Bio 178 Lecture 3The Nature of Molecules & Chemical Building Blocks of

Life

Reading

• Chapters 2 & 3

Quiz Material

• Questions on P 34 & 60

• Chapter 2 &3 Quizzes on Text Website (www.mhhe.com/raven7)

Outline

• Hydrogen bonds (cntd.)• Water• Acids, Bases, and Buffers

Hydrogen Bonds

Properties

• Weak and transient

20X weaker than covalent

• Directional

Because they are weak they are only effective over short distances.

Importance

• Stabilize the shape of many biological molecules

• Form tight bonds when there are many

Chemical Reactions

Involve the breaking and formation of chemical bonds.

Properties of Water*The polarity of water (and hence its ability to form hydrogen

bonds) is responsible for its unique physical properties.

• Cohesion and Adhesion

Cohesion

Eg. Surface Tension

Adhesion

Eg. Capillary action

• Heat Storage

Water heats up slowly and maintains its temperature for long periods due to:

(a) High Specific Heat

• Measures amount of heat that must be absorbed/lost by 1 g of a substance to change its temp by 1 C.

• Requires a lot of energy to break the hydrogen bonds so that water molecules can move freely and increase temperature.

• Importance - homeostasis.

(b) High Heat of Vaporization

• 586 calories are required to change 1 g liquid water into a gas.

• Importance - sweating.

• Ice Formation

Crystalline lattice less dense than liquid water.

Importance - barrier that protects aquatic life from cold air above.

• Powerful Solvent

Responsible for hydrophobic exclusion and the shape of amphipathic molecules.

Ionization of Water• IonizationOccurs when the covalent bonds within a water molecule break spontaneously:

H2O H+ + OH-

• MolesThe number of grams of a substance that contains 6.02 X 1023 molecules.

Eg. 12 g carbon contains 6.02 X 1023 molecules of carbon.

Molecular mass of C (mass per mole):

12 g

• Concentration

Moles per liter = M

Concentration of H+ in pure water at 25 C = 10-7 M

• pH

pH = -log[H+]

Where log = exponent of [H+]

Example - Neutral Solution

What is the pH?

pH = -log[H+]

= -log(10-7)

= -(-7) = 7

N.B. pH is a logarithmic scale

Example: How does the [H+] differ between a solution of pH 3 and 6 (how many times more concentrated is the solution of pH 3)?

Acid

A substance that dissociates in water to increase [H+].

Base

A substance that combines with H+ when dissolved in water.

Buffer

A substance that minimizes changes in pH by acting as a reservoir for H+.

Importance - Maintains pH of cells around 7.