Bio 178 Lecture 3 The 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 (

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 Ionization Occurs when the covalent bonds within a water molecule break spontaneously: H 2 O  H + + OH - Moles The number of grams of a substance that contains 6.02 X molecules. Eg. 12 g carbon contains 6.02 X 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 = 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.