LECTURE 2
: CHEMISTRY FOR BIOLOGY
I. Atoms - the basic chemical unit.
A. There are 92 naturally occurring atoms.
B. Atoms are not usually found singly.
1. They forms elements — e.g. H2, O2
2. They form compounds -- H2O, NaCl.
C. 95% of living material is made up of only 4 different atoms:
1. 19% carbon (C) 65% oxygen (O), 9% hydrogen (H), 3% nitrogen (N)>
3. Remaining 4% made up of 30 different elements.
II. Atomic Structure.
A. The nucleus.
1. Protoms.
2. Neutrons,
B. Electrons.
C. Figure 2.5 -- helium (He) atom.
D. The 92 different kinds of atoms have different number of protons and neutrons.
1. The number of protons an atom has = atomic number.
2. The number of protons + the number of neutrons = mass number.
3. A helium atom has an atomic number of 2 — and a mass number of 4.
E. Isotopes -- atoms of an element having different mass numbers.
1. 99% of natural carbon is the carbon-12 (12C) isotope/
2. Other isotopes are the 13C and 14C isotopes.
F. Radioactive isotopes.
1. Isotopes whose nuclei are unstable — emit radiation.
2. Various practical uses for radiation.
III. Electrons and Chemical Bonding.
A. Electrons move around nucleus in levels called electron shells.
1. Inner most shell holds 2 electrons.
2. Second and third shells each hold 8 electrons.
B. Reactivity is determined by how many electrons are in the outermost shell.
1. Atoms that have full outer electron shells are inert = non-reactive
2. Atoms that do not have full outer electron shells are reactive.
3. Examples.
C. Atoms combine and are held together by chemical bonds.
D. There are three types of chemical bonds:
1. ionic bond.
2. covalent bond.
3. hydrogen bond.
E. Ionic bonding — electrons are gained and lost to form ions.
1. Ions are atoms that have gained or lost electrons.
2. Example of ionic bonding: NaCl (Figure 2.8)
F. Covalent bonding: atoms may share electrons with other atoms to fill outer shell.
G. Non-polar covalent bonds -- equal sharing of electrons -- e.g. H2, O2, N2
H. Polar covalent bonds: unequal sharing of electrons -- e.g. NH3, CO2, H2O.
1. Atoms of different sizes means unequal electron sharing.
2. Example: water () — Figure 2.11.
I. Hydrogen bonds.
1. Bonding between molecules.
2. Poles of one molecule attracted to poles of another,
3. Example — water molecules — Figure 2.11.
IV. Water — "the stuff of life."
A. Hydrogen bonding gives water biologically important characteristics.
B. Cohesion — why straws work.
C. Surface tension —walking on water!
D. High specific heat.
1. Climate moderation.
2. Characteristics of ice — makes hockey possible!
V. Acids, Bases and pH.
A. Acids.
1. Compounds release large numbers of H+ ions in water.
2. Examples: HCl, H2SO4
B. Bases.
1. Compounds pickup large numbers of H+ ions in water.
2. Examples: NaOH, KOH
C. pH — measures concentration of H+ ions.
1. Scale for measuring strengths of acids and bases.
2. pH values ranges from 0 to 14.
3. Pure water is neutral and has pH = 7.0.
4. Acids have pHs less than 7.0.
5. Bases have pHs greater than 7.0.
VI. Biological importance of pH.
A. Organisms must maintain relatively constant pH.
B. pH values of common substances (Figure 2.17).
C. Maintaining pH balance — buffers.
D. Ecosystems and acid rain (Figure2.18).
Next time: Molecules of Life (Chapter 3).