|Introduction to Cell & Molecular Biology (BIOL121) - Dr. S.G. Saupe (email@example.com); Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321|
to General Chemistry
Life is chemically unique
This is one of the characteristics of life. Recall that we can determine if something is, or was, alive simply by analyzing its elemental composition.
Imagine sampling atoms throughout the universe. If you could do so, 91 out of 100 atoms (91%) sampled would be hydrogen, 9.1% helium, 0.6 % oxygen, 0.04% nitrogen. A similar study of materials in the earth's crust would give yield an elemental composition of: 47% oxygen, 28% silicon, 7.9% aluminum, and 4.5% iron. Values for seawater: H 66%, O 33%, Cl 0.33%, Na 0.3%.
Now, consider an organism like a cat. Place it in a blender, set the selector to puree, and then sample the atoms. The approximate elemental composition would be: 63% hydrogen, 25.5% oxygen, 9.6% carbon and 1.4% nitrogen. These four elements make up 95-99% of the elements in any organism. In addition, there are approximately 20 other elements are also required by living organisms, but in relatively small amounts. What can we conclude from our experiment with the pureed cat?
What's so special about C, H, N, O?
III. A look at bonding
A chemical bond is an attraction that occurs between atoms to forms aggregates (molecules). The bonding ability (and chemical properties) of an atom is largely determined by the number of subatomic particles.
IV. Life fractionates isotopes
This is another characteristic of life. We expect organisms, such as plants, to have the same concentration of isotopes that exists in nature. As it turns out, they don't - usually they have a lower isotopic concentration. Thus, plants are able to discriminate against (fractionate) radioactive isotopes and absorb non-radioactive ones.
V. Chemical Properties
The arrangement of the electrons in the atom ultimately determines the bonding ability and hence, chemical properties, of the element. Points to consider:
Electrons have discrete energy levels. The greater the energy, the further away from the nucleus it will be. Electrons can move between energy levels if they gain/loose the necessary energy.
Two electrons maximum can occupy the first energy level. They are closest to the nucleus and have a spherical orbital (1s).
Eight electrons maximum can occupy the second energy level. They can occupy any of four orbitals, each orbital holds a max of 2 electrons. One of the orbitals is spherical (2s), the other three are shaped-like dogbones (2p).
If there are more than 10 electrons, they occupy higher energy levels.
An atom is most stable at its minimal energy state, i.e., the electrons fill the innermost shells, and the orbitals are complete.
The bonding ability of the atom is determined by the number of electrons in the outer shell
Atoms "stabilize themselves" by sharing electrons or transferring electrons to other atoms to complete their outer shells.
The following table summarizes the bonding ability of C, O, N, and H. You should remember the atomic number of these elements and then be able to reconstruct the table based on this information
|Table 1: Electron composition of the elements of life|
|Element||Atomic #||Electrons in Inner Shell||Electrons in Outer Shell||
Electrons required to fill outer shell (valence)
VI. Covalent Bonds
This is a bond formed by a pair of shared electrons. Each pair of shared electrons are symbolized with a dash "-". Covalent bonds can be:
VII. Ionic bonds
Transfer of electron from one atom to another with a mutual attraction of the ions (charged). Cations are positively charged, anions are negatively charged (remember � anion = N for negative). e.g., Sodium (11Na) + chlorine (17Cl) = salt
Weak electrostatic attraction between a hydrogen covalently bound to an electronegative atom and another electronegative atom. More on this during the water lecture.
IX. The importance of covalent bonds
A. Proper strength. Covalent bonds are not too strong nor too weak.
|Table 2: Comparison of bond types|
|Bond Type||Bond energy (kJ mol-1)|
|Covalent||158 � 828|
582 � 1004
8 - 42
B. Covalent bonds are not disrupted in water (aqueous solution). Think what would happen every time we showered if we were made of, say, salt.
C. Allow a great diversity of chemical structures (i.e., single, double and triple bonds; various shapes including long chains, rings, and branches)
D. Strength of covalent bond inversely related to atomic weight. C,N,H,O are the lightest elements capable of forming covalent bonds - and thus they form the strongest covalent bonds.
X. Why is carbon so special (fitness)?
More different types of molecules are based on a carbon skeleton than any other element (molecule with carbon � called organic compound; no carbon � inorganic. At one time chemists thought that carbon containing molecules could only be made by living things. Louis Pasteur helped to disprove this idea).
Why carbon? (especially considering
that silicon is 146x more abundant in the earth�s crust and has 4 unshared
electrons just like carbon):
Some scientists claim that hydrogen, because it is
so plentiful, is the basic building block of the universe.
I dispute that. I say that there is plenty more stupidity than hydrogen, and that
is the basic building block of the universe.
Last updated: July 14, 2009
� Copyright by SG Saupe
Last updated: July 14, 2009 � Copyright by SG Saupe