Ch 2.  Molecules of Life                                             BI 101, 9/06

 A basic knowledge of chemistry is needed in order to understand life at the cellular level. 

 Key Concepts to Know:   (figure and page numbers are for your textbook)

I.  All Substances Consist of 1 of more Elements, Figure 2.1

1. Element - a substance that can't be broken down to substances with different properties.  The Periodic Table lists 92 naturally occurring elements, or types of atoms.  The variety of elements composing life is much less, e.g,

CHOPKNS CaFe Mg B Mn CuZn Mo Cl & Na
(See Hopkins Cafe managed by mine cousin Mo, clean & neat)

The elements C,H,N,O,P, & S comprise 98% of most organisms; take away water and Carbon accounts for over 50% of your body.. 

2.  Atom - smallest unit of an element & is an element; usually consisting of negatively charged electrons orbiting a nucleus of positively charged protons and neutral neutrons.

  II.  The atoms of an element have the same # of protons

    # of protons = atomic number, unique for each element.
    # electrons is usually is same as # protons, but as you will see the number of electrons can vary.

ex. Carbon, atomic number = 6, thus 6 protons (p), 6 electrons (e-)
                 Oxygen, atomic number = 8, thus 8 p, 8 e-

 III.  The # of neutrons may vary between atoms of the same element.

An atoms of an element w/a specified number of neutrons is an isotope

         ex. ¹²C , 6p + 6n  &   ¹⁴C , 6p + 8n    Some isotopes are radioactive, e.g. ¹⁴C , and can be detected with instrumentation.   Adding ¹⁴C allows the path of given carbon atoms to be traced through a series of chemical reactions .  Uses of Isotopes:  medicine and biochemistry; dating fossils based on relative amount of a radioactive isotope.

 IV.  Some atoms may lose or gain electrons becoming ions, Figure 2.3

        Ex. Na tends to loose an e- & becomes Na⁺; Cl tends to gain an e- &  becomes Cl^-     

ions = charged atoms, e.g. Na⁺, Cl^-, K⁺, H⁺ are important in body fluids--our “electrolytes”

 V.      Atoms react forming molecules due to interactions involving electrons.

           (atoms react in large part based on the attraction of opposite charges)

Molecule - atoms bonded together.  ex. O₂ or  O=O;     H₂  or  H-H;     H₂O  or H-O-H

 VI.     Ionic, covalent, and hydrogen bonds construct the molecules of life, p 18-19.

Ionic bond – a relatively weak bond formed by the electrostatic or electromagnetic attraction between (+) & (-) ions [opposite charges attract]

 Ex. NaCl – table salt, a simple molecule

             Covalent Bond - a bond created by the sharing of electrons between atoms.   

Ex. H₂O, a more complex substance.  Polar covalent bonds = when e- shared unequally between atoms w/in a molecule,

Hydrogen bond - weak bond between slightly positive H atom that is polar covalently bonded elsewhere and a slightly negative atom that is polar covalently bonded to another molecule or to a different part of the same molecule to which the H atom belongs .   

           Hydrogen bonds within large, 3-dimensional molecules hold the 3-dimensional shape of those molecules, e.g. the double helix of DNA is held together by hydrogen bonds and the shape of globular and filamentous protein molecules is held by hydrogen bonds.

Hydrogen bonds between water molecules give water its special properties (p. 20-21).  Water is cohesive (clings to itself) and adhesive (clings to hydrophilic, or "water loving", substances and surfaces; such substances will bear charges and often exhibit polarity, i.e. exhibits both negative and positive charges at opposite ends).  Polar substances dissolve readily is water (ex. sugar).