earths surface is
  submerged in water
• The abundance of water is the main reason the
  Earth is habitable

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

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
Cohesion

• 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
Cohesion

• Helps pull water up through the microscopic
  vessels of plants. Water molecules stick to each
  other and to the walls of the xylem.

21
Surface tension

• Is a measure of how hard it is to break the
  surface of a liquid.

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
Waters 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
Waters 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
• 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 waters high heat of vaporization
• Allows water to cool a surface

26
Ice Floats

• The hydrogen bonds in ice
• Are more ordered than in liquid water, making
  ice less dense

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.

30
Forming solutions with polar solutes.

• Water can also interact with polar molecules such
  as proteins

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

33
Acids and Bases

• An acid
• 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

36
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