Download Enduring Understanding Assignment

Spring Williams
Enduring Understanding
Due Thursday, August 16, 2007
Post-course evidence essay:
Living organisms are complex systems composed of interdependent chemical
structures and processes, and therefore can be understood at a molecular level using the
same chemical principles that apply to non-living objects. Two examples of chemical
principles of nonliving objects that can be used to explain the complexity of living
organisms are polarity and bonding. The chemical principle of polarity in hydrophobic
and hydrophilic molecules can be used to explain a cell and its environment. The
chemical principle of molecular bonding can be used to explain the protein of hair with
and without a permanent.
A molecule has polar bonds if the difference in electronegativity between the
central atom and the atom attached is greater than .5. However, a molecule can have both
polar and nonpolar bonds; depending on the difference in electronegative of each atom
that is attached to the central atom. In contrast, a nonpolar bond would have a difference
in electronegativity of less than .5. To determine if a molecule is polar, the type of
bonds, electronegativity and lone pairs must be considered. A polar molecule must have
polar bonds and/or lone pairs. A molecule is nonpolar if it contains all nonpolar bonds.
The degree of molecular polarity depends on the relative number of polar groups
compared to the size of nonpolar areas. If two molecules with the same polar region and
different nonpolar regions were compared, the molecule with the largest nonpolar region
would be considered less polar.
In Ingrid’s Biology class, we learned about the structure of the cell membrane.
The cell membrane in an animal cell is the barrier that separates the inside of the cell
from its environment. Both the inside of the cell and its environment are composed
mostly of water. The cell membrane is composed of two layers of phospholipids which
have polar and nonpolar regions. The polar region is hydrophilic and is called the head.
The nonpolar region is hydrophobic and is called the tail. The phospholipids arrange
themselves so that one layer of phospholipids has its head facing the environment and the
other layer has its head facing the inside of the cell. This arrangement allows the
hydrophobic tails of each layer to be facing one another. This lipid bilayer forms because
the hydrophobic tails are repelled by water in the intracellular and extracellular matrix.
The amoeba is a unicellular organism that obtains food by the process of engulfing. The
organism is able to wrap its cell membrane around a food particle, until it uses the cell
membrane to bring the food inside of the organism and form a food vacuole. The lipids
from the cell membrane are able to arrange themselves into a vacuole because the
nonpolar tails face inside and the polar heads face the watery environment inside of the
cell and the vacuole. The remaining lipids in the cell membrane reassemble to form a
closed cell membrane.
Amoeba eating a paramecium
(taken from encarta.msn.com/media_461532743_761557743_-1)
An example of an organism’s molecular polarity discussed in class can be
observed in the DNA extraction lab. The purpose of the lab was to isolate DNA from a
plant cell. To reach the DNA, barriers such as membranes needed to be destroyed.
Because the nonpolar region of lipids is significantly larger than the polar region, they are
considered to be nonpolar molecules. In the DNA lab, a nonpolar substance needed to be
used to dissolve the cell membrane. The substance that we used was Edward’s buffer, a
detergent with polar and nonpolar region. The polar region remained in water while the
nonpolar region dissolved the lipids making up the cell and nuclear membranes.
Examples of bonds that were studied in class include but are not limited to peptide
(amide bonds), ionic, and disulfide bonds. Intermolecular forces (IMF’s) are bonds that
work between molecules to hold them together. The three types of IMF’s that we learned
about in class are London forces, dipole-dipole, and hydrogen bonding. Molecules may
be bonded by more than one type of IMF. London forces work between all molecules but
are the only force between nonpolar molecules. Because they are the weakest force, they
are easy to break. Dipole-dipole forces are only between polar molecules. Hydrogen
bonding is found between molecules bonding a Hydrogen atom bonded to Nitrogen,
Oxygen, or Fluorine. They are a strong version of dipole-dipole force.
In class bonds in living organisms were studied in our learning of amino acids.
Amino acids are molecules that join to form protein in an organism. It has specific
characteristics based on its functional group. Amino acid monomers join together by
forming covalent peptide bonds. The peptide bond forms as water is removed when the
amine group from one amino acid bonds with the carboxyl group of the other. This
joining is known as the formation of the amino acid’s primary structure. A protein’s
primary structure is controlled by genes. If there is a gene mutation, a person will have
an incorrect sequence of amino acids (primary structure). The other structures that result
from bonding within and/or between amino acids are secondary, tertiary, and quaternary.
Secondary structures are formed when amino acids are linked together, and within the
chain of amino acids, the backbone of individual amino acids hydrogen bonds with the
backbone of another amino acid. Two examples of the repeated structure that may form
are the alpha helix or the beta sheet. Tertiary structures form when the side chains of
amino acids (within a chain) bond by ionic, disulfide, hydrogen, or dipole-dipole bonds.
Quaternary structures form when there is ionic, hydrogen, dipole-dipole, or disulfide
bonds between subunits of amino acids.
An example of a protein that I thought of during our discussion of protein
structure was keratin. Keratin is a protein that is found in our hair. This protein has
many alpha helixes and disulfide bonds. If a person uses a permanent to get wavy hair, a
reducing agent is used to break some of the disulfide bonds, resulting in a change in the
tertiary structure. Next, the hair may be put on rollers and a neutralizer may be added to
reform disulfide bonds forming the desired tertiary structure (Ophardt, 2003).